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[Bouv6. 
that the moulins of the glaciers of the Alps, where they are known 
to reach down sometimes 500 feet or more to the underlying rock, 
are not stationary but move forward at the same rate as the ice 
itself. 
Mr. Upham, therefore, suggested that the time of the excavation 
of these glacial pot-holes was 'probably the early part of the epoch 
of glaciation, when the ice-sheet was being formed upon the land 
by snow-fall. He thought that upon any hilly country the ice must 
have attained an average depth somewhat exceeding the altitude of 
the hills above the adjoining lowlands before any general motion of 
the ice-sheet could begin. During this process of slow accumulation 
of the ice-sheet, the summer melting upon its surface would produce 
multitudes of rills, rivulets and brooks, which might unite into a 
large stream, and this, pouring through a crevasse and melting out 
a cylindric moulin , might fall a considerable depth to the bed-rock, 
perhaps one or two hundred feet or more upon an area so moder- 
ately uneven as the vicinity of Cohasset, while yet the ice-motion, 
though sufficient to permit the formation of the crevasse, might not 
have gained a definite current to carry the crevasse, moulin , and wa- 
ter-fall away from the spot where they were first formed. We may 
thus explain the continuation of a glacial water-fall in one place 
while it was excavating one of these “ Giant’s Kettles ”or pot-holes. 
After the ice-sheet acquired a current because of the greater thick- 
ness and pressure of its mass, such deep cylindric excavations in the 
bed-rock could not be made ; and during the recession and final dis- 
solution of the ice-sheet it seems probable that its receding border 
had steeper gradients and consequently even more rapid motion than 
in the culmination of the glacial epoch. It is also to be observed that 
the streams formed on the surface of the ice-sheet by the summer 
melting before it was so thick as to have motion would be free from 
drift, so that they could readily find their way through crevasses, 
wearing pot-holes in the rock beneath, and thence flowing in subgla- 
cial courses ; but that the glacial streams during the departure of the 
ice were heavily freighted with the gravel, sand, and clay of the mod- 
ified drift, which must have soon choked up the passages wherever 
these drift-laden streams found crevasses, causing them to flow in 
superficial channels walled and underlaid by ice. 
In support of this view, Mr. Upham referred to the fact that the 
glacial pot-holes are commonly found full of drift, which indicates 
that they were eroded near the beginning rather than the end of a 
