Geology and Landforms of Manitoba

Transcription

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GEOLOGY AND LANDFORMS OF MANITOBA M. Timothy Corkery The geological history of Manitoba
spans more than 3.5 billion years.
By studying the rocks of the prov
ince, geologists can deduce what the
landscape of Man itoba was like mil
lions, or h undreds of millions, of
ears ago. At times seas covered t he
wh ole province; at other times the
province was covered by up t o 1.5
km of ice . Th ere were volcanic is
lands in the area t hat is now Flin
Flon, a n d two continents collided in
the Thompson area. Rocks can re
veal whether it was warm or cold
a nd indicate when our province was
close to the equator. Also, by study
ing fossils we can even deduce
something about the animals that
lived her e long before there were
people.
Geologists partition the history
of the earth in the same way that
historians subdivide and name dif
ferent periods of hum an history,
such as the Bronze Age and the
Renaissance. In the earth 's h istory,
the two major time divisions,
termed eons, are the Precam brian
(the time when there was little liv
ing) and the Phan erozoic (the time
when evidence suggests life was
abundan t). Each of these eons was
hWldreds of millions of years long,
so they are further di vided into
eras: the Precambrian is divided
into the Archean and Proterozoic,
and the Phanerozoic is divided into
the Paleozoic (the time when early
forms of life existed in the seas an d
th en spread onto the land); the
Mesozoic (often called "the age of
reptiles"); and the Cenozoic (the
time when warm-blooded animals
- mammals - became dominant)
(Table 2 .1). The P hanerozoi c eras
are further subdivided into periods.
Manitoba's geological history is so
rich tha t much of th e geological
t ime scale is represented in our
rocks.
The rocks an d minerals that
form ed at different times in the past
occur in different places through out
the province (Figu re 2.1). The oldest
rocks in Manitoba were formed dur
ing the Precambrian eon, and are
exposed in the Precambrian Shield
that stretches from southeastern
Manitoba north westward to Sas
katchewan and the Northwest Ter
ritories. Younger sediment ary rocks
of the Phanerozoic eon are found in
the sou thwest ofthe province and in
the H u dson Bay Lowlands, in the
northeast.
In south-central Manitoba, rocks
deposited in shallow seas during
several periods of the Paleozoic are
found; these periods are, from old
est to youngest, the Ordovician,
Silurian, Devonian, and Mississip
pian. For the Mesozoic era, we have
rocks deposited during the .Jurassic
and Cretaceous periods, and in
some locations th ese are overlain by
Cenozoic rocks from the Tertiary pe
riod. Of most recent origin are gla
cial sediments from the Quaternary
period tha t cover much of the prov-
inee.
HISTORY IN
THE ROCK RECORD
Precambrian
Rocks in the Precambrian Shield
are predominantly igneous in origin
but include areas of metamor
phosed volcanic and sedimentary
rocks called greenstone belts. Broad
reas of igneous rocks were formed
by the cooling and crystallization of
extremely hot melted rock material
THE GEOGRAPHY OF MANITOBA
12
called m agma. After t h e old rocks of
the Precambrian Shield were formed,
they were buried deeply in the earth
an d cha ng d by hea t a nd pressure
to metamorphic rocks. However,
this expla natio n is over implified;
the rocks are a ble to tell us more
about the distant past. They tell of
continents olliding, volcanoes erupt
ing, and gre t wandering rivers in
mountains that may h ave been as
m aj sti c as t he Rockies today. At
times the area t hat is now Manitoba
was even in a different global posi
tio n from tha t which it. occupies to
day.
Table 2.1 Geological Formations of ManiLoba
AGE' ERA
C
E
N
PERIOD
EPOCH
QUATER·
..;fa}'T
NARY
I
~
TERl IAAY
C
140
Clay, sand, gravel.
TURTl£ MTN.
120
Shale. clav and saM LI(tOile b. d. - localed onl In TUllia MDunla!
30
Sana and saMSIOns, glO<llllsli gray - \Qcaled only In TuM'e Mounlain
'lC .,.:.
p'lf(Wj[
Alm~ G
..
T
lJP1'fR
V ERM.LIO ~
A
OI!'""l>:\
RIVEfl
C
E
Z
0
FAVEL
0
U
ASHVillE S
C'r,rlO
SWAH RIVER
J
S
I
"
u
<
.5
RESTON
,
uPf'f-!l
IU~!.·
AMA!1ANHf
..,ff
I ..HI fTal.E~
,~
ot()
300
O'.H'lEX
VANIAN
CHARLES
"I
g'"
•I
'"
,;
"I
D
·
D
A
l
E
0
0
E
V
Z
0
I
C
I
~
A
~I
0100
SILURIAN
A
0
0
MISSION
~
l OOGEPOt.E
§
A
BandM green ~ha19 al'!2 calcareous sand5.IQl"le
SandS llrmes~one yaI1-co[ouII:td shale
Limestone bull. and ,Mias. gray
WMe aoll)'dnla and'or g~m snd banoed dolomila aM ,hale
Red sha.11.!: to slClstona dolomrbc. 011 prOOuctng
Ca,bor.3l. breCCia If.c~y.ndB5J le {cryp!o·e<plosion
Permlan- Tna-59i (?)
M.s~".
anh,dnlo and
SlruClu r~1
~O!omit a
120
i.JmesIOM. lig hl bu ~. ool<1,c, 10, s.IIlelOus Ilagmonl.1 eIl.tty.
bands 01shale and anhydnlo 011 ploouclng
165
LImestone and argllacea'-l$ llmeswne. Ilgll' brown aM {edisl1
mollled Zones 0' shaly. oolilic C,,"Oldal and cherty II....lon'
0·1 pro<lue,ng
BMKEN
20
Two blacl< shalo ' ''''•••obaralod by ,:lIslone. 0,1SIlOw
J5
Red. siltstone anD sl1ale, dolommc
NISKU
~
DlJP£ROW
170
120
Umesl"". ~ doIom,". yellow'll'.Y I.,., I.'...., porOU$. some aMytjnl8
Limeslone .od dOlom,l. ero'I. r.eous and . nhvdn l ~ rn otec,,"
eycll",,1 Shale. l.mosloM and dotomile. anhydnle
~g
i&
~
~
SOUAISR!V~
H!fll;'
I OA"t,~,H;\Y
~
~ ...Rtt
". 65
i.JrnoslOO. aM dal<mle. porous, . nhydtM
120
Seu, po,a." and anhydrl'e, dolomll. ,nlerbedded
IOCIII
75
,"a). led & green
!2
Dolomlle. IIghl jello""h brown. ,.01\.
Limestol'le toss'llIerous htgh·eaJerum 001 mil. and shale bn~ rod
Ifl TEl1I..A KE GROUP
135
Dolomu•. yellowish-orange '0 greyis/t·Y9!Ow loss,IIlerous .lIty ZOnE>
!;TONEWAU.
I~
~
f'/INNIPEGOSIS
'"
~ RN
/EiUI"(tf...
W'WW, TONY
'.K)UNTAII<
(j"r,~
(, ~.
SlLK:PtI.
""
Oolomile greyish-yellow bedde<J Dolom.I••
3C
y.llowiS"·~ r. y,
<h. ly
Dolomlle dusl<I.\'eUow, losstl,l&rQus
Snale red'1ll1len, ' os""f,,,o"s. IIme9lOo" t>ands
~n.u·-,
"'-I~Tc..nfn
Fa
Doloml!tc limeston e m !lled. end dolomlle
WINN'PEG
eo
DEAOWooD
60
ShaleJl!""n w' ''L .anelsIO,," 'nlerlledd«l
:::ian an saOO&one Quartzose
GlaU~t' ~ ~~:on" aM ,.IIS10n•• and ,hal., green g's)'
10 blue very
• 01SW Maniloba only
.
flEDfllVEfl
':';'T~ .":'u HEJ.C
N
C~ MBR IMoJ
and QUartz sand pynllc snale - gray non ·c;.aI(.8rE<1us
OVG~!..lf, !l'flETON
V
C
I
2Q
C ~ Y ON
2
"
500
• .:.r
~
ST MARTIN
P E N~SYl
•S
450
SandS ll1"lI~
MEUTA
Ip~;&n
JOO
~
Shale. oark g re ~ f1on·ca. lcar~ous , :)Ilfy GUatl2 Sflnd or sandstone
75
200
' .... R
200
25()
11 5
WAS>\AOA A
C
Gray shalo non-calcarS()IJS lOcallronSiona.
benlonl" Mar tlase. gas louod
S~al. da,. yrey e," bonaceous non·ealcaJ""US. l>emonne bands
t55 I S~ale. oroy sDec1<1ed calca,,,"s OOnlonllic: s l"'~ l ly oel,ol,l.roo,
IS~a'EU1lir1< ~ril'L11Qn~us~ =. Uo~s~1 samlJ n~ SI·l
Gr. y s~. l o ,'illn heavy calco reous Sll.cl<~
40
bandS Dlllma'loM and ben'onll'
310
tStr/1llEs;..:U:;J
tR£~iCfQUS
R
"""'"
00''0·lo(llttt.
E
U
15')
1.\lN.
,,·U\'fX~
E
5
0
~.a l
t :.cFl,;,r
C
R
100
1J1der
t.1~·fl'
BOISSEVAlN
C
A rchean
The rock record of Manitoba tarts
in the Precambri n eon about 3.5
billio n years tigO . In t he eastern
part of the Precambrian Shield, in
the area known as the S uperior
Geologic Provi nce, rocks were
form ed dur ing t h Archean era and
re more than 2.5 billion years old.
Early in the Arch an , from abo ut
2.7 to 3 billion years ago, th ere was
no large continent as there is today.
At th t time ocean basins exis ted
with black pillowed basalts l being
formed al ong ocean ridges, mu ch
li ke mid-ocea n spreading rid ges
found in the Atlantic and Pacifi to
day. Also. chains of volcan ic islands
kn~wn as is1an 1 a rcs were being
formed. Remn ants of rocks formed
during these time a re preserverl in
th greenstone bel ts . Very old areas
of continental gr ani es are pre
served indicating that so me conti
n ntal land mas es must also h av
existed in the early Arch ean er a.
Very slo wly, t he force tha t move
continen ts today were a t work mov
in g th var ious segmen ts of the
em t togeth er to form th Superior
Craton (Contin nt) by a bout 2.7 bil
lion years ago.
Geolo<yical forces are im percep ti
bly slow but never idle, and .7 bil
lion years ago
major geol ogical
event called an orogeny began. Dur
in g the next 60 m illion years h igh
moun ta ins were thrust up and
great rivers ra n through de p val
ley . Grave l an d sand deposited by
th se r ivers are preserved in some
of the green 'Lone b It . Also during
t h is p riod of mountain buildin g,
'alled the I{enoran Orogeny, many
of th e granitic rocks of the Superior
GLACL'.L DRIFT
OS
M
8ASIC lrTHOlOGY
'I
~():£I,'(
Z
0
50
n«'.
