The other characteristic peculiarity of these _roches moutonnees_
consists in the direction of the glacier-scratches, which ascend the
slope to its summit in a direct line on one side, while they deviate to
the right and left on the other sides of the knoll, more or less
obliquely according to its steepness. Occasionally, large boulders may
be found perched on the very summit of such prominences. Their position
is inexplicable by the supposition of currents as the cause of their
transportation. Any current strong enough to carry a boulder to such a
height would of course sweep it on with it. This phenomenon finds,
however, an easy explanation in the glacial theory. The thickness of
such a sheet of ice is of course less above such a hill or mound than
over the lower levels adjoining it. Not only will the ice melt,
therefore, more readily at this spot, but, as ice is transparent to
heat, the summit of the prominence will become warmed by the rays of the
sun, and will itself facilitate the melting of the ice above it. On the
breaking up of the ice, therefore, such a spot will be the first to
yield, and allow the boulders carried on the back of the glacier to fall
into the hollow thus formed, where they will rest upon the projecting
rock left uncovered.
Pages:
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221