exploring Burrator - Dartmoor National Park

Transcription

T he evolution of a
Moss-strewn clitter slope in broadleaf woodland
Dartmoor
Landscape
exploring Burrator
© Peter Keene
© Peter Keene
Clitter on the flank of Sharpitor
For south west England enquiries phone Traveline: 0870 608 2 608
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visit www.devon.gov.uk/devonbus or phone 01392 382800
P
P (cars only)
Parking
P
Dartmoor granite
Devonian country rock
(mainly slates and sandstone, some lava)
P
500 metres
y
railwa
Riv
er
M
ea
vy
12
B3 2
+
De
von
por
t
Sharpitor
Lea
t
550 yards
The approach in this booklet is to concentrate on a series of viewpoints, all
close to, or within easy walking distance of, roadside parking places. Convenient
parking and access to every viewpoint is clearly marked on the inside rear
cover map. Walking to one or any number of sites, and in any order, should
bring some satisfaction and reward.
Each viewpoint is regarded as a separate little excursion concentrating on
one aspect of the landscape of the moor. However, as may be seen from the
map, all the selected sites are clustered in the Burrator area, encouraging the
prospect of visiting more than one site. In fact, if you intend to go to several
of the viewpoints, there may be an advantage in doing the Sharpitor walk
first, as the focus here is on broader aspects of the evolution of the upland
landscape of Dartmoor.
Princetown
(2 miles)
Toilets
antled
Dism
Many of those visiting the moor are in cars, and with friends or family who
may not wander too far into the moor. Yet, to appreciate and understand the
nature of the landscape, we need to be there, standing on the tors or looking
closely at the granite from which this landscape was made. In that way, what
is being discussed can best be judged against what you see.
Viewpoints with description in text
Walks to viewpoints
Footpath and cycle route
Leather Tor
Peek Hill
Leather Tor
Pit
Cross
Gate
B3
Inn
Norsworthy
Bridge
t
Lea
or t
onp
Dev
To
Walkhampton
21
P
2
DOUSLAND
ton
ver
Yel mile)
(1
Yennadon Down
The booklet is suitable for anyone who is inquisitive about the scenery
surrounding them. Some key words describing the Dartmoor landscape are
in italics and are explained in a short glossary on page 34. In addition,
Information Boxes found throughout the booklet carry more detailed discussion
which provide a resource which can be referred to during any of the walks
or later.
Dismantled railway
The smooth rolling uplands of Dartmoor, broken by distinctive bare rocky
outcrops, give both enjoyment and an inner sense of contentment to many
who walk, cycle, ride or drive across this sweeping moor. This booklet is
written for those who experience this pleasure yet also seek some explanation
for the underlying geology and landforms over which they pass.
B3
212
About this book
P
Burrator
Reservoir
Pillow
Mounds
Yellowmead Down
Sheeps Tor
Burrator
Dam
Burrator
Quarry
Sheepstor Dam
+ Sheepstor Village
P
+ Inn
Meavy
Front cover: Burrator Reservoir with surrounding viewpoints. © South West Water
River
Meav
y
or Brook
Sheepst
© Crown Copyright. All rights reserved. Dartmoor National Park LA 08908L/01/01
Use of this image is limited to viewing on-line and printing one copy.
T he evolution of a
Dartmoor
Landscape
exploring Burrator
Written by Peter Keene
Walking to the Peek Hill viewpoint
1
Contents
Viewpoints around Burrator
Each viewpoint listed below can be reached by a short walk from roadside parking (see inside
rear cover map). Every listed viewpoint is regarded as a separate excursion concentrating on
one aspect of the physical landscape of the moor so any number of viewpoints can be visited
in any order. At each location attention is drawn to some interesting aspects related to the
evolution of the moorland landscape.
Viewpoints
Focus
Page
Sharpitor
The evolution of a granite landscape
with tors
3
Burrator Dam
The human impact
10
Burrator Quarries
Looking at rocks in close-up
13
Norsworthy Bridge to
Leather Tor Pit
Finding evidence for violent environmental change
19
Burrator Dam to Sheeps Tor
A longer walk designed for those wishing to
venture a little further on to the moor, scrambling
up to one of the more spectacular local tors.
This walk may also be regarded as a review,
pulling together aspects of landscape
interpretation introduced on the other walks.
26
Information boxes
More detailed information on the evolution of the physical landscape is presented in tinted
Information Boxes (as listed below). These boxes are not essential reference material to the
casual enjoyment of the walks but are designed to provide an interesting and more detailed
discussion or explanation for some of the intriguing landscape features visited. Any box can
be usefully referred to during any of the walks or after.
Box
Topic
Dartmoor is born – a timescale
The origin of the tors
Plymouth’s water supply before the reservoir
Identifying the main minerals in granite
Landscape processes
Newleycombe Brook - a stream at work
Slope movements in a cold climate
A pit reveals environmental change
Jointing in granite
A
B
C
D
E
F
G
H
I
Page
4
6-7
10
15-16
19
20-21
22
23
29-30
Other information
2
Some key words describing the Dartmoor landscape34
Walking with Moor Care, Safety for the walker, and 35
the Geological Fieldwork Code
inside
rear
cover
Area map
Sharpitor
400 metres
The crest of the tor is a gentle walk
of about 400 metres from the roadside
parking areas on the B3212. It is about
3 miles (4.8 km) from Princetown and
the same distance from Yelverton (see
inside rear cover map). Sharpitor: grid
reference SX 560 703.
0
350
P
2
B321
0
33
P
37
0
36
0
390
410m
38
12
0
Sharpitor
400m
Peek Hill
Leather Tor
340
Yelverton
2 miles
310
The short answer is GRANITE. Most
upland surfaces are slowly reduced in
height as they are eroded by slope
processes and rivers. However, the
granite of Dartmoor has proved more
resistant to this attack than the
surrounding, softer, sedimentary rocks.
The result is that, after a long period of
time, the granite moor stands higher than
the surrounding Devon countryside.
Despite its present elevation, Dartmoor
granite originally formed over two miles
(3km) beneath the earth’s surface.
Overleaf an Information Box illustrates
the birth of the moor in greater detail.
Photo: Leather Tor and the high moor surface
© Peter Keene
Viewpoint - Sharpitor
Beyond Burrator Reservoir the land drops
steeply away from the moor. Twelve
miles (19km) away, and 410 metres
lower, is Plymouth Sound and the English
Channel. In other directions, for example
south-east (see photograph), the relief
of the moor is relatively gentle. Although
this is the ‘high moor’, the horizon looks
quite level with many of the rounded
summits reaching similar heights. With
this view it is not difficult to reconstruct
the ancient tableland or plateau into
which the rivers and streams of
Dartmoor subsequently eroded their
valleys. Although it has been dissected
by numerous streams radiating from the
moor, Dartmoor stands proud of the
surrounding Devon landscape. Why?
35
0
33
B32
Standing on the peak of Sharpitor you
are 410 metres above sea level. On any
clear day you will have sweeping views
in all directions. “There is nothing I love
so much as that which stretches before me
and out of sight.” (André Breton)
Princetown
2 miles
The evolution of a granite landscape with tors
3
Box A
Dartmoor is born - a timescale
GEOLOGICAL PERIODS (Age in millions of years before present)
DEVONIAN & EARLY CARBONIFEROUS
In a vast east-west trough, sands and muds were deposited, compacting to form
many beds of sandstone and mudstone. Some limestones and lavas were also
present.
LATE CARBONIFEROUS
Continental collision: two super-continents (Laurasia to the north and Gondwana
to the south) collide. The east-west trough between them is closed, intensely
folding the sediments into a great mountain range. The mountains have long
gone but the spectacular folding can still be seen on the coast (e.g. Hartland
Quay, North Devon).
Melting mantle: deep within the roots of these (Variscan) mountains, temperatures
rose to about1000°C and parts of the mantle melted to form a large molten
underground reservoir (batholith) of magma.
Dartmoor pluton: in places the magma rose to within a couple of miles of the
surface. One of these ‘plutons’ cooled to become the granite mass of Dartmoor.
Alteration and mineralisation: the great heat and pressure altered (metamorphosed)
the Devonian and Carboniferous ‘country rock’ surrounding the moor whilst,
within the cooling granite, circulating hot water and vapours led to the local
concentrations of minerals both on the moor and in the surrounding ‘metamorphic
aureole’.
Jointing formed: the cooling granite contracted and a strong jointing pattern
developed (more detail on jointing in Box I, pages 29-30).
