OR I`G I N 0 F POT HO LE SIN G LAC I ATE D RE G ION S

ORIGIN OF POTHOLES IN GLACIATED REGIONS
11
OR I'G I N 0 F POT HO LE SIN G LAC I ATE D RE G ION S
By C. G.
HIG GINS
(Unive rsity of California, Davis, California , U .S.A.)
ABSTRAC T.
A review of the inadeq uacies of the g lacial rnoulin hypothes is fo r the origin o f po tholes.
RESUME. Une cri tiq ue des insuffisances de I' hyp oth ese d es moulins glac ia ires pour expliq ue r la p rove nance des
nlarmites de geant.
FIR ST proposed by Charpentier 1 and later popularized by Bragger and Reusch 2, 0 the glacial
moulin explanation for the orig in of large potholes was widely and, for the most part, uncritically
accepted fo r almost a century. In the early 1930's several writers 3, 4 began to doubt the moulin
hypothesis and b y the early 1950's most writers had abandoned it in favor of the view that large
potholes are formed by eddy currents in sub-ae rial or subglacial streams. However, a vigorous
exchange of correspondence in this Journal in 1951- 52 5,6, 7,8 showed that the mouIin hypothesis
still retained some ardent advocates and provoked the present review of the subject.
Briefl y, the m oulin hypothesis holds that certain deep, cylindrical bedrock cavities , or "potholes" * , were eroded by melt water torrents plunging vertically down through glacial crevasses or
" moulins"t to th e glacier bed, thence flowing away as subgl acial streams. Traditionally, the
hypothesis concerns only erosive effects at the b ase of the waterfall and disregards any effects of
the resultant subglacial stream.
Earl y w rite rs recognized that potholes are formed in mode rn sub-aerial streams and that
"these differ in appearance from m oulin-formed potholes onl y in that the latter lack flaring edges
and stream-worn bedrock surfaces between them" 9. Thus committed to emphasizing the
differences between stream-worn and supposedly m oulin-formed potholes, and already predisposed to finding different orig ins for them, these writers neglected stream potholes and concentrated their attention on alleged m oulin potholes in efforts to find features that would confirm the
m oulin hypothesis. As a result , they fail ed to recognize that m any of the features they cited as
being uniqu e with moulin potholes are also characteristic of many stream-worn potholes:
gigantic size (" giant's kettl es ")
large b oul de rs, supposedl y " grinders" , in the potholes
lack of fl aring edges
spiral flutes and grooves on the walls
half-potholes .
The recent, excell ent observations on modern stream-worn potholes b y Angeby 10, 11 and others
show clearl y that these features cannot be cited as evidence for moulin origin. On the contrary,
according to th e principle of U ni formitarianism, they suggest formation by running water.
The remaining " unique" features of supposed m oulin potholes:
occurrence in glaciated areas
occurren ce on hillslopes or hilltops, high above modern drainage
occurrence in clu sters on convex slopes where glacial crevasses (or subglac ial strea m
rapid s) might be expected to have form ed
glacial striae and polish on interve ning b edrock surfaces
potholes fill ed with glacial till
• Au thors have ca ll ed such fea tu res "potholes", "po t-ho les" and "pot ho les" . The latter term is a lso lIsed synonym ously with "swa ll ow ho les" in reference to solutional "s ink -ho les " in li mes ton e, a nd all three term s have been applied
to ~eve ra l o ther sorts of bed rock dep ress io ns. I n a now com ic exc ha nge of co rresponde nce in Science, V a l. 10, 1899, seve ral
wnte rs (0. H. H e rs hey, p. 58; F. F. Hil de r , p . 88; O. C. Farr ington , p. , 87; and G. P . Merri ll , p . 298) so lem nl y debated
whet her the term "pot-hole" was offens ive to good tas te a n d s h ould be rep laced by t he Spanish w o rd , "remolino".
L arge potho les have bee n called "giant's ke ttl es (or cauldro ns), ' in va ri ous lang uages : " n1 armites de geant" J " Ri esentopfe
(-kessel)" , " j atteg rytor", etc. (The s pelling of the wo rd po tho le a ppears to have varied on both s id es o f the Atlantic.
Earlier U.S. write rs favo red "pot -ho le"-the spell ing in G rea t B rita in- but recent ones have use d Hpo tholc" rather
consistently.)
t Some write r s (e.g., Gilbert, G. K ., ref. '4, p. 3 ' 7- 20) h ave restricted t he te rm "moulin " to the w aterfall itself or
e ven to t he water fall erosion process. Som e others have u sed it b roadl y to incl ud e a n y crevasse, w h eth e r o r not enl arged
by abl ation, ncc uoied b y a waterfall.
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JOURNAL OF GLACIOLOGY
do not, in themselves, 'show that the holes were bored by englacial waterfalls, but only that the
potholes were formed during (or before) glaciation. The same features would be expected to be
associated with potholes that were formed as normal eddy-holes by subglacial or latero-glacial
streams.