Tot> soil dun a sand•
.J,r,g')t;(;F.IE
0
MEMBER
FO~ MATlON
S5(j
PflECAMBAlAN IroN)
Ad d and bas", cry'lall'n. !fld ,... .tamorph;c rockS
'miAlons 01 years OOlor. p'.S,,"1
Source: Geological H ig h way Map of Ma nitoba 1994, 2nd ed. (Winnipeg: Manitoba
Minerals Division , 1994).
Geolog] Province were fo rm d, and
tremend us heat a nd pressure
metamorphosed the rocks beneath
the high mountains.
Thus, y th e en d of the Archean,
2.5 billion year s ago, the Superior
Province of Manitoba h ad ch anged
from oceani c basin and islands i n
some unnam ed sea - similar to t he
East Indies of today - into a conti
n n t with mountain s a long the
western edge, near wh ere Thorn son is now 10caLed.
Proterozoic
Th Proterozoic era sLart d some
2. 5 billion years ago . Th re is evi
dence th at a portion of the western
margin of t h Superi r Geologic
Province was rifted away we tward
about 2.2 billion years ago, creating
an oceanic basin in the sa me fash
ion th a t th At lantic Ocean has
opened a nd spread to separate the
Americas from Africa and Europe.
With in this new ocean the cycle of
ocean fl oor spreadi ng, islan arc de
13
Geology and L andforms of Ma nito ba
..
.
W01LAl1TOM /
il
!
zoic and created a chain of moun
tains. Again the rocks were melted
and metamorphosed dur ing another
mountain-bui lding event, called the
Trans-Hudson Orogeny, which closed
the Precambrian eon in Manitoba.
Th e geological cycle of continen
t a l r ifting a nd ocean floor spread
ing, clos ure of th e ocean basin by
subduction of the oceanic plate, and
continental collision concurrent with
a mou ntain-building event has been
repeated many times thr oughout
geologi cal history. It has now come to
be known as the Wilson Cycle, after
the famous Canadian geologist J .
Tuzo Wilson , wh ose pioneering
work led to t he theory of plate tec
tonics.
IIEJAHILIHI
Tb
I
SEAL RIVER
Tl
;
/
/
CHIPEWYAH
Tb
1
1
tYNNlAK~
Lynn· TV,
lake
lEAF RAPIDS
'
TO
(- -_
Thom~SDn
.
t
~
-t,lIPEIUOR
II.ISSEYNEW
Ta
~.:r
•
'TI
DillI.
0..., ,
PIKWITONfL
Y
STU
Sb
'cf#
GODS
LAKE
Sg
N
•
MO lSO~
LAkE
Sb
___
IS LAND
f
A
~=~
'00
km
LAKE
Sg
CE NOZOIC
•
Tertiary D
Cretaceous BERENS
IIIVEIl
Sb
M ESOZOIC
.. UCHI
Sg
.. ",.oW'C
PRE CAMBRIAN
{
j
Jurassic Devonian Silurian ••
Ordovician Precambria n T
T ran s· Hudson Orogen
S
Supenor Provlnc:e
Major subdivIsions of the Pre<:ambri an In Manlloba: .
Batnolithic granrie Sedimental)' gne.ss Granite greenstone •
~
.. Fig ure 2.1 Generalized Geology of Ma nitoba (Source: Modified from Geological H ighway
Map of Mani toba 1994, 2n d ed. /Winnipeg: Man itoba Minerals Di vision, 1994J)
velopment, and sedimentation b~
gan agam .
Th e fa mous Thompson Nickel
Belt was for ming along th e west
coast of t he S uperior Con tinent.
The sands of th e day were sitting on
the gr anit es and gneisses of the
worn-down mou ntains from the
Ken oran t ime about 600 m ill ion
yea r s before. Out in the seas to the
west, deposits of mu d a n d sand
wer e building u p on the bottom, and
new chains of volca nic isla n ds were
bein g formed in the basin. DUling
thj s time th e F Un Flon , Lynn Lake,
an d Th ompson ore deposit s were
formed.
As in t h e Arch ean , pressures
built up and another old Arch ean
continen t to t he northwest drifte d
towards the S upelior Cr at on . In t he
same way t h at India ran into Asia
a nd push ed u p t he Himalayas , the
Hearne-Rae Cr aton squeezed th e
sedimen ts , volcan ics, a nd ore de
posit s formed d uring t he P rotero
Phanerozoic
By th e beginning of the Phanerozoic
eon, t he area th a t is now Manitoba
had been eroded down to a rela
tively flat to undula ting peneplain
(almost a plain ) located in the cen
tre of t h e continent. Sediments de
posited since t hen have not been
dist urbed except by periods of ero
sion when t h e land was above sea
level , because Manit oba, unlike the
mou ntainou s areas of Alberta and
British Columbia, has not been dis
turbed by a more recent orogeny.
Th is h as resulted in the preserva
tion of abundant fossils a nd sedi
ment a.r y featu res in the r ocks of th e
Paleozoic era.
All t he youn ger (Ph a nerozoic)
rock s in Man itoba are sedimentary.
Th er e are two types of sedimentary
rocks: clastic rocks an d chemical
rocks. A clastic rock is for med wh en
an older rock is (1) broken into frag
men t.s, (2) transported by wa ter or
a ir, (3) deposit ed to form a sediment
such a s a beach sand , a n d t h en (4)
lithified (made into a solid rock),
typica lly by cement ation . Clastic
r ock s are widespre a d in southwest
ern Manitoba.
A chemical rock forms by precipi
tation of atoms or mol ecules from a
solution with in a n ocean or lake.
This can occur wh en organisms
such as cor al or clams make th eir
shell s ; alternatively, if too m uch of a
certain su bstan ce is dissolved in
t h e solu tion, it precipitates to for m
a rock. Chem ical rocks a re abun
dant in the Mani toba Lowla n ds of
south -cen tral Man itoba and in t h e
14
ERA
A
T" ~I.
Mo unl.ll n
Wlnnlpe~ River AI
Red Allier
E
c
1001)
Honzontal Scale
o
..
50
l raadM
km
1500
,
A
1
~----------- + -----
Figure 2 .2 Geulogical Cross ·section of S outhern Manitoba (Source: Modified from Geological Highway Map of Manitoba 1994, 2nd ed.
(Winn ipeg: Manitoba Mi nerals Diuisin l1 , 19 47)
Hudson Bay Lowlands.
Manitoba h as rocks from most
periods in the Phanerozoic eon (Fig
ure 2.2), but there are some gaps.
F r instance, there are n o rocks
from the Penn sylvanian, Permian,
or Triassic periods, nor are there
any from the Upper Jurassic. Dur
ing these imes all of Manitoba was
above sea level as it is today, and as
now, the rocks that were deposited
du ring the previous periods were
eroded by river and wind a tivity.
Thus during these times, not only
were no new sedimentary rocks de
posited in our geological record but
we were actually losing pieces of the
record . When a new sh Bow cratonic
sea again covered the centr al part of
North America during t.he J urassic
period, the sedimentary record re
sumed, with he sediments being
deposited on the eroded surface of
the older Paleozoic strata . This type
of contact is called an unconformity
(Figure 2.2 .
Paleozoic
Paleozoic sedimentary rocks cover
the Precambrian Shield in the Hud
son Bay Lowlands and in south
western Manitoba. These rocks ac
cum ulated in depressed areas in the
earth's crust known as sedimentary
basins, two of which influenced
sedimentation in Manitoba: the
Hudson Bay Basin centred in Hud
son Bay, and the Willi ton Basin,
centred in northwestern North Da
kota.
The Willist on Basin developed
fyo m the south as the seas advanced
over the slow ly subsiding Precam
brian Shield. This advance of the
sea, termed a transgression, left de
posits of sand t ne (dominated by
the quartz-rich andstone of t he
Winnipeg Formation) as it flooded
northward (Table 2.1). As the Pre
cambrian Shield continued to sub
side, sediments accumulated on the
shallow bott.om of the ancient sea at
a rate that kept pace with the sink
ing of the crust below. These
sediments formed the dolomites,
limestones, an d interbedded shales
of the Red River Formation . The
rocks dip gently t wards the centre
of the basin in North Dakota, where
subsidence was gre test (Figure
2.2) . The Paleozoic rocks of the
Williston Basin contribute to Mani
toba's mineral ind ustry through
produ ts such as silica sand, dolo
mi tic limestone for building stone,
dolomite, and high-calcium lime
stone for cement; ubsurface deposits
of salt and potash are potential prod
ucls (Chapter 16). In addition, some
of the rocks serve as reservoirs for
petroleum.
At this t ime Manitoba was at a
tropical latitude, c vered by warm,
shallow seas that teemed wi th newly
developing life. As a result, numer
ous fo ssils such as corals, tr ilobites,
and brachiopods are found in Pa
leozoic rocks. It was during this era
that an explosion of life for ms oc
cu rred. However, this does not indi
cate that li~ began in the early
Paleozoic, but rat.her that a wide ar
ray of animals evolved hard parts
that could be preserved as fossils. It
is an unanswered questi n why the
sudden explosion of t hese life forms
occ uned t t his time.
P al ozoic rocks in Manitoba con
tain represen tatives of all the major
phyla of the animal kingdom (Table
2.2). A partial list of fossils from the
Stony Mountain Formation (Ordo
vi cian) includes several kinds of
coral, snails, brachiopod , crinoids,
sponges , trilobites, nautiloid cepha
lopods, and even some of t he earli
est armoured fishes.
15
Geology and Landform.,> ofMan itoba
Table 2.2 Major Phyl a of the Animal Ki ngdom
Phylum
Anim als Included
Porifera
Sponges
oelenterata
•
•
•
•
•
Corals, sea ane mones
Mollusca
Clams, snails, squid
Annelida
Wor ms of all shapes an d sizes
Arthropoda
Trilobites (extinct), crustaceans (crabs, crayfish , shrim p, bamacles), insects,
chelicerates (spiders, scorpions, ticks, mi tes) Echi nod ermata
Starfish. sea cucumbers. san d dolJ arf!, sea urchi ns. crinoids Chordata
Al most a ll a ni ma ls with a backbone (fish , a mphibians. d inosaur s. reptiles, birds, mammals )
Mesozoic
At the conclusion of the Paleozoic
era, t he ma rine sedime ntary rocks
were raised above sea level and
eroded. This erosional surface was
characterized by th e development
of sinkholes and caves in limes tone,
and by the development of hills and
valleys. Later, downward move
ment of the earth's crust led to a re
turn of shallow seas and to the accu
mulation of Mesozoic sediments on
the erosional s urface. The con tact
between the eroded Paleozoic rocks
and th e base of the Mesozoic rocks
is another example of an uncon
formity. Mesozoic sediments, which
formed red siltst ones, sandstones,
shales, and gypsum, were deposited
in ancient seas that covered Mani
toba from about 64 to 225 million
years ago. Distant volcanic activity,
probably in western North America,
spread volcan ic ash, which was
later altered to beds of bent onite,
across Manitoba. Gypsum, bento
nite, brick clay, and shale are im
portant mineral products from the
rock fo rmations of the Mesozoic era
(Chapter 16).