PERMO-TRIASSIC
Dartmoor granite unroofed: there followed a period of rapid erosion. The granite
had cooled about two miles (3km) underground and yet New Red Sandstone
sediments deposited near Exeter in the Triassic already included fragments of
Dartmoor granite, showing that by this time the granite must have already been
exposed at the surface by erosion.
4
TERTIARY
Any sediments, such as a layer of chalk, which subsequently might have covered
the granite have long been removed by erosion. At some time the granite and
the surrounding landscape of Devon and Cornwall was planed-off by a series of
plateau-like erosion surfaces. These surfaces, modified by uplift, tilting and
faulting, form the distinctive plateau landscape of Dartmoor into which the high
moor valleys have been carved.
QUATERNARY (The last two million years)
The Quaternary has been dominated by dramatic climatic oscillations which
frequently plunged Dartmoor into cold arctic-like conditions. It can be argued
that much of the physical landscape of Dartmoor is the result of the modifications
which took place under these conditions.
AGE
Standing on solid rock
Identifying solid granite
Look around your feet to confirm that
you are in fact standing on ‘solid granite’.
To do this you need, first, to be able to
recognise that the rock is granite. This
may not be as easy as it sounds, even
when bare rock is exposed.
In winter the climate at 410 metres is
severe, yet the rocks are covered in moss
and lichens which discolour the stone
and make it difficult to see individual rock
crystals. The photograph on page 13
shows what local granite looks like when
‘clean’.
Is the rock solid? Again, not as easy a
question as it sounds. It is a characteristic
of granite that it is criss-crossed by bold
and widely spaced joints that give most
tors the appearance of gigantic irregular
blocks. More confusion might be caused
by the great scatter of granite boulders
(clitter) that surrounds most tors, and
over which you must have scrambled to
reach this summit. The photograph below
shows the clitter field surrounding
Sharpitor, here emphasised by a
powdering of snow.
The granite tors are the most distinctive
and well-known feature of the Dartmoor
landscape. Many of the surrounding hills
have tors either on their summits or
flanks. How were these remarkable
landmarks created? Box B on pages 6
and 7 explains.
Clitter on Sharpitor
© Peter Keene
Dartmoor is made of granite, yet over
much of the landscape the solid rock
itself is obscured by soil, unconsolidated
debris or vegetation. Exceptions to this
are the tors, where solid granite is
exposed at the surface.
5
Box B
The origin of the tors
Isolated rocky outcrops develop on many hard rocks cut by numerous joints.
The origin and evolution of granite tors have been a matter of intense debate.
However, there is strong support for models which emphasise the critical
importance of climate in controlling the style of weathering and the movement
of debris down the hillsides.
Warm wet phases
Massive blocks of granite, created by the jointing pattern, are often surprisingly
well-rounded, a characteristic which is even more pronounced underground,
where the joints between the rounded blocks (corestones) are filled with
decomposed, weathered granite (growan). Deep chemical weathering can
be an effective process whenever the climate is humid enough and warm
enough to support chemical activity.
Tropical forest environment
Warm wet conditions very favourable to deep chemical weathering existed
on Dartmoor within much of the Tertiary (page 4). This supports the idea
that the ‘rotted’ weathered granite (growan), so common on Dartmoor, was
created at this time. The sketch below visualises what Dartmoor might have
looked like at this time, with a dense cover of trees and vegetation. In this
environment, warm water, acid-rich from rotting vegetation, could percolate
down the joints and interact with the granite, particularly the relatively weak
feldspars (page 14) leaving the quartz (sand) relatively sound.
Temperate phases
Close examination of growan has revealed that many of its characteristics
match moderate chemical weathering in a more temperate climate such as
we are experiencing today. It is likely that deep chemical weathering reoccurs
whenever the climate becomes warm and wet enough to encourage chemical
activity. Whatever its history, growan is very widespread and much of the
buried granite is covered by this partly decomposed layer, as is suggested
in the diagram below.
6
When
the climate is
warm, acid-rich water
from the thick vegetation
cover trickles down into the granite
along its joints, rotting the surrounding
granite into rounded blocks, or corestones.
(continued on page 7)
Box B
(continued)
Tors in the Ice Age
Towards the end of the Tertiary (two million years ago) the climate
deteriorated, plunging Britain into the Pleistocene Ice Age. The graph overleaf
demonstrates that this was not an age of unrelenting cold but a series of very
cold ‘glacial’ stages separated by warmer ‘interglacial’ stages. During some
of these cold stages, ice sheets spread over much of Britain. At their maximum
extent they reached south as far as the North Devon coast.
Periglaciation: conditions on the
margins of an ice cap
Only 18,000 years ago, a 1,200 metre thick ice sheet spread from northern
England as far south as the Bristol Channel. However, southern England
remained ice sheet free. Dartmoor, only 70 miles (112 km) from the ice
sheet edge was ‘periglacial’, experiencing cold tundra or steppe-like conditions
with only short summer thaws. Water penetrating into restricted granite
joints repeatedly froze and melted, a process thought to be capable of forcing
granite blocks apart (frost wedging). Where the texture had been already
weakened by chemical weathering, water could penetrate the grain structure
of the granite causing it to break up (granular disintegration). During the
coldest stages, the deeper subsoil remained frozen solid, even in summer
(permafrost), so that when the seasonal thaw melted the surface snow and
ice, the water could not percolate into the frozen ground. Some of this
summer meltwater escaped by running down the hill-slope as ‘sheet wash’.
Cold stage slope movement: solifluction
Cold
stages in the
Ice Age caused
movement of loose debris
on the slopes (solifluction),
which stripped away weathered debris
to expose the granite corestones as tors.
Clitter, smaller blocks of granite, litter the slopes.
Within the shallow zone of seasonally-melted ground, meltwater lubricated
the soil and rock debris to such an extent that it became a pasty mush, which,
under the force of gravity, slowly sludged down any slope. This mass movement
process, known as ‘solifluction’, was extremely effective in transporting debris
down-slope, denuding the granite corestones and exposing them as ‘tors’
which were then susceptible to rock failure and frost attack. Blocks of granite,
detached from the tor or from hillside exposures, collected around the tors
or were rafted down-slope by solifluction to become aprons of clitter
surrounding the tor. More details on solifluction (head) are on pages 22 [Box
G] and 23 [Box H].
7
WARM
The climate of southern
Britain fluctuated violently
In the ‘recent’ past.
record of a cold past
surface appearance of the moor.
Dartmoor tors can thus be regarded as
relics left over from a colder past.
COLD
17
16
600,000
15
400,000
14
Years before present
temperature ° C
850,000
13
200,000
Short interglacials
when the climate was
as today’s
12
A natural landscape?
today
A fossil landscape?
The very cold climate, which favoured
the rapid movement of loose debris down
hill slopes, so exposing the tors, ended
abruptly about 10,000 years ago. The
climatic improvement which followed
heralded the present warm interglacial
climate (see above). The warmer climate
effectively ended rapid slope movement
(solifluction). Look at the clitter
surrounding Sharpitor. The lichencovered stones and well-vegetated slopes
show little sign of being on the move
today. Similarly, the dominance of
mechanical frost action was replaced
by chemical weathering
associated with
percolating water.
This chemical weathering
does little to change the
8
The fluctuations in the graph opposite,
imply a number of cold stages when
debris was progressively stripped to
expose the tors. These cold stages
alternated with less dramatic interglacial
stages when chemical weathering was
resumed.
The notion of the moor as a ‘fossil’ tundra
landscape is perhaps reinforced by the
very bleakness of this high moor. This is
misleading, for this is not a ‘natural’
landscape. By 8,000 years ago oak and
hazel forest had invaded the moor to a
height of about 410 metres, the height
of Sharpitor. Mixed farming took place
up to 450 metres in the later Bronze Age
(4,000 years ago) by which time human
impact on the moor was significant and
much of the forest cover had been
removed.
Despite a deterioration in climate after
3,000 years ago causing farming to retreat
© Peter Keene
A lone survivor. Perhaps the tender first
shoots were protected from grazing animals
by clitter.
to the lower more sheltered land, the
moor has been a ‘managed’ environment
ever since. This includes, of course, the
return of the forests in the form of conifer
plantations both surrounding Burrator
Reservoir and below you to the east in
the valley of the River Meavy.
Walking to Peek Hill you leave the granite
crossing onto the country rock (see rear
cover map).