There is no direct evidence that moulin waterfalls form potholes. Neither is there, as noted
above , any indirect evidence that they might have done so. Furthermore, there are grave mechanical
difficulties in the moulin hypothesis. In addition to minor objections, "the main objection to the
hypothesis lies in the difficulty of conceiving the moulin as existing long enough in the necessary
definite form at the same spot, or as reforming in the same manner and at the same spot often
enough to account for the work accomplished" 12. After discussing this difficulty thoroughly,
Alexander concluded that it was insurmountable 13.
However, even if moulin torrents could remain long enough in one spot to erode the bedrock,
it is doubtful that th ey could produce the deep, cylindrical potholes ascribed to them. Gilbert
noted that these requ ire "whirling water. Mere impact of the plunging water is not sufficient if
there is no enclosure or obstruction to determine a whirl" 14. Alexander's hydraulic experiments
confirmed this.
Alexander concluded that deep, spirally -grooved potholes are formed only when the incoming
water jet enters the pothole obliquely at a low angle and spirals down the walls, rotates the scouring
tools at the bottom, and exits up the center'*'. He found that a high-angle or vertical jet plunges
down th e center to the bottom of the hole, fans out along the bottom and exits radially upward
along the walls, eroding a broad, flaring plunge-pool depression .
Conditions necessary to produce oblique deflection of a moulin torrent at the bottom of a moulin
must be comparatively rare, so that only a correspondingly small proportion of moulin-formed
potholes would be of the deep, cylindrical, eddy-hole type; the majority would be flaring basins
of the plunge-pool type. However, in the classic areas of supposed moulin erosion, deep potholes
predominate just as exactl y simi lar potholes predominate where modern, vigorous streams flow
over bedrock obstructions.
In short, there is neither direct nor indirect evidence that moulin action forms potholes.
Furthermore, in the unlikely event that moulin torrents could remain long enough or often enough
at one spot, they would actually produce potholes quite different from those that have been ascribed
to them . On the other hand , all fea tures of potholes ascribed to moulin erosion may readily be
explained as natural consequences of erosion by sub-, en-, or latero-glacial streams, as has been so
ably demonstrated by Angeby and Alexander. In view of these arguments, continued entertainment of the moulin hypoth esis seems a needless extravagance.
MS. received 8 February 1956
.. Angeby (ref. 10, p . 25), in an experiment with an act ual spi rall y-gmoved pothole 7'9 m . deep, confirmed Alexander's
labo ratory observations on spira l flov.r. Alexander found no basis for the content ion of Brogger and Reusch, op. cit .,
p. 765 (ef. S treifT-Becker, ref. 7) that a high-angle jet produces spiral flow passing upward aga inst the walls.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
I l.
12.
13 .
14.
Charpentier, Jean de. E ssai s llr les glaciers et SIIY le terrain erratiqu e du bass';'z du Rhone . Lausannc, Marc Ducloux,
18 41. 363 p .
Bragger, W . C., mId Reusch , H. H . Giant' s k e ttles at Christiana. Quarterly Joumal Geological Society, Vo!. 30, 1874,
p . 758- 7 1. See Ange by's bib li ography (re f. IQ) fo r Danish and German editions of this paper.
Geer de, Gerard. Stoc kholm straktens kvartargeolog i. Sveriges Geologislw UndersOimillg, Avhalldli,;gar och Uppsatser ,
Ser. Ba , No. 12 , 1932, p. 21.
Alexander, H. S. Pothole e ro s ion. Journal of G eology, Vo!. 40, 193 2, p. 305- 37.
StreifT-Becker, R. Pot-holes and glacier mills. Journal of Glaciology, Vo!. I, No. 9, 1951 , p. 4 88- ,-}0.
I-Iollin gworth , S. E. Pot-holes and glacier mill s ; some comments on Dr . StreifT-Becker's article. Journal of Glaciology,
Vo !. I, No. 9, 1951 , p . 490 .
S treifT-Bccker, R. Letter in Journal of Glaciology, Vo!. I , No. 10, 1951 , p . 582.
Fregri, Knut. On th e origi n of pot-holes. Journal of Glaciology, Vo!. 2, No. I I, 1952, p. 24-25 .
Upham , Warren . Giant's kettles eroded by moulin torrents . Bulletin, G eological Society of America, Vo!. 12, 1900,
.. p. 25 - 44·
Angcby, O. Pothole erosion in recent wat e r fa ll s. LUlld Studies ill Geography, Ser. A, No. 2 , 195 I , p. 1- 34. [ English
s ummary of a longer paper, "Evorsionen i recenta va ttenfal~ ) " Lund, 195 I.]
- - R ecent, subglacial an d laterog lacial pot h o le-erosion (evorsion). Lund Studies in Geography , Ser. A, No . 3,
195 2, p. 14- 24.
Alexander, H. S. op. cit. , p. 310.
- - op. cit., p. 311 - 12.
Gi lbert , G. K. Mou lin wor k under glaciers. Bulletill of the Geographical Society of America, Vo!. 17, 1906, p. 319.