With the r eturn of the shallow
seas, m ost of the species that had
previously lived in the region re
turned, wit h some noteworthy ex
ceptions; for example, trilobites and
graptolites, which would ha ve done
well in a similar environ ment , were
extinct by th is time. However, fos
sils of large marine vertebrates,
such as mosasaurs and plesiosa u rs ,
are foun d in Mesozoic strata. This
was th e age of reptiles, and al
tho ugh fossils of dinosaurs h ave not
been fou n d in the predominantl
marine strata .in Manitoba, the
r ocks of equivalent age to the west
have abundant dinosa ur remains.
There is still a possibili ty of finding
fossil dinosaurs in Man itoba, in the
eas tern margins of the deltaic sys
tem s of rivers flowing from the
mountains to the west.
Cenozoic
Only a small portion of Manitoba
con tains rocks of early Cenozoic
time, which began abo ut 64 million
years ago. Paleocene strata of the
Turtle Mountain Formation are
lilnited to a relatively small isolated
area capping the topographic high
of 'furtle Mountain, in southwest
ern Manitoba. These strata, con
sisting primarily of fine sandy and
silty shales, rest directly on Mes
ozoic rocks.
The past 55 million years h as
been a t ime of relative geological
st ability in central North America.
The region was uplifted from the
sea and became a fl at plain. There
was little sedimentation, and ero
sional patterns began to develop on
the plains. Animal life became di
verse and abundant; paleonto
logical digs in 10-milljon-year-old
sediment s in the cen tral plains of
the Un ited States indicate that doz
en s of h oofed species, similar to
those found in t he African plains of
today, were present . Gr azing ani
mals such as horses, pronghorns,
and camels occupied the grass
lands, which they shar ed with vari
eties of elephant, rhinoceros, and
tapir. The presence of fossil tor
toises and alligators, which could
not survive extended periods of cold
weather, indicates a warm climate.
Carnivores such as sabre-toothed
cats, bears, dogs , and small weasels
found no lack of food on these
grassy plains. Even an unusual sa
bre-toothed deer found a niche in
the environment.
As the climate ch anged with ap
proach of the Ice Age, so did the ani
mal life. Woolly mammoth, bison,
and other cold-tolerant species took
the place of the savanna-dwelling
populations. Possibly the latter
were finally driven to extinction as
colder winters and dry conditions
reduced their range.
PREGLACIAL TOPOGRAPHY
AND DRAINAGE
The land was uplifted about 50 mil
lion years ago, and the Mesozoic
seas ret reated from Manitoba; even
H udson Bay may have been el
evated above sea level. This newly
exposed, generally fi at former sea
floor was su bj ected to erosion by riv
ers. Erosional patterns that persist
today began to develop on t he t hick
Cretaceous shales th at covered
most of southern Manitoba 55 mil
lion years ago. The la nd then, as
now, sloped away to the east from
the Rocky Mountains, and rivers
fl owed eastward across the Prai
ries, down this gentle slope towards
the Canadian Shield (Figure 2.3).
Some were diverted northward into
the Mackenzie watershed, others
sou th to the Mississippi watershed,
and possibly some flowed across the
16
Shield to dr ain to the al'ea wher e
NORr~W [ ll m ilT 11[1
H ud on Bay is today,
H is diffi cult Lo locate these an
Preglacial
Channel
cient river and stream valleys be
cause they have been deeply buried
beneath glacial deposits, but they
h ad 50 million years t erode the
Mesozoic sh les th at may have x
tended as far east a the On tario
border.2 Before t he onset of glad
bon , the shales were eroded back t o
the Manitoba Escarpmen t, and the
eastward-flowing streams carved
deep valJ eys, producing the em
",
b yments in the escarpment tha t
are now occupied by the Assini
boine, the Valley River, and the
Swan River.
The Man itoba E scarpment is,
6111i!ff
("OWMBlj
the n, a preglacial feature. It was
not significantly eroded by glacia
tion be ause of the ero 'ion-resist
an t na ture of the overlying hard
gray Oda nah shale . This shale, with
its h igh silica cont n t derived from
volcanic ash and the remains of sili
MONU~i
ceous microorganisms, formed a re
sistan t caprock to the Ma nitoba Es FIg ure 2 ..'3 Preglacial Drainage of the Prairies (Source: Modified from J .B. Bird, The
carpmen and prevented it from be N a tural Landscapes of CanHdl'! (Toronto: Wiley Publishers of Canada, 1972j, 117)
ing reduced to the level of centr a1
a nd east rn Manitoba . The escarp ditions are so cold that continen tal When he climate changes again
ment generally forms the eastern glaciers expan d thousands of kilo and w rms enough so tha t the win
most edge of Cret aceous r ocks in metres from high latit udes, and al ter addition of ice is less than the
pine glaciers build up and flow summer reduction, the glacier slow
the province.
down from the mountain. This ly stops advancin g an d the edges
h appens when the annual addition melt away faster than the i e can
GLACIATI ON
of winter snow is greater t han the flow.
melt ing or subli ma tion in the sum
Glacial P eriods
At this point the glacier is said to
Period of glaciation are not re mer. Even small amounts of annual be receding; as the weight of the
stricted to the time we call the lee addition of now, which t urns to ice, glacier is r emoved, t he lands rise
Age. Duri ng t he 4. 5-biIlion -year
m build u p over h usands of back to their equilibrium position
history of the earth , global climatic years. The resulting ice, which in by a process called isostatic reo
conditions h ave ch anged many our area is estim ted to have been bound . This r ebound is fa st at fir st
s much as 3 k m thick over H udson and then slows. It is not complete
tim s; indeed, maj or fluct ua tions
are the norm r at her than the excep Bay a nd possibly 2 k.m thick over even today, and in the area of
tion. While Manitoba was basking Winnipeg will try to fo rm a level Church ill the r ate of rise is still
in the sunny conditions and warm surface just as water seeks the low abou t 70 em per century.
As the ice a dva nces over the
st poin t as gravity pulls it d wn
seas of the P aleozoic era, other con
tinen ts, such as Africa an d So uth ward, Th us, a s the ice thickens by groun d, it picks up loose gravel and
Am erica, experi enced repeated gla an nua l addi tion of snow, th e mar soil, which are then frozen into its
cial periods. This reflects the global gins are pu hed from behind and base, prodUCing a n effective abra
position of tl continent as they th e ice advances as t he glacier flows
i e as t he moving ice erodes and
resh apes the land below. Glacially
were moved by pi t e tectonics dur to reach an equilibrium.
ing th ese times . However, pr oxi mity
As well as spreading out over th e polished outcrops wi th straight
to the earth's polar regions did no t land surface, the immense weight of grooves called striation s or cres
always mean ice nd cold; th re is ice de'presses th e earth's crust; as cent-shaped "fr actures cal led chatter
evidence of grea for ests similar to the ensity of ice i abou t one-third marks are one re sult of th move
those on the we i coast of Canada that of the rocks, it is estim ted ment of the debris-laden ice over
near the Nort h Pole du r ing Mes t hat 3 m of ice will depr ess th e earth the rocks .
E roded material tha t is picked
ozoic times, indicating a global abou t 1 m. So in the regi n f Hud
son Bay, the earth's crust may have up by the glacier is transpor ted and
warm spelL
During a glacia ion, climatic con- been depres ed by as much as 1,000 m. 1a ter deposited as till, an unsorted,
Geology and Landform s of Man itoba
17
unlayered mass of debris. Water
from the melting glacier also de
posits sediments known as outwash
deposits, and still other materials
are deposited along the shores and
on the floors of glacially dammed
lakes.
During the last several m illion
years of the Pleistocene epoch , E u
rope, Asia, North America , and An t
ardica have been in a relatively
cold period - often so cold th at vast
areas were covered by repeated ad
vances of the continent al ice sheets.
Studies indicate that there have
been numerous periods of glacia
tion, each lasting thousands of
years, the most recent of which,
termed the Wisconsinan, began
about 75,000 years ago a nd ended
about 8,000 years ago.
Ice sheets did not con tinuo usl
cover all of Can ada during this time
but fluctuated north to the Arctic
and south into the United States as
the climate varied. Ice from the last
glaciation receded about 7,000
8,000 years ago, after a 15,000-ycar
period of continually moving glacial
ice. The erosion and deposition from
these episodes of glaciation and the
significant sedimentary deposits
formed by meltwater as the glacier
continued their slow 3,500-year
process of melting and retreat are
responsible for most of Manitoba's
present-day landscape.
marks the eastern end of the
Brandon H ills. G But perhaps the
best known esker is that at Bird
Hill , 16 km northeast of Winnipeg.
Here a ru gh , narrow ridge of sand
and gravel extends 6.51m1 east from
Birds Hill and then merges into a
delta-shaped plateau that extends
over a broad area underlying Birds
Hill Park. The esker is the main
source of aggregate material for the
city of Winn ipeg.
End moraines are thought to
have been deposited at the edge of
ice sheet s when forward motion bal·
anced wastage. No fewer than 17
Glacial Deposition
, have been mapped throughout the
Much of Manitoba is covered by gla province, al though t.he best known
cial, glaciofluvial, and glaciolacus are in sou thern Manitoba. Io The
trine deposits, although especially Darl ingford moraine extends from
in the southeast, east, a nd parts of north of Brandon through the
t he north, bedrock is exposed at the Bran don Hills to the Tiger Hill s a nd
surface. Till and glaciofluvial depos east to Pembina Mountain . Further
its are widespread, as are many of n orth, The Pas moraine exLends
the la ndforms commonly associated south from near The Pas, then east
with deposition by a major ice sheet.. ward between Cedar Lake and Lake
Hummocky st agnation moraine 4 Winnipegosis, and on to form Long
(ground moraine) covers large parts Point in Lake Winnipeg (Figure 2.4).
of Turtle Mountain, Riding Moun
Glaci ofluvial outwash sediments
tain, Duck Mou n tain, and the Por deposited by glacial meltwater are
cu pine H ills. In the area between also extensive. They give rise to flat
Deloraine and Waskada in the terrain on which former outwash
southwest , this ground moraine has stream courses can often be seen in
a distinctive circular r idge pattern, air photographs, such as in the area
whereas further east , particulary between Pil ot Mou nd and Crystal
northwest of Killarney, low till City.
ridges 1.5-6 m hi gh trend nort heast
to south west.