Leather Tor
Peek Hill has its own tor. The significance
of this is that it demonstrates that, given
a solid rock with plenty of joints, tors can
develop on rocks other than granite.
Should you wish to extend this landscape
walk, Leather Tor is only 200 metres to
the south-east.
The jointing patterns in the granite are
well developed here. There is more
information on jointing in Box I (pages
29-30).
The rocks of Peek Hill were probably
once mudstones and lava but they
became baked slates by being so close
to the granite before it cooled.
Burrator Quarries (page 16) are also on
the junction between the granite and the
Devonian country rock. In the old quarry
face there, it is possible to see the junction
in cross-section.
Peek Hill
Burrator district as shown on an early Ordnance
Survey map (circa 1850)
About the same distance to the southwest over level ground is Peek Hill.
9
Burrator Dam
The human impact
Access to Burrator is best from
Dousland village on the B3212
Princetown to Yelverton Road (see
inside rear cover map). At the
crossroads in Dousland, take the road
south-east (signposted to Burrator and
Meavy). In under a mile (1.6 km) a
road swings left to Burrator Reservoir
and Dam (grid reference SX 552 680).
Although no landscape in the British Isles
is truly ‘natural’, one attraction of standing
on a Dartmoor tor is the feeling of
untamed wilderness, where human
impact is muted.
Burrator Dam, the resulting reservoir
and the afforestation surrounding the
lake, have created a very different
‘artificial’ landscape, although one which
has its own attraction and its own intimate
relationship to the moor.
Box C
Plymouth’s water supply before the reservoir
Plymouth expanded rapidly as a strategic naval port in the 16th century and
soon outgrew its local well-based drinking water supply. Nearby Dartmoor,
an upland of impermeable granite and high rainfall (1,500mm or 60 inches
per year) became a potentially attractive source of water.
Drake’s Leat
Viewpoint - Burrator Dam
In 1585 an Act of Pariament authorised the town of Plymouth to divert, for
its own use, the waters of the River Meavy. The Meavy passes through the
Burrator Gorge draining a good part of the south-west flank of Dartmoor.
The water was soft and pure and did not peat stain even after heavy rain.
It also continued to flow in dry weather.
10
Sir Francis Drake (mayor of Plymouth at the time,1590) was instrumental
in driving the scheme through. Drake’s Leat (Plymouth Leat) diverted some
of the River Meavy at a point now covered by the reservoir, although the
course of the leat can still be traced downstream of the dam. The Ordnance
Survey map (circa 1850), on page 9, shows the leat as a thin black line,
skirting the western side of the gorge between the road and the valley floor.
Plymouth was 10 miles (16 km) away as the crow flies, but the leat was an
open gravity-controlled channel and so had to maintain a regular gradient.
The resulting contour-hugging channel took 18 miles (29km) to reach
Plymouth. This was a considerable engineering feat as the valley floor here
is only 200 metres above sea level – an average gradient of 1 in 145 or 0.7%.
Devonport Leat
In 1793, responding to continued drinking water demands, the Devonport
Leat was opened. It tapped the Meavy catchment on its eastern heights and
then follows the northern flank of the valley (see map on page 9). This leat
is again gravity-controlled and it is worth a walk to see the smooth speed
of its rushing waters. Although now superseded by the Burrator Reservoir,
the leat still supplies water to Dousland Water Treatment Works for the
Yelverton area.
Choosing a dam site
2
21
PRINCETOWN
B3
2 Miles (3 km)
1000 metres
Walkhampton
A3
86
B3
Inn
21
2
Dousland
21
2
TAVISTOCK
5 miles (8 km)
B3
Dam
Burrator
Reservoir
Yelverton
Sheepstor
Inn
Meavy
A38
6
PLYMOUTH
10 miles (16 km)
The 19th century expansion of Plymouth
prompted a search for a reservoir site in
the Meavy catchment to replace the
supply from Drake’s Leat which suffered
from evaporation in summer and became
frozen in winter.
Where would you site a dam? With the
benefit of hindsight you might suggest,
“Right here”. The map ‘hatching’ on page
9 shows a narrow gorge at this point.
Looking down-stream from the parapet
of the dam gives you a good impression
of the gorge through which the Meavy
tumbled - an ideal site for a dam?
In fact, the two sites initially suggested
in the 1880s were higher up the
catchment. The favoured site was the
old Head Weir of Drake’s Leat, now in
the middle of the reservoir. A 13 metres
earth embankment was planned, holding
1,360,000m 3 of water. However,
geological investigations revealed ‘rotten’
unconsolidated weathered granite at this
site to a depth of 30 metres (100 feet).
The second choice was further upstream,
across Hart Tor Brook (GR: SX 562 715),
but overwhelming public opposition
blocked this option.
The present dam was therefore a
compromise site, rather than one
determined purely by engineering or
geological considerations.
The foundations of the Burrator Reservoir. February 1896 © South West Water
Getting to Burrator Dam
11
Building the dam
Work began in 1893 and took five years
to complete. Unlike most modern dams,
it is of gravity construction, relying for
its strength on the sheer bulk of the dam
itself rather than on an arch or buttress
wall. The extensive use of granite, both
as rubble blocks (some of 8 tonnes)
within the dam and as masonry blocks
on the dam face, was in part to provide
rock of sufficient density to make such
a gravity construction sound.
Before the foundations were built, a
trench, in places 15 metres deep, was
cut into the solid granite along the line
of the dam. The photograph (page11)
shows this trench in February 1896. The
massive natural joints were cleaned and
filled with concrete.
The granite for the dam came from a
quarry 200 metres upstream, now on
the floor of the reservoir. Some 60,000
tonnes of granite were removed. The
hole increased the capacity of the
reservoir, which, when opened in 1898,
was 2,720,000m3.
Twenty-five years later (1923) work
began to raise the dam height by three
metres, increasing the reservoir capacity
to 4,650,000m3. The original quarry now
being drowned, granite for new masonry
was cut into the hill in a roadside quarry
only 200 metres from here, on the road
back to Dousland (Lower Burrator
Quarry).
Clean blocks of this stone are well
displayed in the roadside parapets of the
dam. The photograph opposite shows
one of these granite blocks in detail. Box
D (pages 15 and 16) describes the
common crystals which you can identify
in any granite block here.
Burrator
Reservoir
P
+
Quarry
P
from Dousland
(B3212)
T Burrator
Dam
River Meavy
Old Burrator
& Sheepstor
Halt
250m
12
dismantled railway
(cycle way)
Burrator Dam - Burrator Quarries and old rail halt
Burrator Quarries
Looking at rocks in close-up
Feldspar
Mica
Quartz
A close up of a granite block in the parapet of Burrator Dam.
Access to Burrator Quarries is best
from Dousland village on the B3212
south-east (sign-posted to Burrator
and Meavy). In under a mile (1.6 km)
a road swings left to Burrator Quarries,
the reservoir and dam.
The floor of the disused upper quarry,
on the left as you curve down the hill
towards the dam, has a spacious
parking area (grid reference SX 550
677). Alternatively you may prefer to
park by the dam, 300 metres further
on, where the walk starts at the
roadside wall of the dam.
Granite exposed
The granite exposed in the parapet of the
dam was quarried only 200 metres from
here (we will pass the quarry shortly).
Although some local variations occur in
Dartmoor granite, the composition of the
granite in this wall is typical. Granite is an
example of an igneous rock, one that has
cooled from molten material. As such it
is made up of minerals in a crystalline
form. The crystals here can be picked out
with the naked eye.
Large interlocking crystals
Why does granite have large interlocking
crystals? Crystal size is controlled by the
rate of cooling of the magma, the molten
material from which igneous rocks form.
Viewpoint - Burrator Quarries
© Peter Keene
13
Slow cooling at depth in the Earth’s crust
gives plenty of time for crystals to grow.
Those crystals which were the first to
form in the molten material, could grow
and expand without interference from
other crystals. Those formed later on
had to make do with whatever space
was available. Eventually all the available
space is filled with angular interlocking
crystals.
Mica, feldspar and quartz
The photograph, on page 13, shows part
of a block of granite in the parapet wall
on the northern side of the road across
the dam. It displays well the three main
minerals of which granite is composed.
The large crystal of feldspar measures
3cm by 1cm.
Choose any block of granite in the wall.
Can you recognise the three minerals?
Box D, opposite, provides some detail
to help identification and a little more
information about each mineral.
14
Having examined the wall and perhaps
admired the view, return to the
western end of the dam and walk south
along the road, away from the lake
(toilets on the left). After some 200
metres, on the right, is the tall face of
an old abandoned roadside quarry.
