The basic cause is the upheaval of the land--a fact
which is beyond controversy, the other agencies are very plain
and recognisable consequences of the upheaval. There are,
however, many geologists who do not think this explanation
adequate.
It is pointed out, in the first place, that the glaciation seems
to have come long after the elevation. The difficulty does not
seem to be insurmountable. The reduction of the atmospheric
vapour would be a gradual process, beginning with the later part
of the elevation and culminating long afterwards. The reduction
of the carbon-dioxide would be even more gradual. It is
impossible to say how long it would take these processes to reach
a very effective stage, but it is equally impossible to show that
the interval between the upheaval and the glaciation is greater
than the theory demands.
It is also said that we cannot on these principles understand the
repeated advance and retreat of the ice-sheet.
This objection, again, seems to fail. It is an established fact
that the land sank very considerably during the Ice-Age, and has
risen again since the ice disappeared. We find that the crust in
places sank so low that an arctic ocean bathed the slopes of some
of the Welsh mountains; and American geologists say that their
land has risen in places from 2000 to 3000 feet (Chamberlin)
since the burden of ice was lifted from it.
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