[Illustration: Fig. 254.--Left valve of _Pecten Islandicus_, Glacial
and Recent.]
III. POST-GLACIAL DEPOSITS.--As the intense cold of the Glacial
period became gradually mitigated, and temperate conditions of
climate were once more re-established, various deposits were
formed in the northern hemisphere, which are found to contain
the remains of extinct Mammals, and which, therefore, are clearly
of Post-Pliocene age. To these deposits the general name of
_Post-Glacial_ formations is given; but it is obvious that, from
the nature of the case, and with our present limited knowledge,
we cannot draw a rigid line of demarcation between the deposits
formed towards the close of the Glacial period, or during warm
"interglacial" periods, and those laid down after the ice had
fairly disappeared. Indeed it is extremely improbable that any
such rigid line of demarcation should ever have existed; and it
is far more likely that the Glacial and Post-Glacial periods,
and their corresponding deposits, shade into one another by an
imperceptible gradation. Accepting this reservation, we may group
together, under the general head of "Post-Glacial Deposits,"
most of the so-called "Valley-gravels," "Brick-earths," and
"Cave-deposits," together with some "raised beaches" and various
deposits of peat. Though not strictly within the compass of this
work, a few words may be said here as to the origin and mode of
formation of the Brick-earths, Valley-gravels, and Cave-deposits,
as the subject will thus be rendered more clearly intelligible.
Pages:
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504