Abandoned roadside quarry
Do not approach the quarry face but
stand on the grassy floor close to the
road. All observations made about this
quarry can be appreciated from a safe
distance. Do not walk under the
quarry face.
This is the quarry that, in 1923, provided
the blocks of granite used to increase
the height of the dam (page 12) and build
the parapet.
Box D
Identifying the main minerals in granite
Mica The little black dots
Small, black gleaming plates (thin flakes) with a splendent lustre. This black
or bronze-like biotite mica is rich in the metallic elements of iron and
magnesium which modify the colour of the mica when it is attacked by
chemical weathering. The mineral is relatively soft (hardness 2.5 to 3) and
even softer when attacked by chemical weathering. The mica weathers into
clay which lacks the strength and coherence of the original mineral. Some
of the black dots you see may be tourmaline crystals. These also occur locally
in veins, as in Burrator Quarry where they will be discussed under that
heading. Tourmaline is similar in colour to biotite mica but a lot harder
(hardness 7) and, when the crystals are well-formed, develop characteristic
curved three-sided prisms.
Feldspar The large white rectangles
The white rectangular feldspar crystals clearly dominate the example shown
in the photograph on page 13. They would have begun early on in the process
of crystallisation and grew steadily within the molten ‘soup’. Its texture, large
feldspar crystals set in a finer-grained background, is characteristic of many
South West granites. The large crystals are called phenocrysts and the whole
texture, porphyritic. Some of the smaller white crystals are also feldspar but
with a more opaque, irregular shape.
China clay (kaolinite) is the end product of intense alteration largely derived
from the decomposition of feldspar. This is considered to have occurred in
some of the granite by the action of hot circulating solutions after the granite
was emplaced.
Quartz Colourless and glassy, here without any distinctive shape
This hard mineral (hardness 7) has a vitreous lustre. In the sample shown
on page 13 it seems to have moulded itself around the other minerals and
is without any distinctive shape although when in veins its crystal structure
can be quite distinctive.
Quartz (silica SiO2) is generally very stable and resistant to weathering. When
chemical weathering has altered the feldspar and mica content of the granite,
the quartz retains its strength and coherence. At the foot of blocks of granite
exposed either to chemical weathering (rain water) or physical weathering
(frost action producing granular disintegration), it is common to find a residual
accumulation of sand (quartz). The same sand may be found on the beds of
most Dartmoor streams, on its way through the drainage system to Plymouth
Sound.
All the minerals in the blocks remained hard but feldspar is particularly
vulnerable to chemical breakdown and, in chemically weathered granite that
you might come across elsewhere, the feldspars become soft enough to be
scratched with your finger nail – it is on its way to becoming a clay.
15
Lower Burrator Quarry
Not surprisingly, this quarry face exposes
granite which is identical to that which
we saw at the dam, a typical Dartmoor
granite. However, some 75 years have
elapsed since this face was worked. The
rock surface has weathered. It is covered
with lichens and, in particular where
water is seeping from joints, blackcoloured algae. The best place to see a
clean granite face today is, in fact, back
at the dam.
Joints parallel with quarry faces
Joints parallel with land surface
Quarry
Jointing
The quarry face is cut by a series of
massive, well-developed joints, similar to
those seen on the photograph on page
11. These represent fractures formed by
stresses set up in the molten rock as it
cooled and contracted.
A second group of joints are those which
formed when erosion removed overlying
rocks to expose the granite (page 4). The
reduction in the confining pressure was
so great that the rocks expanded and a
series of fractures (joints) developed
roughly parallel to the land surface. These
sheet structures can be seen in most
tors. In this quarry it is likely that many
of the horizontal joints running across
the quarry face were formed in this way.
16
A quarryman’s nightmare
Sheet jointing (pressure release) is a
process which is operating today. When
this ‘cube’ (the quarry) was cut from the
hillside, the release of pressure on the
rocks behind was such that a series of
potentially dangerous sheet joints would
have developed. As too many quarrymen
know to their cost, this can lead to
accidents when a whole quarry face peels
away and crashes to the quarry floor.
(More detail on jointing, Box I, pages
29-30)
Joints developing around a quarry due to
pressure release.
Box D
(continued)
Tourmaline
During the final stages of cooling of the
magma, the circulation of active, hot
fluids and volatile gases led to the
deposition of veins of important minerals
such as tin, copper, lead and zinc, both
within and on the margins of the granite.
Veins of non-economic minerals, such as
quartz and tourmaline, were even more
common.
Tourmaline is a hard, black mineral
deposited by super-heated boric vapours
that followed the joints and fissures in
the already consolidated granite - proof
that the joints were already there when
the vapours arrived. Small clusters of
hard black crystals and thin veins
(stringers) are common. The wholesale
replacement of mica and feldspars can
result in very resistant masses of
tourmaline and quartz.
The Upper Burrator Quarry
Now continue to walk up the roadside
and, after a few metres, turn right into
the wide parking area in front of the
Upper Burrator Quarry. This quarry
has a relatively low rear wall and is
safe to approach to examine the rock
closely.
The contrast between the two quarries
could hardly be greater. We have
crossed the boundary between the
Dartmoor granite and the surrounding
Devonian country rock into which the
granite was intruded (page 4). What
makes this site so special (it is a Site
of Special Scientific Interest) is that
the junction between these two rock
types is exposed in the quarry face.
The country rock is best seen towards
the left-hand (south) end of the long rear
wall of the quarry. These rocks were
once muddy marine sediments which
compacted into a mudstone. Later, uplift
and folding altered this sediment into a
slate. Later still, when the granite was
intruded, the immediate proximity of the
molten granite meant that the Devonian
slate became baked to a hornfels.
The contact is not clear-cut and to
appreciate what is happening involves
getting close up to the exposure. The
contact is sharp but very irregular and
consists of a series of intrusive veins of
fine pink granites which have penetrated
the country rock along joints. Thin veins
of tourmaline (black) and quartz (white)
are also recognisable.
The veins of granite have much smaller
crystals than the granite from the Lower
Quarry (photograph, page 13). Here, in
the Upper Quarry, the molten granite
The northern end of the quarry, showing
the line of a large vein of fine-grained
granite.
seeping into the country rocks in thin
veins, cooled relatively quickly so had
little time to develop large crystals before
it solidified. The component crystals mica, feldspar and quartz - can still be
picked out, but the thinner the vein, the
finer the granite crystals.
This is the outermost frontier of the
granitic invasion. Perhaps, to the
geologist, this is like visiting Hadrian’s
wall, the feeling of being on the edge of
the Roman Empire.
Walking along the face from south to
north, the altered slates are more and
more affected by the proximity of the
invading granite. The actual contact
zone is towards the right-hand end of
the quarry.
17
From the northern end of the quarry
there is an easy scramble upslope to
reach, within a minute or so, a level
track which used to be a railway line.
Try locating yourself on the map on page
9. This may not be an easy exercise. The
first edition of the Ordnance Survey map
(circa 1850) predates the dam and the
reservoir. The site of the Burrator Dam
was close to Sheepstor Bridge. Contours
are not included but the steep gorge
below you is shown by tight hatching.
The Princetown Railway
The map also predates the railway (now
a cycleway). A ‘railway’ is shown on the
map as a parallel black line but the route
shown is to the west of here. This was
the Plymouth and Dartmoor Railway, a
horse-drawn tramway to Princetown,
completed in 1826. In 1881 this tramway
was acquired by the Great Western
Railway in preparation for the opening
of a full ‘steam’ train service to
Princetown. This line was fully opened
in 1893.
The dismantled railway line that you are
standing by was a rail loop around
Yennadon Down, added to the original
Yelverton to Princetown railway by the
GWR because the new steam trains could
not take the tight curves which were
part of the old tramway system. The
steam railway closed in 1956.
Solid black lines on the old map show
the route of Drake’s Leat and Devonport
Leat (see Information Box C, page 10).
The panorama
From the little outcrop of jointed
granite nearby, there is a good view
of the reservoir and surrounding
countryside. Keep clear of the
unfenced back wall of the quarry. Most
of the viewpoints shown on the inside
rear cover map can be seen from this
spot.
Why was the Burrator dam built in this
location? The topography may give you
a good enough reason but other priorities
were also involved (see page 11).
Now the option is to return to the
road by the way you came or follow
the dismantled railway a little way,
passing the old Burrator Halt (see
photo below) and taking the path
which slants down the hillside to the
right. Once on the lakeside road, turn
right to complete the circuit.