Glacial Lakes
Classic "inver ted-s poon-shaped" Some of the most distinctive scen
drumlins are not common in south
ry of Manitoba result s from the
ern Manitoba, but rock-cored stream fact that glacial lakes existed in the
line hills have been mapped i n the area at the end of the Wisconsinan.
Holla nd!'I\-ehernelNoire Dame de Evidence for the existence of these
Lourdes area,5 and dr umlins and lakes t akes fOUl' forms: st.randlines,
drumlinlike ridges have been map which mat'k former lakeshores;
ped in the area between Binscarth spillways, which carried water from
an d Russel1. 6 Simila r feature s are one glacial lake to another; deltas,
widespread throughout northern where rivers en tered the lakes and
Manitoba.
deposit ed sedimen ts; and lake bot
Eskers and ka me complexes are tom sediments, deposited beyond
also a bundant in both the north and the immediate shores of t he lakes.
the south. Numerous small eskers
Lake Agassiz, the largest of the
wer e mapped by Elson 7 in the area lakes, probably came into existence
south and west of Baldur and in the about 13,000 B.P. as the resu lt of the
area between Cartwright and Crystal merging of a number of smaller
City, and by Klassen8 east of the lakes (Figure 2 .5 ),11 It owed its ex
Assiniboine River sou thwest of istence to ice damming the north
Bir tle. The Arrow Hills, a promi ward-flowing, preglacial drainage.
nent landfor m rising 50 m above With various advances and retreats
the surro unding landscape north of t he ice, the lake rose and fell so
west of Oak Lake, are proba bly a n that a t different tim es it emptied
esker. A prominen t esker also sou th into the Mississippi syste m,
Results of Glaciation
The effects of the glaciation of
Manitoba can be grouped under
four headings: glacial erosion, gla
cial deposition, creation of glacial
lakes and the alteration of the
drainage system, and the results of
isostatic depression and r ebound.
Glacial Erosion
The preglacial topography of Mani
toba was devoid of high mount ains
and deep valleys; consequently, the
potential for alpine glaciation pro
ducing spectacular landforms such
as in the mountains of Albert a a nd
British Columbia did not exist. Nev
ertheless, glacial erosion played a
role in the evolution of the land
scape of Manitoba, part icularly of
the north. Here the ice scraped off
the surface materials, leaving be
hind extensive exposures of bedrock
once the ice melted. Glacial scour
ing and etching t ook the fo rm of
striations and gouges on a small
scale, and t he larger north-south
a nd northwest-southeast flutings in
the Westlake a nd Int erlake regions.
Over the rest of southern Manitoba,
eviden ce of glacial erosion is largely
absent, or at least hidden by glacial
deposition . However, at one stage
ice fl owing from the Hudson Bay re
gion may h ave been blocked by the
Ma ni toba Escarpment.3 As thi s ice
was ch annelled to the southeast, it
may have scoured the edge of the
escarpmen t..
18
east in to the Gl'eat Lakes system ,
and northwest in to t he Mack enzie
system. 12 Finally th e pres nt out! t
to the north a long the Nelson River
became availabl , leaving Lake
Winnipeg, Winnipeg sis, and Mani
toba as remn ant s of the former gla
cial lake (Figure 1.1. 1 on page 6).
Other named glacial lakes wer e
Lake Sour is and Lake Hind in
southwestern Manitoba .
When it was at its highest, Lake
Agas iz extended far into the A.qsini
b in' embayment , and strandlines
were formed al ng the Manitoba Es
carpmen t . Beaches of this stage,
known as the Herm an , are found
along the east side of Pembina
Mou ntai n a nd Riding Mountain. At
a lat r date the lake tood at the Up
per Campbell level, during which
time a prominent beach was formed
that extends from the United St Les
border n or1.h we. t along the Mani
toba Escarpment to the Sask atch
ewan border. Known as th e Arden
Ridge in the Neepawa area, it deter
mines the southeast-northwe t di
rection of High way 352 in thi . area,
and is also followed by High ay 10
further north .
The steep-sided , flat-floored spill
ways are a second piece of evide n e
for the form er exi.sten ce of glacial
lake . They are occupied by rivers
that are mi fits, £lowing in valleys
far loo large for the present dis
ch arge. Th e Assiniboine Valley west
of Brandon is one example, as is the
Qu'Appelle Valley, which join s the
A s iniboine near St. Lazare. These
valley provide a start lin g contrast
to the general flatness of the 'Ul'
rounding prairies. They were cre
ated, possibly in a very short period,
by glacial meltwater fl owing from
one glacia l lake to another.13 In the
south, t he Souris and Pembin a ri v
ers occupy spillways for mllch of
heir length.
Deltas were deposited into the
glacial lakes. Elson lists 33 deltas
deposited into Lake Agass iz, the
largest of which is the massive
Assiniboine Delt a .14 A broad pre
glacial embaym ent in the M nitoba
Escarpm ent was invaded by glacia l
LakeAga siz, and h ere a delta, with
its apex near what is now Bran don,
was deposited. The delta exten ds
ea tward almost to Portage la PI' i
rie, a dista nce of a pproximately 120
N
~
A
I
..",
,
~~ ~~~~
HUDWIN
,
HummockY
Moraine
Ground]
_
End Morain e
.L...L
Escarp ment
.'
,
"
'. \ ~
~
•
Winnipeg"
RED RIVER
VAll£Y '.:'
'<'
""':::.
.•
S",nbac
MILHER RIDGE· :
BEDFOR D HILLS ,
~
u,k,"f'
"It'
\\ runb
.tt.
Figure 2.4 Major Glacial Depositional Features o!'So utheTll Mani to ba (Sou rce: Modi fi ed
(rom Geol ogical Highway Ma p of Ma nitoba 1994, 2nd ed. (Winnipeg: Man itoba Mineral,;
Divi8wn, 1994jJ
km, a nd has a maximum width
from north to south of 75 km. T he
bulk of the sediments in th e delta
en tered Lake Agassiz by way of th e
Assiniboin e/Qu'AppelJe system. I S
The final piece of evidence for t he
glacial lakes is the exis tence oflake
floor ediments, principally fin e
a nd, silt, and clay, which cover
part s of Manitoba and gi ve rise to
fla t ten·ain . eposition into Lake
Agassiz produced the excessively
fla t lan d around Winnipeg, n orth of
POI' age la Prairie, a n d around Dau
phin a n d Swan River. Much of the
sedimenta tion is in the form of
varved clays, sediments display' ng
regular alternations of t h in lam in a
tions nd somewh a t thicker layers.
The alternati ons may represent
sea onal deposiLion , with the
coarse-gra ined layers bein g depos
ited in summer, when the lake was
open, and the fi ne-grained layers
accumula ting in quiet conditions
during the win t.er, when the lake
was frozen .
Isos tatic Depression and Rebound
The weight of the ice caused depres
sion of the earth's crust , which re
boun ded towards its former position
once the ice melted. Depression and
subsequent r ebound was greatest
wh ere the ice was t hickest. In north
ern Manitoba the Hudson Bay Low
la nds were depressed relative to sea
level, resulting in a t ransgression of
th sea (called the Tyrrell Sea as
fa r inland as 200 km from the
presen t sh ore of Hudson Bay.
Stran dlines forme d along the
shore and h ave ince been elevated
to h eights a mu ch as 183 m above
Geology and Landforms of Ma nitoba
19
,
:/
(,' r
n-'"
---.
,"
I
-
H udsoll
,,",,:
.j,",;
1 ,-
Bay
';L
..l
+r
MANITOBA
/
,
~. "
----,
'-'
Lake Agassiz Basin
__I
Lake SOUriS Basin
DAKOTA
Major Lake Agassiz
Sedimentation Basin
_
-- - <
Tyrrell Sea Sediment
MINNESOTA
"" -. - - - -
........ Outlet
122
Elevation of the Manne
Limit
,"l melTBS)
,- .
'.:..! ,
'1/
().
100
200
~.
;
... - - -
Figure 2.5 The Extent of Glacial La ke Agassiz (So urce: E. Nielse n et 01 . Surfi cial Geological Ma p of Manitoba [Winnipeg: Prouincp of Ma nitoba , Depart ment ofE nergy and M ines, 19877J sea level on the Manitoba- North
west Territories border. 16 Further
south, the Agassiz strandlines were
tilted up to the north; for example,
Herman beach es are found at eleva
tions of 389 m above sea level west
of Morden but rise to 410 m above
sea level wes t of NeepawaY Also,
the upper Assiniboine Delta slopes
down to the south as a result of
isostatic rebound, so that the mod
ern Assiniboine fl ows along it
southern edge.
HOLOCENE
DEVELOPMENT
Nelson River into Hudson Bay. Sev
eral rivers of soulhern Manitoba
have changed course during post
glacial time. Perhaps the most re
m arkable is the Souris, which once
flowed southeast in the direction
now followed by the Pembina, but
which now takes a sharp turn to the
northeast t o join the Assiniboine
(Figure 2.6). The bend is explained
as an elbow of capture, the former
Souris having been captured by a
tributary of the Assiniboine. Below
the elbow, the Souris flows in a se
ries of incised meanders best seen
at the village of Wawanesa (Figure
2. 7).
With th e northwar d retreat of the
ice front, the drainage pattern of
Manitoba gradually evolved to the
present situation , in which the
main flow is to the n orth via the
In addition , the misfit r ivers of
southern Manitoba - the Assini
boine, the Souris, and the Pembina
- ha ve adopted a meandering form
on the floor of the spillways that
they occupy. Meander development
and abandonment is rapid; the
Assiniboine has created 26 cutoffS
in the area between the Shellmouth
Dam and Portage 1a Prairie in the
hundred years since the first maps
ofihe area were made.
Another Holocene development
has been the dissec\,ion of lhe Mani
toba Escarpment by small creek
draining to the Manitoba Lowlands.
