The
comparatively short distance over which they are carried, and the
suddenness of the transportation, allow no time for the abrasion which
produces the smooth surfaces of water-worn pebbles or the polished and
scratched surfaces of glacier-worn ones. In the latter case, we have
seen that the pebbles, being so set in the ice as to expose only one
side, may be only partially polished, while others, more loosely held
and turning in their sockets, may receive the same high polish on every
side. In such a case the lines will intersect one another, in
consequence of the different position in which the stone has been held
at different times. No such appearances exist in the water-worn pebbles:
their blunt surfaces, smoothed and rounded uniformly by the action of
the water in which they have been rolled or tossed about, present
everywhere the same aspect.
The correlation between these different loose materials and the position
in which they are found helps us also to detect their origin. The loose
materials bearing glacier-marks are always found resting upon surfaces
which have been worn, abraded, and engraved in the same manner, while
the water-worn pebbles are everywhere found resting upon rocks the
abrasion of which may be traced to water. It is true that in some
localities, as, for instance, in the gravel-pit of Mount Auburn, near
Cambridge, large masses of glacier-worn pebbles alternate with
beach-shingle; but it is easy to show that there was here a glacier
advancing into the sea, crowding its front moraine and the materials
carried under it over and into the shingle washed up by the waves upon
the beach.
Pages:
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226