Burrator Halt (1955 photograph by Eric
Hemery, © Pauline Hemery).
18
Norsworthy Bridge to Leather Tor Pit
Finding evidence for violent environmental change
Access to Norsworthy Bridge is best from Dousland village on the B3212
Princetown to Yelverton Road (see inside rear cover map). At the crossroads in
Dousland, take the road south-east (sign-posted to Burrator and Meavy). In
under a mile a road swings left to Burrator Reservoir.
Passing the dam on your right, keep straight on, ignoring any junctions coming
in from the left. After some 1.5 miles (2.4km) the road crosses two stone bridges
spanning two streams in quick succession. Stop just beyond the second bridge
where there is convenient parking for cars or minibus. The walk starts by the
side of the stream, Newleycombe Brook, which flows under the bridge closest
to the car park.
Natural processes (Box E) are always
changing the landscape. For example,
material weathered from the tors is
transported down-slope, either as solid
debris or in solution. Slopes lead to the
streams that will eventually transport this
‘load’ off the moor.
All the material that travels down slopes
anywhere in the stream catchment above
this point will eventually find its way
under this bridge. Dartmoor is gradually
being carried away. Boulders and stones
are trundled or bounced along the stream
bed. Finer material such as silts and clays
Walk from Norsworthy
Bridge to Leather Tor Pit
0m
36
500 metres
Leather Tor
330
m
at
rt Le
onpo
Dev
De
vo
np
or
t
Cross
Gate
Old
Newleycombe
hardwood Brook
Newer
Norsworthy
hardwood
plantation Bridge
P
Ri
240m
from Burrator
Dam,
Dousland and
Yelverton
eavy
River M
270m
Le
at
Pit
300m
Burrator
Reservoir
rM
ve
vy
ea
From
Sheepstor
& Burrator
Dam
are kept in suspension by the turbulence
of the water. Some dissolved minerals
or other chemicals are carried in solution.
What is today’s load?
Look at the stream closely. Do you think
the stream is moving any of its load at
this moment?
Box E
Landscape processes
The landscape is constantly being modified
by a variety of processes acting upon it.
These include:Weathering processes whereby
rock decomposes or disintegrates in situ
and becomes available for transportation
by other processes.
Transport processes involving
agents such as running water, wind and
gravity, which transfer debris from one
location to another.
Erosion processes where debris
in transport wears down the land surface.
This material is then added to the load
being transported.
Depositional processes where
the transported material is laid down as
accumulations of debris. This happens when
the transporting agent, for example a
stream, no longer has the energy to move
all the load which it has acquired. A stream
loses power when it enters the sea, a lake
or, temporarily, when sediments enter a
sheltered backwater.
Viewpoint - Norsworthy Bridge . . .
Carrying away the moor
19
Box F
Newleycombe Brook - a stream at work
Bedload
The most obvious sediments visible are the boulders and stones in the stream
bed. Much of this bedload is covered in lichens or algae, hinting that it doesn’t
spend much time trundling along the stream bed. The boulders are in
‘temporary store’, waiting for storm water (spate) to move them downstream
a little further. Such storms may only happen between 5 and 25 days a year.
Quartz sand is also a common sediment on the stream floor. This too is only
stirred into movement by the turbulent water of spate. Although prominent,
sediment movement along the stream bed probably only accounts for about
10% of the total ‘load’ transported by the stream.
Suspended load
Fine sediments such as silts and clays can be carried along in suspension by
currents flowing as slowly as 1mm per second. The main stream at your feet
is certainly flowing faster than that and yet the water probably looks clear
and therefore is not carrying a solid suspended load. Why is this?
Platy silt and clay particles tend to stick to the stream floor and it requires
a considerable velocity of water (such as that resulting from a rain storm)
to pluck these particles from the bottom. However, once in suspension, the
fine sediment can be supported easily by the turbulent water and carried
downstream even if the stream loses a lot of its power. The suspended load
will only sink to the channel floor if, by chance, it is diverted into a quieter
backwater.
20
The clear water, seen in the stream, simply means that all the fine sediment
lifted into suspension by the last storm has passed through here and that the
flow of water is not sufficient to entrain new sediment from the stream bed.
The flow of the stream at this moment is probably fast enough to carry fine
sediment, if only it were freed from the stream bed. With a stick, try stirring
up a little fine sediment from a backwater. Does the current carry it away?
If the water was not clear in the first place, it is probably raining!
Dissolved load
Newleycombe Brook may appear clear, but it is full of chemicals which have
been dissolved from the surrounding hillsides and have entered the stream
in solution. Long term monitoring of these streams, by Plymouth University,
indicates that chemical weathering and the removal of material by solution
is an important process in the present climate.
Measurements of contemporary weathering rates suggest that the volume
of material being removed in solution, mainly due to the dissolution of
feldspar, is three times greater than that removed by mechanical processes
such as bedload and suspended load transport.
Box F
(continued)
w a t e r
s u r f a c e
STREAM FLOW
DISSOLVED LOAD
Invisible chemicals in
solution are being
carried by the stream
at all times
SUSPENDED LOAD (mud, silt, clay)
Muddy water full of fine sediments picked
up by fast flowing water. These sediments
are slow to settle, even in calm water.
Pure stream water
Environmental change
Natural chemicals and minerals in solution
dominate the load carried by
Newleycombe Brook today. This is
typical of most streams in the temperate
climate of present-day Britain but this
has not always been the case.
The visual evidence of such change is all
the more pleasing because the ‘clues’ are
there for all to see and require no
particular skill to appreciate their
significance. However, it does help to
have an idea of what to look out for! If
the weather permits, now might be a
convenient time to sit by the brook and
read the panel on the next two pages
before setting off to look at the exposure
in Leather Tor Pit.
A small pit dug into the hillside about
300 metres from here provides visual
evidence for some of the dramatic
environmental changes which gripped
Dartmoor over the last few thousand
years.
BEDLOAD Stream bed stones, only moved by storms (when the river is in spate).
21
Box G
Slope movements in a cold climate
During the coldest stages of the last Ice Age (page 8) Dartmoor was an
arctic-like tundra and the ground was deep-frozen. Only the top metre or
so of the ground would seasonally thaw in the desperately short summers.
Water, melting from the winter’s snowfall, together with melting groundice would not be able to percolate down into the still-frozen deeper ground
(permafrost) and so the upper ‘active layer’ would soon become saturated
with water. That which could not be held in the saturated ground would
flow down the slope as a surface sheet of water (run-off), carrying small
grains of debris with it.
As today, the ground surface around the tors at that time consisted of
dislocated loose slabs of granite (clitter) and smaller angular rock fragments,
all mixed within a matrix of finer debris. Lubricated with meltwater, this
pasty mess oozed downslope at a rate of anywhere between 10 to 100 cm
each summer season. This cold stage slope process is known as gelifluction,
or solifluction. These moving slopes commonly settled on lower less steep
ground as layers of angular debris called ‘head’.
This layer of head often buried and disturbed the weathered ‘rotten’ granite
which lay beneath. The weathered granite, growan, was the product of
earlier warmer conditions pre-dating the head (Box B, pages 6 and 7).
Head deposits overlying growan together give clear visual evidence of
environmental change on the moor. The section (opposite) is typical of what
lies under the soil all over the moor. The description gives enough information
for you to recognise these layers when you come across them exposed
elsewhere on the moor. Of course, every layer may not be present in the
exposure you examine elsewhere.
22
SOLIFLUCTION ON DARTMOOR
A cross-section of a Dartmoor hillside during a cold stage of the last Ice Age.
ACTIVE LAYER - The surface layer, a metre or so deep, froze each winter
but thawed each summer to produce a saturated mess of debris which slid
gently down any slope.
Angular stones
embedded in a
Clitter rafted downslope
matrix of finer
material slipped
gently downhill a
few centimetres
each summer.
Movement down
the slope decreased
with depth.
AYER
of
r limit
Lowe
EL
CTIV
the A
PERMAFROST LAYER - The deep sub-soil and rock remained frozen
throughout the summer so 'active layer' water could not escape by sinking
down through this impermeable barrier.
Box H
A pit reveals environmental change
4
3
2
1
5 SOIL Soil penetrated by roots. A block of clitter protrudes.
Environment indicated: mild chemical weathering in a temperate
climate such as we have experienced for the last 10,000 years.
4 HEAD An unsorted scattered jumble of angular granite
fragments embedded within a background (matrix) of mixed
finer material including quartz-sand and clays. The are no
indications of these particles having been moved by streams.