In the Riding Mountain area, "the
escarpment slope region is . .. char
acterized by deeply incised inter
mittent streams. Stream incision
of between 30 m and 60 mare com
mon."lh At the base of the escarp
ment, deposition of eroded shale
has produced a series of low-angle
alluvial fans . Fans of similar origin
occur where small streams flowing
from Pembina Mountain reach the
lowlands between Morden and
Rathwall
A prominent feature of the landcape of southern Manitoba are
sand dunes developed on lake
deposited sediments and on ouL
wash . Most marked are those on the
sands of the Assinihoine Delta,
which, north of the Assiniboine River,
have been blown into dunes ofvarious
types. Some, southwest of Epinette
Creek, are long, seif-like dunes that
trend northwest-southeast. These
are nOw stabilized by a discontinuous
cover of vegetation, but in the Bald
Head Hills is an area of active
d unes with arcuate plans, steep
southeast faces , and gentle north
west slopes 19 (Figure 4.1.2 on page
57). Although the area of active
du nes has decreased during th
past 20 years, it has been calculated
that the dunes are moving south
eastward at a rate of 20 em a year. 20
Other, less well-known dune areas
are the Laude)' Sand Hills on sand
deposited into Lake Hind, and an
area west of St. Lazare developed
on glaciofluvial sands.
PRESENT-DAY
TOPOGRAPHY
Manitoba can be divided int.o four
physiographic regions (Figure 2.8)
each of which shows the effects of
Pleistocene glaciation.
Hudson Bay Lowlands
The Hudson Bay Lowlands consist
.
20
THE GEOGRAPHY OF MANITOB A
N
A
G_~
ftl,t (
~(I~ c,5 ~ Waw~ne$a
/
•Mill oarei
_
-
End Mora ine
- 6811'11.6 01 G l iFI.~I.Q J
- - Lako Spillway
Figu re 2.6 The Souris Elbo w o{ Capture
of an undulating plain oflow eleva
tion. Sou th of th Churchill River,
limestone till covers Paleozoic
strata, whereas in t he north of the
region it overlies t he Preca mbrian .
The whole area is overlain by m a
rine clays. In this region oflittle re
lief, the major elements of relief a re
strandlin es, former position s of the
Tyrrell Sea sh ore. Ad ditional relief
feat ures are t he valleys of the
Ch urchill and Nelson ri ers, both of
which have cut through the till and in
to b drock. Erosion by t he contin ntal
ice sheet deranged the drainage, re
sulting in a maze of swamps, lakes,
and st.reams. The region is still rising
because of isosta tic reboun d. On 1
Jul 1757, Samuel Hearne, a Hud
son's Bay Company explorer, carved
his n a me and t he ye a r into quartz
ites while si ting by a moor ing ring
on the shor of Hu dson B y. Today
thi ring is s veral metres above t he
highest tide levels.
Precambrian Shield
The Precamb rian Shield is the larg
est phy iographic region, an are a of
uneven or hu mmocky terrain . Th e
cen tral part of the Shield is occu
pied by t he Nelson depression or
trough, which slo pes t owards Hud
son B y and is drain ed by the
Churchill, Nelson , n d Hayes riv
ers. Local relief is created by lakes
a nd rivers th a t are generally en
t renched by 15 to 30 m. In north
western Manitoba the surface of the
Fig ure 2.7 Inci::;ed Meander and an Abandoned Meander of the Souris River at Wawanesa
(Photograph: Na tional Air Photo Library [NAPL] A16404·15;
Preca mbrian Shield is hilly, with
rock outcrops and numerous glacial
la ndforms such a s eskers and mo
raine ridges. The regi on is covered
by the great expanse of boreal fo rest
and thousands oflakes and rivers.
Manitoba Lowlands
The Ma nitoba Lowlands are th e
fl attest part of the province, relief
being generally less th an 8 m. The
lowlands are located sout h west of
the Precambrian Shield nd are
bounded on the west by th e Man i
toba E scar pment (Figure 2.9) . Th e
region is developed prima rily on
gently dipping Paleozoic limestone
an d dolomite strata, and h as been
modifi ed by gl acial erosion a nd
mantled by glacial deposits. I e is
drained by the Saskatchewan , Red,
and lower Assi niboine rivers, which
cut across t he structure of the un
derlying strata_
Of note in this region are Lakes
Winnipeg, Winni pegosis , a nd Mani
toba, all remnants of Lake Agas iz
(Case St udy LIon page 6). South of
Ma nitoba's Great Lakes, bedrock is
deeply buried beneat h t he silty
clays of Lake Agassiz tha t form one
of the fl a ttest and most prosperous
farming areas in Canada. In the
Interlake region to the north, topog
raphy is more bedrock-controlled,
with lim stone outcrop s forming
plateaus, rarely more t han 30 m
high, often covered by a t.h in layer of
glacial deposit, . Here t he lowland is
for est d and h as extensive areas of
muskeg a n d str ing bogs.
Throughout most of th region,
th ere are sca "tered moraine as
well as beach ridges of former Lake
Agassiz. The Assiniboine River has
cut a ch annel t hrough the sands of
the Assiniboine Delta, a nd is one of
many en trench ed stream s th at dis
sect the region. Th e top of the
Assiniboine Delta has been wind
blown t.o form extensive sand d unes
th at re part ially fixed by vegeta·
tion (Case Study 4.1 on page 56) . In
the extreme southeast, towards
Geology and Landforms of Ma nitoba
21
Southwest Uplands
r '\
1 A Precambrian Shield
\
3A
3B
3C
3D
3E
3F
3G
3H
31
Porcupine Hill s
Swan River Plain
Duck Mou nlain
Val ley River Plain
Ri ding Mountain
Minnedosa-R eslon Till Plain
Upper Asslniboi ne Delta
Souris Plain
Tig er Hills
3J Pembi na Mounio in
3K Boissevain Ti ll Pla in
3L Turlle Mountain
Manitoba Lowlands
I.~
2A
2B
2C
20
Interlake-We sllake Plain
Red River Pla m
Lower Assiniboine Delta
Soulheas t SeeMn
~~
km
Hudson S ay
Lowlands
lA
__ Precambllan
1-
Shield
Man.toba
Ltlw'ands
•
Soulhwesl
Uplands
Sel kirk ~
o_~ , oo
28
.W''''~''
Figure 2.8 Physiographic R egions of
Manitoba
"'
Lake of the Woods , sandy morainic
deposits impart variable relief and
drainage, and large areas are peat
bogs and swamp.
Southwest Uplands
The Southwest Uplands form the
western edge of the Saskatchewan
Plains and are underlain by gently
westward-dipping Mesozoic strat a.
The region is characterized by the
Porcupine Hills, Duck Mountain,
Riding Mountain, and Pembina
Mount ain , whose western margin s
form the Manitoba E scarpment. Be
tween the "mount ains" are broa d
valleys , fo rmed by the work of
preglacial rivers much larger than
the present misfi t stream s. The sur
face of these mountains, wh ich at
t.heir highest rise 500 m above t he
Manitoba Lowlands, is a relatively
flat plateau. Bedrock is exposed along
the escarpment face, but on the pla
teaus it is covered by great t hick
nesses of glacial deposits, reaching
250 m on Duck Mountain. 21
In the south, Tur tle Mountain
(757 m above sea level) is an ero
sional remnant , a topogr aphic fea
ture th at remained after erosion
had redu ced the su rrounding area.
It is capped by rocks of Paleocene
age that are covered by thick (u p to
150 m) glacial drift.22
__
~
;;
_~
3l
3K

-
' ";1
t
.
Siernbach
.
20
,
I
j
I
(i
________ -;,_
Figure 2. 9 Physiographic Regions 0/ Southern Manitoba (Source: After TR. Wei r, ed. ,
Economic Atl as of Manitoba [Winnipeg: Pro vince of Manitoba, Department of lnrlll.~ try and
Commerce, 1960))
CONCLU SION
The presen t physiography of Mani
toba h as been shaped by 3.5 billion
years of geological history. Geologi
cal activity con tin ued fro m the
early Precam brian volcanism and
mountain-building events for a bil
lion ye ars to gr ind the Precambrian
Shield flat, so that t he shallow seas
of the P aleozoic and Mesozoic could
lay down the limestones of t he
Manitoba Lowlands and the shales
of the South west Upla nds. More re
cently, glacial per iods left a m ant,]e
of deposits over m uch of the prov
ince and depressed the land to allow
the rivers t o run across the Shield
to Hudson Bay.
Each event has left its finger
print on the la nd. The P recambrian
produced early volcanism, left as
greenstone belts th at have provided
areas of mineral wealth . Then t he
major Kenoran Orogeny about 2. 7
billion years ago produced the ex
tensive regions of granite and
gneiss that fo rm the bulk of t he Su
per ior Province. The cycle was r e
peated 1.7- 1.8 billion years ago,
forming the rocks of the Trans
Hudson Orogen in northwestern
Manitoba. There followed a long pe
riod of general quiescence dur ing
which t h e Precambrian Shield was
eroded down to a rela tively fl a t
plain that extends far to the west,
deep beneath the Rockies.
Th roughout the Paleozoic and
Mesozoic eras, continental drift
placed Manitoba near the equ ator.
At t his time the Williston Basin
slowly subsided and was fill ed with
carbonate rocks, an d newly evolv
ing life forms fill ed the seas . DUTing
t he late Mississippian period the
sea receded, the waters became
br ackish and dissolved salts killed
off the animal life, and evaporites
and the last of the Paleozoic
dolomi tes were deposited. Then the
lan d emerged again from the wa
ters t o be part ially eroded away.
La t er in the Meso zo ic t he seas
22
returned , bringing more sedim nts
and n w life forms in the wa ters,
and land animals appeared in areas
at the west rn edge of lh ~ province.
By the end of the Cretaceous and
earl. Tertiary, the land was once
again emerging from t he seas. T he
continent of North America slowly
drifted northward to its present 10
'ation. Land animals, many of them
mammal , occupied t he land, an d
the stage was set for the Ice Age,
during which the details of Manito
ba's landscape were produced . Gla
cial el'osi n and deposition h ave
produced a varie ty of land~ rms.
Also, damming of the pregiaci I
drainage by the ice resu lted in a
number of glacial lake. , which in
t urn have left behind their own dis-
tinctive assemblage of landforms .
The period since the Ice Age is
ut an insta nt in geological t im .
Neverthel S5 , some details h ave
been added to the physiographic
map, such as altera tions in r iver
OlU'ses, D rm tion of sand dun es
and uplift of glacially depressed
areas. The last, at least, continue .
NOTES
1. When lava Rows are expell ed (ex
truded ) under \ ater, the outer surface
f l h e lava cools qui<:kly, forming a
th in crust . Li qu id parenL lava breaks
through the au t to for m a series of
pillowli ke masses. Contin ua hon of llie
process produces a series of in tercon
nected pIllows. h e signHicance of
pill ow If! at! is th at they ind icate
underwater ormation.
2. B .B. Banna t ne a nd J .T. Tellet·,
"~o l ogy IJfMa ni toba befo re the Ice
Age," in NCtt u rat Heritagp of Ma ni·
tuba: Legor'Y () f the Ice Age, ed . J .T.
