The Hilton Falls glacial pothole is believed to have formed during
the glacial retreat some 12,000-14,000 years ago during the Tioga
period, one of three glacial maxima (the other two are known as the
Tahoe and Tenaya periods) of the Wisconsin Glacial Period.
The earliest theories on the formation of glacial potholes
hypothesized that "moulins de glacier" (glacial mills) formed where
water flowing atop glaciers and ice sheets as surface streams fell
into holes in the ice and impacted upon the ground below. This
theory was first proposed by Bršgger and Reusch (1874) and
was generally accepted until the 1950's. However, starting in the
1930's, scientists started to question this theory, largely because
they couldn't see how glaciers or large ice sheets could remain
stable enough for the water to carve out such large and deep
potholes, e.g. Alexander (1932).
Today, the generally accepted belief is that sub-glacial (under
the glacier), or ice marginal (beside the glacier) streams enlarged
weakened areas of the underlying bedrock. Detritus (pieces of rocks
also known as clasts) carried in these streams acted as the cutting
tool that became trapped in the weakened bedrock. Over time, as the
water continued to flow, these clasts continued their circular and
downward cutting action enlarging the holes, sometimes both in
depth and width. This continuous abrasive action often resulted in
the clasts themselves being worn down to very fine particles.
Logging Requirements:
To log this Earthcache, perform the following tasks and
determine the answers to the questions below. Email me your
answers, DO NOT include them with your Found It log.
- For the purpose of this exercise, imagine that the pothole is
completely intact, that part of its exterior wall is not missing.
Using a measuring tape, metre stick or a good eye, determine, in
metres, the approximate height, width and depth of the pothole.
Once you have these figures, calculate, in Litres, the volume of
water that would be required to fill this pothole assuming it was
completely intact. Send me your measurements and the volume that
you have calculated. Remember that 1 m3 = 1000 L.
- The bedrock upon which you are standing is sedimentary. Take a
look at the layers within the cross section of the pothole. Is
there a difference in thickness in these layers from top to bottom?
If so how do they vary?
- In terms of geologic processes, provide a possible explanation
as to why part of the pothole wall is missing.
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Pothole Formation
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Pothole Cross section
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Literature Cited:
Bršgger, W. C., and Reusch, H. H. 1874. Giants' kettles at
Christiania. Quarterly Journal of the Geological Society of London,
30: (120), 750 - 771.
Alexander, H.S. 1932. Pothole erosion. Journal of Geology. 40 (4):
305-337