For example, there has been NO sorting into beds of sand,
clay, gravel etc. In a stream the stones would have settled
quickly to the bottom where they would be in contact with
(touching) other stones. In this diagram a block of clitter which
must have been rafted down slope is shown integrated within
the head.
Environment indicated: thick deposits of head are associated
with solifluction processes during very cold climatic episodes
such as experienced on Dartmoor during the Ice Age (page 8).
3 GROWAN (DISTURBED) This exposure of growan
(weathered granite) still looks like granite (page 13) but it
crumbles to the touch and the interlocking crystal structure of
sound granite has been disordered and disturbed by slope
movements. Sometimes growan has been integrated into slope
movements creating ‘bedded growan’ (see left) with hints of
some surface water (run-off) on the slopes.
Environment indicated: growan suggests mild chemical
weathering of a warm or temperate climate. However, this
example has been disturbed by subsequent cold-environment
slope movements (see above).
2 GROWAN (IN SITU) Rotted granite. It still looks like
granite but is incoherent and crumbles to the touch. Chemical
weathering has caused some decay (softening) of the feldspar
crystals which weakens the rock, allowing mechanical
disintegration to occur. The interlocking crystal structure of
granite has not been disturbed. For example, the black vein of
tourmaline is still as it was in the sound granite. Growan
excavated at Sheepstor Dam was 30 metres deep.
Environment indicated: the mild chemical weathering of granite
to produce growan is associated with warmer or temperate
conditions pre-dating the last cold stage of the Ice Age.
1 SOUND GRANITE Hard angular interlocking crystals
of mica, feldspar and quartz (page 15). A hard black vein of
tourmaline is shown (see page 16). Weathering first shows up
along hair-line joints as a faint discolouration or iron-staining as
the biotite mica is chemically attacked by mildly acidic ground
water.
Clitter slope
Growan
Head
Characteristics of a weathered tor
Angularity of sound
granite increasing
with depth
55
23
Leather Tor Pit
24
Cross Newleycombe Brook by the
stone bridge and then, after a few
metres, cross a second bridge which
spans the upper reaches of the River
Meavy. Almost immediately on the
right there is a stile and footpath which
gently climbs the hill keeping the River
Meavy to your right.
What did you see?
After 200 metres, you emerge onto a
forestry track. Walk ahead, up-slope.
Behind a small but steep bank, runs
Devonport Leat (Box C, page 10). Turn
right to follow the leat upstream. After
less than 100 metres, cross the leat by
a flat granite ‘clapper’ bridge and
enter a small quarry, Leather Tor Pit.
2 The lowest layer presently exposed
is undisturbed weathered granite (see
growan, Box B, page 6). The granite is
‘rotten’ with a crumbly feel but the
interlocking crystal structure is still intact.
Undisturbed veins of black tourmaline
(Box D, page 16) and reddish, weathered
veins can both be followed up across the
growan.
Finding the evidence
3 The same growan continues upwards
but gradually becomes more disturbed.
The discoloured weathered veins show
how some of the growan has been
distorted, curving into near-horizontal
beds. We might assume that this material
has been influenced by solifluction, the
down-slope mass movement which was
so important in the cold stages of the Ice
Age.
If you would like to try some
interpretation, the evidence is in front
of you. Perhaps using the section on the
previous page as a model and, assuming
there are three or four layers that might
be recognised, examine the rear wall of
the pit closely for visual clues that the
dramatic environmental changes
previously suggested did actually happen.
Do not disturb the face of the pit. When
you are satisfied, read on.
© Peter Keene
Devonport Leat by Leather Tor Pit
1 Leather Tor Pit is an old gravel pit. At
one time sound (solid) granite was
exposed in the floor but this had become
covered with debris, much of which has
fallen away from the walls due to winter
frosts.
The bedded nature of the growan has
also been interpreted as showing the
influence of surface meltwater flowing
down the hillside in early summer.
At Cross Gate the leat passes beneath
the road. Turn left to follow the road
downhill. Within 500 metres you are
back at Norsworthy Bridge.
4 Above the growan things change
abruptly. There are angular granite stones
embedded in a dark unsorted background
of loose material. This is typical of ‘head’,
a deposit laid down by solifluction slope
movements in very cold conditions (Box
G, page 22).
5 The head grades up into a thin layer
of stony soil and subsoil.
Now retrace your steps to Devonport
Leat and follow the south bank
westwards for 300 metres.
The concrete hut by the path is a gauging
station for measuring the water discharge
along the leat.
An inactive landscape?
Clitter frozen in time
Leather Tor Pit provides convincing
evidence of a much colder past, yet the
landscape about us contains similar clues.
For example, the moss-covered boulders
within the woods noted earlier were
once active clitter slopes, granite boulders
embedded within the once-mobile head
deposits which moved downslope over
the bare tundra-cold hillside.
When the climate improved (10,000
years ago) and solifluction became much
less active, the movement of head and
rafted clitter virtually ceased. They remain
on the hillside today, frozen in time.
© Peter Keene
Beside the leat, the well-vegetated slopes
and sheltered woodland with moss or
lichen coated boulders give the
impression of an inactive landscape. This
is misleading for, as we have seen, there
are unseen processes at work wherever
air or water penetrate the soil. However,
because most of this weathering is
chemical, the material is dissolved in
ground water and removed in solution
without any apparent change in the
appearance of the hillside.
25
Burrator Dam to Sheeps Tor
This longer walk is about 3 miles (5km) there and back. It is designed for those who
wish to venture a little further onto the moor, scrambling up one of the more
spectacular local tors. This walk may also be regarded as a review, pulling together
aspects of landscape interpretation introduced on the other walks.
Walk from Burrator Dam to Sheepstor
250m
250
m
301m
Beechcroft
Plantation
m
350
Burrator
Reservoir
Pillow
Mounds
Sheeps
369m
Tor
P
lk
wa
T Burrator
Sheepstor
Dam
300m
Dam
Pit
500m
P
Quarry
+Sheepstor
Village
Viewpoint - Burrator Dam to Sheeps Tor
from Dousland (B3212)
26
Access to Burrator is best from
Dousland village on the B3212
south-east (signposted to Burrator and
Meavy). In under a mile (1.6 km) a
road swings left to Burrator Reservoir
and dam (grid reference SX 552 680).
If you would like to know more about
Burrator Dam before setting off, read
pages 10 -12.
Walk eastwards across the dam and
follow the road, keeping the reservoir
on your left. Take care as this road has
no footpath. Ignore the first stile. After
about 500 metres the second stile on
the left gives access to a wide, gated
track that almost immediately rises
onto the low Sheepstor Dam. Take the
track halfway across the dam and
pause.
Sheepstor Dam
Sheepstor Dam was built at the same
time as the main Burrator Dam, which
opened in 1898. It was recognised that
when Burrator Reservoir was filled, water
would escape over a low watershed and
into Sheepstor Brook. This water would
be lost to the reservoir and so a low
earth bank with a clay-filled core was
proposed to prevent water escaping
through Sheepstor Brook (see map on
the inside rear cover).
Dam trouble
Although Sheepstor Dam was only a
modest structure, it still needed good
sound foundations. Two trial pits had
found sound granite but, as soon as work
began, excavations revealed widespread,
Narrator
Plantation
Solid granite was located at a depth of
30 metres, necessitating the digging of a
trench 32 metres deep which, when
infilled with concrete, provided the
foundations for the modest stone-faced
bank upon which you are standing.
Afforestation
The local landscape was dramatically
changed not only by the flooding of the
valley but also by the afforestation of the
surrounding slopes. The level of
treatment of the public water supply at
that time was such that it was considered
essential to keep the water in reservoirs
as pure as possible.
Plymouth Corporation bought all the
land within the catchment and in the
1920s conifers were planted on many of
the surrounding slopes. This helped to
retain the relatively thin soil and also
acted as a filter for water percolating
down the slope. The link between stands
of conifers and the acidification of lake
water was not recognised at that time.
When severe storms blew down many
acres of mature conifers in 1990,
replanting was largely with deciduous,
broad-leafed trees but, without effective
management, these have not held their
own against self-seeded, fast-growing
conifers which, as you may see as you
walk, threaten to overwhelm the slowergrowing deciduous trees.
Now move on to the end of the dam.
Here there is a choice. Either continue
around the reservoir to the left, or
take a brief diversion (a pit stop) some
100 metres into the woods to see the
sort of soft granite that caused such
problems for the builders of the
Sheepstor Dam.