Teller (Winni peg: Ma nitoba Museum
of Man a nd Nal ur', ] 984), 7- 21.
3 E. Nielsen et aI., Surfic io.l Gf'ologic.:ol
Map o{Maniloba, Map 1 ·1 (Winn i
peg: Provi nce of Mani toba, Depart
ment of Energy and Mines, 1981).
4 When the ice s heets were no lo nge r
bein g reple nished by sn wfall , the j 'C
disappeared by stagnation and
dow wast.ing. Sedimen ts conta ined
with in the ice were deposited on t h e
land belnw, re ' ul tin g in an in:egu lar
lopography of hills n d dep ressions
referred to as hum.mocky groun d
moraint: 01' hum mQl'ky stagnation
moraine.
5. J .A. Elso n, "Surficial geology, Brandon
west ofprincip I m eridian. Ma nitoba,
Map ] 067 A," in E .C. Ha ist ' ad,
Gl'(]ulid Water Re ources of the and L. Clayton , eds ., Glacial L ake
Agassiz, S pecial P a per 26 (St . John's,
B ra ndon Map-Area, Mafl itoba, Memoir 300 (Ottawa: Geological
NF: Geological ssociation of Canada ,
1 83), HI7-209.
Survey of Canada, 1960).
14. J.A. E lson , ~Geo l ogy of glacial La ke
6 R. W, Klasse n, Pleistocene Geology and
Agass i z,~ in W,J. Ma er- Oakes, ed.,
Geomorphology of the R iding Mou n ·
Li{e, Land rind Water (Wi n ni peg:
lain and Duck Muunlain Areas,
n ivers ity of Manitoba Press, ] 967),
Man itoba-Sashatchewan , Memoir 3913
(Ottawa: Geological S urvey of Canada,
37- 95.
1979 ).
15. Klassen , Pleistocene Geology.
16. Nie lsen et aL, Surficial Geological
7. Ison , "Su m inl geology."
Map of Manito ba.
8. Klassen, Pleistocene Geology.
9. J . WelRted a nd H . oung, "Geology 17. Ibid.
and orib,; n of t he Bnmdon Hill s, 18. RA McGinn , "A genera l de cription
sou th west Ma nitoba, " Canadian of the eas tern slopes of R iding
Mounta in" (unp u bli shed ma n cri pt,
J ournal of Earth Sciences 17(1980) 94 - 51.
Brandon U ni versity, 1983).
19. W.J . Brown , "Geomorphology Field
10. Th ese are mapped a nd na med in
I ielsen et aI. , S urficial Geological
Trip to Southw t Man itoba; ' in
Depart ment oj'Geography, University
Map of Manitoba.
11 . Ibid .
or Ma n itoba Field Guide for the
12. l'. G. F is her a nd D. G. Sm ith , "North
Canadian Association of Geo oTaphers
Ann ual Mee ti nf{, ,Ju ne 1970 (Winni
w st outlet of glacia l Lake Agassiz:
peg: U ni versity of Ma nitoba, Depart
t he Clearwater- Lake Ath abaska
ment of Geography, 1970), 43- 71.
I!pillway valleys," Abstract in Pro
20. M.H. War d, "Veget t ive Colonization
gramme with Abstracts, T hird
and Succes ion an d t h Impacts of
l nle rnatiulw l Geomorphology Co nfer
ence (Hamilton . ON: McMllilter
't'ra mpli n g in the Ca rberr y Sand Hills,
Ma nitoba" (M.Sc. t h s is, University of
Univers ity, 1993), 139.
]\I[ n itoba , 1980 ).
13. A.E. Ke how a nd L. Clay ton , "Lat e
21. Nielsen et aI. , S urfici al Geological
Wiscon sina n fl oods and development
Map uf Man itoba.
of t he Souris-Pembina sp ill way
ystem in Sru;katchewan, No rth
22. Ibid.
Dakota a nd Man itoba," in .J .T. Tell er
Geology and Landforms ofManitoba
Case Study 2.1
Karst in Manitoba
Geraldine Sweet
"Karst" is a term used to describe
landscapes developed both on the
surface and underground in regions
of soluble rock. Typical features
Include pavements, depressions,
caves, and springs. In Manitoba,
throughout the southern Interlake
region (the area between Lake
Winnipeg and Lakes Manitoba and
Winnipegosis) and along the Hudson
Bay coast are extensive areas of
limestone, dolomitized limestone,
and dolostone, all of which are
soluble to some degree in acid-rich,
terrestrial water. These rocks occur at
the surface or are covered by a
variable layer of glacial debris.
Anhydrite (gypsum), l one of the
most soluble of all rocks, is exposed
around Gypsumville. Although there
are probably karst features wherever
these rocks occur (Figure 2.1.1), they
are masked in the south and In the
far north by the glacial overburden.
There is evidence of large sinks in the
bedrock below the city of Winnipeg,
and large infilled cavities have been
encountered in cores from the
Devonian rocks west of Lake Mani
toba.
Surficial karst features are clearly
defined, and cave passages reach the
surface and can be explored in three
regions in the province.
(1) Near Lake St. Martin (Figure
2.1.1), exposures of gypsum result in
a well-developed landscape of
sinkholes, trenches,l and shallow
caves. Because gypsum dissolves
easily in water, this kind of karst is
usually quickly formed and relatively
short-lived. The present landscape is
thought to be postglacial, the rock
being so fragile that the ice would
probably have destroyed any surface
features . However, in some of the
caves there is evidence of intense
folding of the bedrock that may have
been glacially induced.
Because the rock is so malleable,
the caves vary considerably in form,
but the most common are long,
narrow phreatiC tubes,3which
formed below the piezometric
surface (water table)4 but which are
now dry because of a lowering of the
23
local water table. In fact, the longest
cave yet found in Manitoba, Laby
rinth Cave near Gypsumville, which
measures over 180 m, is of this type.
Often the tubes intersect, creating
small chambers at the intersections.
The land surface in the Lake St.
Martin area resembles the classic
cockpit karst of tropical regions, with
many well-developed sinks coalesc
ing, creating a densely pitted surface.
The reason for the density of sinks
an indicator of advanced develop
ment - is the extremely soluble
nature of the rock.
(2) There is some surface expres
sion of karst in the southern part of
the Interlake, where the now-famous
snake dens at Narcisse are examples
of collapse sinks. Water below the
surface has dissolved the rock,
creating cavities whose roofs have
subsequently collapsed. There are
much larger depressions of this
nature in places like Broad Valley, and
farmers throughout the area tell tales
of pits and trenches they have filled
with rocks.
At Inwood, Stony Mountain, and
Garson, the limestone has been
excavated for building material,
exposing extensive enlarged joint
systems in the walls of the quarries,
but the main area of cave develop
ment is near Dallas, where a number
of explorable caves have been
discovered . Some are simply long
trenches, widened by running water
just below ground level, probably
during the immediate postglacial
period, when running water was
more abundant. Some are easily
accessible through breaks in the roof.
Others are more difficult to find;
entrances are typically a small hole,
followed by a short drop into the
cave proper. St. George's Bat Cave is
of this type: it opens out into a
sizable cavity about 5 m high and 165
m long . It even has a side passage
and some stalactites, formed by
secondary precipitation. 5 Altogether
eight smaJl caves have been found in
this a rea, and exploration by the
Speleological Society of Manitoba is
continuing.
(3) The most extensive area of
both surface and underground
features is between Grand Rapids
and William Lake, in the northern
'" rr
---
~ ~".'-:'~r",
S'If'lQOr.
RI\#r
Figure 2 1.1 [,ocation of Kars t Areas in
Mo.1!Itoba
Interlake (Figure 2.1. 2) . Here the
bedrock is exposed over hundreds of
square kilometres, with only a thin till
cover in the lower areas. The result is
a region of well-developed karst
pavements, so named because the
surface resembles paving stones . In
places, blocks have subsided or have
been dissolved to produce shallow
bedrock sinks. In others, deep
trenches, or Lanjones, are the result
of joint enlargement by both chemi
cal and physical processes.
An escarpment marking the
boundary between the Silurian and
Ordovician outcrops has a series of
springs along the base indicating the
underground drains of many of the
lakes on top of the escarpment.
There are numerous collapse features
similar to those at Narcisse, and
many small, bell-shaped cavities.
Caves were first discovered in the
northern Interlake by hunters and
trappers, but in the last 10 years a
plan of exploration by Manitoba
cavers has yielded many new cavities,
and more are being found each year.
Most are found within the top 20 m
of the bedrock and range consider
ably in shape and size, although they
are all small by world standards.
There seems to be a pattern to the
development of the caves. In most
cases the main excavation has oc
curred in the relatively soft and
THE GEOGRAPHY OF MAN ITOBA 24
vuggy dolostone of the Atikameg
uniV part of the Silurian, usually
found 1- 3 m below the surface and
overlain by thinly layered, fine
grained rock. The Atikameg
dolostone is easily dissolved and
cavities have formed . later the roof
collapsed, and in many cases the cave
was enlarged along one or several
fractures .
In Dale's Cave, passages are
phreatic, having formed below the
piezometric surface. Most of the
caves are still actively forming, as
there is often much water available
during spring melt when it is particu
larly aggressive, and therefore
solution is rapid. In early spring
Moosearm Pit (Figure 2.1.3) has over
a metre of meltwater sitting in it,
until ice in the floor melts and the
water disappears rapidly, suggesting
well -developed drains in the bedrock
below. Few of the caves have stalac
tites, probably because cave forma
tion is stJII active, but there is some
secondary precipitation in the form
of the aptly named cave popcorn and
dogtooth spar.
Almost all the caves show evidence
of physical weathering - frost action
- and the bedrock is ge nerally
shattered to a depth of at least 30 m.
This is the result of postglacial
pressure release and may account for
the lack of an integrated cave system,
as water can move easily almost
anywhere in the rock. In the 1960s
Manitoba Hydro decided to build a
dam on the Saskatchewan River at
Grand Rapids (Chapter 18). More
than a third of the cost was devoted
to grouting the rock in the forebay,
to prevent water from escaping
underground . Photographs taken at
the time show long, wide trenches in
the bedrock.
At Steep Rock, on the east side of
lake Manitoba, there are a whole
series of "sea" caves. Also, along the
shore of Lake Winnipeg, lake water
has eroded the carbonate cliffs to
create deep undercuts, cavities,
arches, and stacks, a type of develop
ment found to a greater or lesser
extent on the shores of a number of
lakes. In addition there are several
locations in Grass River Provincial
Park and on Clearwater Lake, north
east of The Pas, where large sections
of cliff have collapsed, forming
cavelike passages. The latter are not
..... .,
DALE 'S CAV E ...
I I
'.
/
./.,
// ,-·::·
.. t, ! ...
/~/
/
...
