The change from one to another of those glacial conditions was slow as
we count time. When the great cycle of snow years, called the Glacial
Period, was nearly complete in California, the ice-mantle, wasting from
season to season faster than it was renewed, began to withdraw from the
lowlands and gradually became shallower everywhere. Then the highest
of the Sierra domes and dividing ridges, containing distinct glaciers
between them, began to appear above the icy sea. These first river-like
glaciers remained united in one continuous sheet toward the summit of
the Range for many centuries. But as the snow-fall diminished, and the
climate became milder, this upper part of the ice-sheet was also in
turn separated into smaller distinct glaciers, and these again into
still smaller ones, while at the same time all were growing shorter and
shallower, though fluctuations of the climate now and then occurred
that brought their receding ends to a standstill, or even enabled them
to advance for a few tens or hundreds of years.
Meanwhile, hardy, home-seeking plants and animals, after long waiting,
flocked to their appointed places, pushing bravely on higher and higher,
along every sun-warmed slope, closely following the retreating ice,
which, like shreds of summer clouds, at length vanished from the
new-born mountains, leaving them in all their main, telling features
nearly as we find them now.
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