A pit stop
Just after the end of the dam, by a low
concrete-block bunker used for storing
sand or gravel, two paths lead into the
wood. Take the path which swings to
the right. Soon, after about 50 metres
you enter a small clearing.
A side wall exposes the ‘bedrock’. Is this
the rotten granite (growan) that lay
beneath the Sheepstor dam?
A close examination answers that
question. Below is a check list of the main
characteristics of a growan which is still
in-situ (i.e. like the growan under the dam,
not disturbed or transported by later
surface processes).
It crumbles easily in your hand.
The original interlocking crystal texture
of the granite is undisturbed (see Box D,
page 15).
Any black tourmaline veins (page 16)
are still in-situ and have not been broken
up.
The quartz (sand) remains hard but
the pale feldspars have been partly
decomposed in the weakly acidic ground
water.
(Another typical pit section containing
growan is described in Box H, page 23).
When you are ready, return to the
path beside the reservoir.
Follow the track north, keeping the
reservoir on your left. After 150 metres
a gate and stile lead onto a surfaced
road. Turn right. After 100 metres the
narrow lane opens out into a small
grassy area. On the left a stone-walled
path heads directly uphill. After
another 150 metres a gate and stile
lead on to open moorland.
soft weathered granite, growan, which
‘could be dug out manually with a spade
as if it were compacted sand’. (Growan,
Box H, page 23).
27
An eye for landscape
28
© Peter Keene
Ahead lie the slopes of Sheeps Tor. Many
of the characteristics of a typical
Dartmoor tor can be recognised as you
climb towards the summit. Some of the
more distinctive landscape features are
outlined in Information Boxes within this
booklet.
If the weather is pleasant and there is a
convenient boulder to sit against, this
may be an opportunity to prime yourself
on what to look out for as you climb.
You may like to consult the boxes listed
opposite, or look at the diagram which
reviews the range of features which are
typically found on the flanks and summits
of Dartmoor tors.
A granite floored track beneath established
deciduous trees leads you on to the open moor
The origin of tors (Box B, page 6).
Tors in the Ice Age (Box B, page 7).
Moving slopes (Box G, page 22).
Jointing in granite (Box I, page 29-30).
Vertical joints
Sheet joints
(pressure
release)
Avenue
Tor
Clitter
Stepped outcrops
of granite
Head
Head,
valley
deposits
Stream
downcutting
into deposits
Weathered granite
(growan)
What to look out for on the flanks and summits of Dartmoor tors (after Gerrard 1983)
Box I
Jointing in granite
The castle-like appearance of many tors is clearly related to the imposing
joints that define the great blocks of granite which make up the rock mass,
some the size of houses. It seems likely that these massive joints developed
as a result of the initial cooling and contraction of the once molten granite.
Tourmaline veins (page 16), related to the volatile gasses which penetrated
the granite at a late stage in its cooling, do not cross these joints, suggesting
that the main joint pattern was already established by this early stage.
A route for weathering
Joints provide routes by which air and water can penetrate otherwise solid
granite encouraging both chemical and mechanical weathering along the joint
planes. Chemical attack, in particular the carbonic acid in ground water, can
react with the feldspars in the granite, weakening the structure and making
mechanical granular disintegration easier. Both have the effect of widening
the joints and rounding the edges and corners of blocks. The resulting growan
chokes many of the joints. Water freezing in restricted joints can wedge
granite blocks apart, contributing to the general disintegration of the rock
mass of the tor.
Examples
Sharpitor seems to be in an advanced stage of disintegration (page 3) and
it needs some imagination to reconstruct the tor from the ruin of surrounding
clitter. The nature of the more massive joints is well displayed in Lower
Burrator Quarry (page 16) and in the photograph of the foundations for
Burrator Dam (page 11). On the south facing wall of Sheeps Tor, it is the
vertical joints that are the more pronounced.
Massive rectangular jointing
29
The south facing wall of Sheeps Tor
Sheet joints or pseudo-bedding planes on
Sheeps Tor
© Peter Keene
Sheet jointing on the flanks of Sheeps Tor
© Peter Keene
30
© Peter Keene
Box I
(continued)
Jointing in granite - sheet joints
Where granite is exposed on the flanks of tors, thin sheets of rock can be
seen curving over the hillside roughly matching the surface landscape. These
granite sheets, separated from one another by joint planes, are the result
of the expansion of the granite as pressure was released when overlying
rock was gradually removed by erosion. Because of this they are often called
unloading structures. There are well-developed examples of sheet jointing
on the flanks of Sheeps Tor (see photograph above).
When the horizontal joints are close together, the result is a mass of closelyspaced layers which resemble the beds found in sedimentary rocks and so
are sometimes called pseudo-bedding planes (see photograph above).
The unloading mechanism of pressure release occurs parallel to the rock
surface at a variety of scales whenever rock pressure is released. It is a
problem experienced in modern quarry faces (page 16).
To reach the crest of Sheeps Tor you
could scramble straight up the hill
ahead. However, a gentler route and
one with a greater range of features,
is to bear right (east) and, choosing a
safe route, clamber diagonally up the
slope. Either way, achieve the southern
summit, overlooking Sheepstor village
and church.
Sheeps Tor, south end
On your climb did you recognise the
variety of jointing patterns that dominated
the shape of the hillside on the walk up?
Everywhere the form of the surrounding
rock seems defined by joint patterns.
Avenues
There is evidence that the frequency
with which vertical joints cut the slopehugging sheets of granite increases
towards the curved hilltop. Since frequent
jointing encourages weathering and rock
failure, it is suggested that the closejointed hilltops would be particularly
susceptible to destruction. One piece of
evidence supporting this idea is that the
summits of many tors, including Sheeps
Tor, are missing. Instead they are cut by
grassy hilltop avenues. The destroyed
rocky summit may then have been swept
down slope in the form of clitter to form
an apron of debris at either end of the
avenue.
Clitter fields
The flanks of the hillside are covered in
fields of clitter. Some of these blocks may
have originated from nearby hillside
shelves of granite such as those crossed
on the climb up the western slope of
Sheeps Tor. The pattern of other clitter
spreads suggests that they were derived
from blocks shed by the tor itself and
then swept to their present location by
the extremely mobile slope movements
(solifluction) active in cold stages of the
last Ice Age (Box G, page 22).
Human impact
In the satisfaction of developing an eye
for reading the physical landscape, it is
easy to overlook the human dimension.
For example, when standing beneath the
most impressive southern face of Sheeps
Tor it may not be apparent today that
this was once an active quarry. From
here granite blocks were taken down to
the village of Sheepstor, seen below.
An interesting detail on the quarry floor
are the converging lines of up-ended
stones which were constructed as runs
to lead unsuspecting vermin into, now
vanished, slate traps which once graced
the apex of each funnel. The purpose
was to protect rabbits, at one time an
important ‘crop’ on the moor
hereabouts. To further encourage the
rabbit population, low mounds of
boulders were constructed to provide
ready-made rabbit warrens in an
otherwise inhospitable environment.
Today, these elongated piles of boulders
look for all the world like a natural
phenomenon. On the rear cover map
they are shown to the west of Sheeps
Tor as ‘Pillow Mounds’.
When you are ready, follow the path
straight up the slope until, after about
400m from the stile, there is a granite
boundary post inscribed PCWW
(Plymouth Corporation Water Works)
1917.
31
On the margins of economic farming
Because of its altitude and exposure, Dartmoor has always been a marginal farming
area. Most of the land provides, at best, rough grazing. From the summit of Sheeps
Tor, the view includes both high moor and valley margins. It is an appropriate place
to reflect on the ebb and flow of moorland farming and the extent to which its
margins have fluctuated, driven by both climatic modifications and economic necessity.
Colonisation
Soon after the end of the Ice Age, 10,000
years ago, woodland crept up the
sheltered valley below you, eventually
seeping on to the moor. 8,000 years ago,
woodland was lapping high isolated hills
such as Sheeps Tor and supporting small
hunting communities. By 4,000 years ago
most of the trees had been cleared and
Bronze Age people grazed sheep and
cattle across the moor. The foundations
of their circular stone-based huts (hut
circles) can still be seen on the moor
today.
Fluctuating conditions
32
Between 3,000 and 2,000 years ago,
colder, wetter conditions forced farmers
to abandon the high moor. Peat spread
over the high, wet plateau surfaces.