"
•••
' .
/ /
"
/~/
~
/
{{ l "
II,;~
'/ "
I I. •
*.
~ / .
',,,
, I ...
/1
... '
I
I
/ I
I I
I
I
•
I
•• ' ,
I
I I
' , ' . , '•
. ',
•
J I •
• •••
I I I /A. ....
I I
.. "
,
I
..
••
MO OSEARM
PIT
I I
II
LAKE
..I-L
...
Escarpment
..
Cave
*
Spring
Small Sink
Pavement
10
0
km
,
j
Figure 2.1.2 Karst Features of the Grand Rapids- William Lake Area
really caves, despite the names given
them by the Department of Natural
Resources, but the collapse has
occurred as a result of solutional
widening of the natural joints of the
rock.
Although the caves in Manitoba
are quite small, they are geologically,
hydrologically, and biologically
interesting . So far no evidence of
human occupation has been found,
other than the odd beer bottle or tin
can, but many of them are occupied
by colonies of Little Brown Bats
(Myotis lucifus) . Caves have an ambi
ent temperature close to the annual
mean of the area, between -2°C and
+1°C, so in winter they are warmer
than the air; in summer, cold er. In
many caves the roof is higher than
the entrance, so "warm" air Is
trapped . The bats take advantage of
this and hibernate rather than
migrate south. Some species of moth
do the same. Larger mammals, such
as porcupines and bears, sometimes
use the caves as dens.
Over a hundred individual caves
have been found in the province.
Perhaps in the future something
larger will be found, as the environ
ment is certainly conducive to karst
development. In the meantime,
Manitoba's caves provide a second,
almost unknown landscape, under
ground.
25
Geology and Landforms ofMan itoba
~
j;1>
j _~
,.
~
~-.,"-
~ie
4. A glossa ry of tec hnica l terms in included
at th e end of this case study.
5. "S peleoth em " is the collective name for
all fo rms of secondary precipit ation in
caves. Speleoth ems are formed w hen
calCium sul phate-ri ch w aters enter
open ca ves and the p ressu re release
causes deposition that cl ings to the
cave w all in much the sam e way as the
"fur" in a kettle.
6. Diffe rent m odes of form ation p rod uce
diffe rent fabri cs or textures in the rock .
M uch of th e carbonate fou nd in
Manito ba w as form ed in w arm, shallow
seas, wh ere sediment w as trapped in
alga l mats. The resulti ng rock is crumbly
and full of spaces, or vu gs.
,§f~ r··
_.;
Estimated spring
meltwater level
Observed spring
meltwater level
~
tFItrl~
Figure 2.1 .3 Cross-profile of Mooseann Pit. in the Grand R apids-William Lake Area
(Source: From a survey by P Voitovici, C. S weet, et al.)
NOTES
1. Gypsum is the hydrous form
(CaSO•.2HP ) of anhydrite, a mineral
sometime s found in large enough
masses to form a rock. It will dissolve in
any liquid.
2. Some of these may simply be enlarge
ment of joints. In tropical environments
th e term "l'anjone" is common, and
refers to major enlargement by solution
and subsequent coll apse . Some of the
tren ches in the northern Interlake
exhibit similar characteristics.
3. The terms " phreatic" and "vadose"
refer to the location of caviti es in
re lation to the piezometric surface
(water table), and hence th eir m eth od
of form ation . Vadose caviti es are
formed when freely flowing w ate r
comes in contact with soluble rock; the
emph asis is on downcutting, and the
upper parl of the cavity is frequently
above the w ater. Phreati c ca viti es form
below the w ater table; the entire
surfa ce will therefore und ergo so lution
at th e same time, creatin g ro un d tubes
that follow the dip of the rock .
Case Study 2.2
The Brandon Hills
John Welsted and Harvey R. Young
A
~
Om
-', .....
.J
Figure 2.2.1 Locatio n of the Brando n Hills
In an area better known for its
flatness than for hills, the Brandon
Hills come as a surprise to the out
sider. located approximately 20 km
south of Brandon, they rise promi
nently above the surrounding land
scape (Figure 2.2.1). They must "have
been a welcome sight for a party of
settlers led by the Reverend George
Roddick from Pictou County, Nova
Scotia, who in May 1879 crossed the
Assiniboine at Grand Valley three
miles downstream from the present
day Brandon and saw an island of
woodland set amidst a sea of waving
grasses." l Although the surrounding
land was soon pre pared for agricul
ture, the hills offered little agricul
tural potential and have remained an
island of natural vegetation sur
rounded by cultivated land.
GLOSSARY
Zanjone - An enlarged fra cture, generally
associ ated with solution of carbonate
rocks in the tropics.
Cockpit kars t - An area of cl osely linked
depressions with remnant hill s between
them .
Piezometri c surface - The in terface
betw een rock containing freely moving
water and th e saturated stratum (al so
ca lled th e water ta bl e).
Cave popcorn - Small g rowths of calcium
carbonate on ca ve wa lls an d ceilings; a
form of stal actite.
Dogtooth spa r - Seconda ry depositi on of
p ure or nearly pure ca lcium, sh aped like
teeth .
Description
The hills run for approximately 12 km
in a generally east-west direction,
and are about 5 km across from north
to south . In places they rise 90 m
above the surrounding landscape and
reach altitudes of over 480 m above
sea level (Figure 2.2.2). Despite their
limited area, they show considerable
variation in topography, leading us to
divide them into four physiographic
regions: (1) the main body of the
hills, an area of irregular topography
with some definite ridges; (2) the
eastern ridge, a large, sinuous,
generally north-south-trending ridge;
(3) the eastern complex, an area of
mainly north -south-trending ridges
and troughs east of, and adjacent to,
the eastern ridge; and (4) the south
ern ridge, a northwest-southeast
trending ridge. 2
The main body of the hills consists
of a series of subparallel ridges that
THE GEOGRAPHY OF MANITOBA 26
trend northwest-southeast in the
west, more nearly west-east in the
centre, and swing around to become
almost north -south In the east. These
are mainly wooded (poplar, scrub
oak, and Manitoba maple), but some
of the more prominent ones have
only a sparse grass cover. A profile
drawn across them along the hydro
road reveals that the topography is
hummocky, with the south side of the
hills being steeper than the north
side (Figure 2.2.3) . Glacial till is
exposed in the road cuts along this
minor highway.
The eastern ridge is quite differ
ent. It has been described as looking
"as if an enormous dump truck has
moved along depositing piles of dirt
every few hundred metres."l Many
do not recognize it as a ridge because
most people see it while travelling on
Highway 344, from which its steep
eastern side is clearly visible, al
though there is no indication of an
equally steep western side. Even the
National Topographic Series 1:50,000
map does not show it, which is
surprising because it reaches a
maximum elevation of 477 m above
sea level and is 90 m above the flat
land to the east." The view from the
top is spectacular, stretching to
Brandon In the north, beyond Shilo
to the east, and across an old lake
floor beyond the small hamlet of
Rounthwalte to Wawanesa in the
southeast.
The ridge is symmetrical in cross
profile, with slope angles of over 30
(Figure 2.2.4) . Slow downhill soil
creep has produced small terracettes,
clearly visible only when the light is
in the right direction. "Draws" or
minor gullies on the ridge sides give a
scalloped appearance when seen
from above (Figure 2.2.5) . Gravel pits
at each end reveal that it is underlain
by well-sorted and layered sands and
gravels (glaciofluvial deposits), and
borings made into the ridge crest
indicate patches of till deposited
directly from ice. large boulders
"erratlcs" - are found along the
ridge crest and sides . They are
composed of rocks foreign to the
area and can be grouped into two
types: granitic boulders, generally
spherical and usually covered by dark
green lichens; and dolostone boul
ders, typically slablike and usually
covered by bright orange lichens.
t ~
r.lllJ~
,
t_
" ~,..
'\
:"
"_
LJ
"
/
~s,
I,'
I,.
It· "
~I
'(),
I~
'c'
../'
.y
. ~-::..
~-
~.'" .•. =. ----,
•
II
Figure 2.2.2 Topography of the Brandon Hills
----t<S1
..0 - 1 - - - - - -
~
_-.;::__--+'11 1:
~~o
~ 4~9+---
JI~.J-u-->oo-=-,oa..-.........,---r--,
I<I'l
~"'
...,.
Do<-J
<II
...,
i
'T'
IOO---"T"----"'---.Im
"r- 'IO(I --'I'
,OOO- -l-t',,'I------,.,.
I
.OO-=--......-"---I
'600 ,.OO!IiJ
M"re,
Figure 2.2.3 Topographic Profile along the Hydro Road
The ridge provides marginal
conditions for plant growth. It is
exposed to wind from all directions,
and the permeable nature of the
underlying sands and g ravels mili
tates against tree growth. Vegetation
at the crest is limited to a sparse
grass cover that is easily disturbed.
However, small, scrubby trees can be
found in the "draws" on the flanks,
where there is more water and
shelter from the wind.
The eastern complex is similar to
the eastern ridge in consisting of a
series of north-south-trending ridges.
These are more obvious on air
photographs than on the ground
(Figure 2.2.5). They also have grass
covered crests, with trees growing in
the intervening troughs. Erratics are
common, including the largest we
have found in the Brandon Hills, a
giant slab of dolostone 3 m long and
1.5 m wide.
The southern ridge is also similar
- both on the ground and on air
photographs - to the eastern ridge.
Erratlcs are of the same type and
occur in the same ratio as on the
eastern ridge . However, this ridge is
lower (approximately 30 m) and less
steep (maximum angles about 20°). It
is also much straighter. The south
west (drier) side is grass-covered,
whereas the northeast (wetter) side
is wooded.
Origin
The Brandon Hills were formed
during the last ice age, probably
towards the end of the Wisconsinan
glacial period. On surficial geology
maps of southern and southwestern
Manitoba, they are shown as end
moraine, part of a system extending
from north of Brandon through the
Brandon Hills and the TIger Hills to
Pe mbina Mountain, a system usually
referred to as the Darlingford Mo·,
raine . The designation "end mo
raine" suggests that they are com·
posed of sediment deposited at the
edge of an ice sheet when it was
stationary, forward movement of the
ice being balanced by melting. While
this Interpretation may be generally
correct, the single term "end mo
raine" does not describe all the
features of the Brandon Hills.
The main body of the hills is most
accurately described as end moraine,
but even here there are uncertainties.
Beneath the area, the layered and
----
27
Geology and Landforms of Manitoba
sorted deposits are clearly of
glaciofluvial origin; that is, they were
deposited by water in association
with ice. These are overlain by till, a
true glacial deposit. Initially we
believed that the ridges of the main
body of the hills resulted from the
irregular deposition of till as the ice
front receded, but we now believe
that they are of glaciofluvial origin
and that the overlying till was depos
ited on preexisting ridges.