However, farms crept up onto the moor
again in the warmer drier conditions of
the medieval period when sheep farming
was central to the Devon economy.
Climatic deterioration in the 14th century
combined with the ravages of the Black
Death caused another retreat from the
moor.
Entrepreneurial farmers in the 19 th
century enclosed large swathes of central
Dartmoor attempting to ‘improve’ the
rough moorland and create rich pastures.
The long stone walls that contained these
‘newtakes’ can still be seen on the high
moor but most of this ‘enclosed’ land
has long since reverted to open moor.
Sheepstor village
The fields which flank Sheepstor
village, demonstrate the present
condition of farming on the fringe
of the moor. Each small laboriously
built stone-walled field represents
an attempt to enclose and improve
land. The green fields of improved
pasture are usually easy to pick out
but notice too the number of fields
invaded by bracken and reverting
to rough pasture.
The rocks speak
Before leaving Sheeps Tor and returning
to Burrator Dam, why not choose a
group of rocks or a distant landscape and
reflect on what that view says to you?
There is pleasure in simply absorbing a
prospect but, as the introduction to this
booklet suggests, there is also a
satisfaction in being able to interpret
what you see.
As an example, look at the photograph
below, which was taken on the western
side of Sheeps Tor. A wealth of detail can
be read into this scene, simply using
information contained somewhere within
this booklet.
In the background, granite, cut by sheet
joints, matches the curve of the hillside.
The sheets are themselves cut by short
near-vertical joints at right angles to the
sheets. These vertical joints are probably
the result of the release of pressure on
the granite (pressure release) as the
overlying rocks were eroded away. The
joints have been exploited and widened
by weathering. Clearly, the resulting
isolated granite blocks have become
candidates for impending collapse.
Blocks collapsing in the past must have
produced the tumble of clitter on the left
of the photograph. The mat of vegetation
partly engulfing the lichen-coated clitter
suggests that today this slope is relatively
stable.
It is almost as if the photograph has
caught that ‘moment’, 10,000 years ago,
when the cold stage ‘engine of
destruction’ was suddenly turned off. In
the temperate climate that followed,
cold stage processes such as frostshattering and rapid slope movements,
solifluction, were largely replaced by
chemical weathering and solution. Visible
rock moving processes were put ‘onhold’ – dormant – but just awaiting a
climatic change which could start the
cold stage machinery up again.
The easiest way to return to Burrator
Dam is to retrace your steps.
© Peter Keene
33
Some key words describing
the Dartmoor landscape
CLITTER
Detached blocks of rock which project above or rest on the surface of the ground.
They commonly form localised spreads.
CORESTONES
Rounded blocks of sound rock surrounded by softer rock which has been partly
decomposed by weathering penetrating along joints.
COUNTRY ROCK
The local rocks into which a mass of igneous rock, in this case Dartmoor granite,
was intruded.
GROWAN
Coarse debris resulting from the partial decomposition of granite by weathering.
HEAD
Originally a local farming term for deep, rubbly subsoil. It is here used to describe
the mantle of unconsolidated material produced by the action of solifluction.
JOINTS
Fractures in a rock that, unlike faults, show no movement on either side of the
fracture. The large rectangular joints in granite are commonly related to contraction
when the rock first cooled from a molten state. Joints parallel to the land surface
(sheet joints) often formed later when the rock expanded as erosion relieves the
pressure caused by the weight of overlying rocks. This process is called pressure
release or unloading.
LOAD
Here used to describe all material transported by a stream and consisting of both
solid debris (bed load and suspended load) and material in solution.
PRESSURE RELEASE see joints.
SOLIFLUCTION
The slow downslope mass movement of surface material and subsoil when saturated
by water. It is most effective in very cold conditions where a permanently frozen
subsoil (permafrost) prevents water from percolating down from the surface. Under
those circumstances it is termed ‘gelifluction’. It is responsible for moving clitter and
huge quantities of ‘head’.
WEATHERING
34
The decay of rocks on, or near the surface of the Earth, often in the presence of air
or water. It is usually the result of mechanical breakdown (e.g. frost action) or chemical
activity (e.g. solution or oxidation).
Walking with Moor Care
Travel with Moor Care – try and share transport and
if possible use public transport. For south west
England timetable enquiries phone Traveline 0870
6082 608;
for DevonBus publications phone 01392 382800.
If you do have to use a car, keep within the 40 mph
speed limit on moorland roads and park sensibly
using only hardened parking areas in wet weather.
Walk only where you have a right, are allowed to do
so or are clearly welcome. Come prepared with an
up-to-date map.
Always use gates and stiles to cross boundaries.
If you are following the line of an eroded path stick
to it and avoid widening it, by walking in single file if
necessary. Respect signs asking you to avoid very
badly eroded paths. Avoid climbing straight up or
down steep hills. Take a winding route to avoid
damage.
When the ground is wet, plan your route carefully
and use hard surfaces where possible thus avoiding
vulnerable, waterlogged moorland paths.
Take care not to cause disturbance to wildlife or
livestock, particularly during the moorland lambing
and bird breeding season (1 March to 15 July) and
during the lambing season on enclosed farmland (1
December to 30 June). Please keep your dog on a
lead during these times, and always under close
control.
Dartmoor is rich in archaeological remains and many
sites are protected by law. Learn how to recognise
archaeological features to ensure that you don’t
disturb them; in particular never move stones, dig,
light fires, or bivouac in or around archaeological
sites.
Respect local byelaws and footpath/bridleway
diversions (as signposted) and follow the Country
Code.
Please remember that all of the National Park is
owned by someone and respect the interests of those
who own the land or make a living from it.
Safety for the walker
Check the weather before you start.
Proper footwear is essential. Most of the moorland
terrain is uneven. Some slopes around tors are
strewn with rocks, which may or may not be
covered with vegetation, and are particularly
hazardous.
Mist is a frequent hazard on Dartmoor. Be sure
you know how to use a map and compass. When
walking, know at all times exactly where you are.
Unless you are experienced, do not walk alone
in very remote country.
Take care when walking along roads and lanes
(some routes in this book do involve walking
where there may be some traffic).
Keep away from all moorland livestock to minimise
disturbance.
If the weather deteriorates do not hesitate to
turn back.
Take your litter home. Litter is not only unsightly
- it can cause fires and may injure people, livestock
and wildlife.
Geological Fieldwork Code
The Geologists' Association has published a Geological Fieldwork Code, copies of which are available from The Geologists'
Association, Burlington House, Piccadilly, London W1V 9AG (Tel: 0171 434 9298). The adherence to this
Code for any geological fieldwork is crucial to safeguard our geological heritage and its scientific interest and value. In
particular, when on Dartmoor please:
always seek permission before entering on to private
land;
avoid undue disturbance to wildlife. Plants and animals
may inadvertently be displaced or destroyed by
careless actions;
when working in remote areas, follow the advice
given in the booklet Safety on Mountains issued by
the British Mountaineering Council, and in particular
inform someone of your intended route;
when exploring underground, be sure you have the
proper equipment and the necessary experience and
appropriate permissions sought. Never go alone.
Report to someone your departure, location,
estimated time below ground and then your actual
return;
remember that rock outcrops are important to
science and should not be hammered or damaged in
any way - please observe and read and do not hammer
indiscriminately;
keep collecting to a minimum. Avoid removing in situ
fossils, rocks or minerals;
never collect from walls, bridges, buildings or other
structures;
take care not to undermine fences, walls, bridges or
other structures;
remember that rock faces, quarry faces and old mine
workings may be highly dangerous - always consider
your personal safety and the safety of others.
35
This booklet has been published by the
Dartmoor National Park Authority and
Devon County Council in collaboration
with Thematic Trails.
Support has been provided by English
Nature and the South West Lakes Trust.
This publication fulfils one of the actions
in Action for Wildlife: The Dartmoor
Biodiversity Action Plan.
Dartmoor National Park Authority
Parke
Bovey Tracey
Devon
TQ13 9JQ
Tel: general information enquires 01822
890414
Fax: 01626 834684
email: [email protected]
Dartmoor National Park Authority on
the Internet: www.dartmoor-npa.gov.uk
Devon County Council
Lucombe House
County Hall
Exeter
Devon
EX2 4QW
Tel: Conservation enquiries
01392 382257
email: [email protected]
© Dartmoor National Park Authority / Devon
County Council / Peter Keene 2001
ISBN 0 905981 111
Clitter on the flank of Sharpitor
36
© Peter Keene

exploring Burrator - Dartmoor National Park

Transcription

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