Both the eastern ridge and the
southern ridge have many of the
characteristics of an esker - a
landform thought to originate by
deposition in tunnels in a glacier or
an ice sheet, the deposits being left
behind in a ridgelike form when the
ice melts. Because the sediments
were deposited by running water,
they are layered and sorted. The till
patches and the scattered erratics on
the ridges would have been depos
ited by the overlying ice as it melted.
Both the eastern ridge and the
southern ridge can be categorized as
eskers, although the eastern ridge in
particular is much higher than any
other esker mapped in southern
Manitoba.s The eastern complex is
sufficiently similar in both composi
tion and topography to the eastern
and southern ridges to suggest that it
has a similar origin.
The description of the Brandon
Hills as an end moraine can be
regarded as the "traditional" expla
nation. However, an alternative is
provided by Aber, who claims that
the hills are glaciotectonic in origin.
He explains the ridges of the main
body as being due to ice thrusting,
with great slabs of previously depos
ited glacial sediment, or bedrock,
being pushed forward by the advanc
ing ice. Despite this explanation for
the main body, he still regards the
eastern and southern ridges as
eskers,6
Whatever the origin of the
Brandon Hills, they are a large ly
unspoiled landform that has main
tained its natural vegetation and that
offers a variety of opportunities for
the outdoors person,
Figu re 2.2.4 T he S teep E as t-facing
( Photawaph: ·fo hn Wel.s t ~ J )
Fig ure 2.2.5 T he Eastern Part af the Bra ndan Hills. The grass-I'o(.'er('ri eastern ridge !l ig h t·
tun pd) crosses th e area frum narth (a t the tap af the ph otagraph) to. sauth ; thi> f!<1s tern
complex is located east of the ridge and highway ,744 i.~ ea"f aOhat. (j)hota{!ra ph: NAPL
Al 64 08·.9)
28
NOTES
1. J. Wel sted and H. You ng, "The Blue Hills
of Brandon," Manitoba Nature (Autumn
19 72):3 2.
2. J. W Isted and H. Young, " Geology and
ori gin of th e Bra ndon Hill s, Southw est
Manitoba, " Canadian journal of Earth
Sciences 17(1980) :942- 51.
3. ) Wel sted and H. You ng, "The Bl ue Hi lls
of Bra ndon," 35 .
4. The relevant m ap is 62G/l 2 Wawan esa .
A survey point located near the centre
of the ridge has the exact height
1566.3 fee t (477.4 m) .
5. Eske rs in the Baldur, Notre Dame de
Lourdes, and Bruxelles area to the east
Case Study 2.3
Evolution of the
lower Assiniboine River
(1) WILLOWBENO PHASE
Lu!.:/!
Yea rs Ago
-)
~
(3) LONG LAKE · HIGH BLUFF
PHASE ?
.\/III1I1O/>
It.,
,
?
\I,wirf)/>
1{"'"1011(1
~
(2 ) flEE ISLANO PHASE
L<lI;,·
7. 030 Years Ago
WF. Ronnie
The "Forks" at the confluence of the
Assiniboine and Red rivers has
become one of Winnipeg's best
known feature s, the focus of major
redevelopment and a source of
international acclaim (Case Study
10.1 on page 150). less well known is
the fact that The Forks has not always
been at its present location; indeed,
there has not always even been a
Forks. About 3,000 years ago, The
Forks was at St. Norbert, 14 km to the
south, where the La Salle River joins
the Red, and for several thousand
years before that the Assiniboine did
not join the Red at all but flowed
north from Portage la Prairie into
lake Manitoba. These and other
routes of the Assinibolne River are
shown schematically In Figure 2.3 .1.
The cause of these dramatic changes
lies with the Portage la Pra irie alluvial
fan, an unusual feature that, al
though barely noticeable on the
ground, has produced an evolution
of the Assiniboine River with few, if
any, counterparts elsewhere in the
world.
Most alluvial fans are small, steep
features composed of coarse materi
als deposited by multichannel
streams with "flashy" (frequently
ephemeral) flow regimes in dry
climates. The fan constructed by the
Assiniboine has none of these charac
teristics. The river Itself is pere nnial,
carries a modest sediment load, and
has a strongly meandering single
channel pattern. The fan it has
produced has a very low gradient, is
composed of relatively fine materials,
and is an order of magnitude larger
than most other contemporary fans .
Only in two crucial respects does it
resemble other fans : the radial
configuration of channels, and its
of the Bran don Hi lls are commonly in
the order of 10m in height.
6. J.S . Aber, "Spectrum of Constructional
Lan dforms," in Genetic Classification of
Glaciogenic Deposits, ed . R.P. Goldthwait
and c.L. Ma ts ch (Ro tterdam : A.A.
Balkema, 1989), 281-92; Figure 4.
Yea rs Ag o \((//11/11/'"
~
--- - ...
)t.
4.520 Years Ago
..,
l'
,
"''''...
,
,-- , , -
,II : .. ,,"
...
....
-'- - - ---
---..
- - - . . .,
(6) Mill PHASE
(5) LA SALLE PHASE
Luke
(4) BLI ND PHASE
u.k,·
1.300 Yea rs Ago
2,980 Years Ago HlII,ilflbCl
.If 1111/, be' ...
~ ""
,,"...- ......> - - - -
, ... ,,"
'\
\
I
I
,.
- -~
I
"
... " - '~
--
~
(7) CURTIS RIDGE PHASE
Lukt'
70 0 Years Ago
• (8) ASSINIBOIN EPHASE
Present
Me//fIIIIP"
\
"
_
.....
I
\
I
," ,
.... ....
I
t'
,
.. '
\
--~
Figure 2.3.1 E volu tion of the Lower As::;iniboine R iver
location at the point where the
Assiniboine emerges from its confin
ing valley onto the l ake Agassiz Plain
(Chapter 2).
The Portage la Pra irie fan was
produced by processes normally
associated with floodplain form ation
by a laterally mobile meandering
stream. Deposition In and near the
river channel repeatedly elevated the
river on an alluvial ridge . As it gr w
higher above the surrounding
Geology and L a ndform s of Ma nitoba
29
c.............. C' Locati on of Fan Profile
Boundary of Allu vial Fan
Escarpment
N
A
~---.!O
km
Figure 2.3.2 Paleochannels of the Lower A ssiniboi lle R iver, S howing the Locotion of Profiles
in Fig ure 2 .3.3
terrain, it became increasingly
unstable and was eventually aban
doned in favour of a new lower
course, a process called avulsion. Each
of these episodes left an abandoned
ridge-channel system, or
paleochannel (Figures 2.3.2 and
2.3.3), with features such as scroll
bars that show clearly on aerial
photographs (Figure 2.3.4).
The Initial courses of the river
were northward to Lake Manitoba.
The oldest paleochannel,
Willowbend, was active before 7000
B.P., or shortly after the recession of
lake Agassiz water from the region.
Drainage into lake Manitoba shifted
to the Flee Island channel and finally,
by 4500 B.P ., to Blind channel, which
remained active at least until 3400
B.P. As the reg ion between present
day Portage la Prairie and Lake
Manitoba became "filled" with
sediments and paleochannels, further
avulsion exploited lower terrain and
steeper gradients to the east, divert
Ing the ancestral Assiniboine away
from lake Manitoba and towards the
Red River.
The firs t eastward route was
established by about 3000 B.P. along
the channel now occupied by the La
Salle River, joining the Red In
present-day St. Norbe rt. By about
1, 300 years ago (in the Mill Phase),
the lower Assiniboine had shifted
northward to join the Red at its
present location at The Forks, but
differed from its modern course on
the fan . The upper fan section of the
river was altered again by 700 B.P. in
the Curtis Ridge Phase. Thus, the
modern route of the Assiniboine
from Portage la Prairie to Winnipeg is
somewhat less than 700 years old. '
Interestingly, maps of the soils in
the Portage la Prairie area reflect this
evolution. Soils adjacent to the
northward channels, being older,
display mature chernozemic A
horizons (Chapter 4), whereas those
south of the present Assinlboine have
immature, thin, organic layers, as
would be expected in much younger
sediments. The shift in outlet from
lake Manitoba to the Red River about
3,000 years ago coincided with a
sharp reduction in sedimentation in
lake Manitoba and increased th e
sediment and water inflow to the
Red . Although the sedimentation
history of lake Winnipeg Is not
known, an Increase in the last 3,000
years would be expected .
Until recently, alluvial ridge
formation continued to elevate the
modern Assiniboine channel on the
fan, producing a potential for future
catastrophic channel abandonment
during large flood events. This
potential was greatly reduced in
1970, however, by the opening of the
Portage Diversion (Case Study 18.2
on page 283) . located just west of
Portage la Prairie, this large artificial
channel enables water to be diverted
from the Assiniboine to lake Mani
toba to reduce flow through Winni
peg during major floods (and inci
dentally to reduce flooding on the
alluvial fan) .
This diversion was possible only
because of the elevation of the river
provided by the fan near Portage la
Prairie. Its value was demonstrated
during truly exceptional floods in
1974 and 1976, when much of the
alluvial fan and lower terrain would
otherwise have been inundated.
More importantly perhaps, given th e
elevated state of the Assiniboine,
these floods might have triggered
avulsive channel abandonment,
which would have required expensive
remedial works. Thus, human modifi ·
cation of the flood regime has
probably ended the period of active
fan formation .
NOTE S
1. Fo r more details, see W. F. Rann ie, L.H.
Thorleifson, and J.T. Tell er, " Holocene
Evo lution of t he Assiniboine River
Paleoch annels an d Portage la Prairie
Allu vial Fans," Canadian Journal of Earth
Sciences 26( 1989): 18 34-4 1; and W. F.
Ranni e, "Th e Po rtage la Prairie
' Flo odp lain Fan'," In Alluvial Fans : A Field
Approach, ed . A. H. Racho cki an d M.
Ch urch (Chi chester, Englan d: John
Wiley and Sons, 1990), 179-93 .
30
A
I ASSUUIDIIlE
~I'-I~
I"
"
. ..
lLE
~A'
8
A&! IN UI OI NF.
~ ~
-
LAA'~
U l '-./ u~,
"'
LA UlLE
C'
Fig ure 2 ..1.3 Cross-pro fil es of the
So uth of'the Assiniboine
RivP.r, S howing Major Alllw ial Ridges
A.~s ilL iboi ne F a ll
Figure 2.3 .4 Paleocha nnels S outh of the Assiniboine River. The Trans-Canada Highway
crosse' the area in the north at the top of' th e photograph. (Photograp h: Ma nitoba Air Photo
Library I MAPLJ MB 89021-J76)