The volume of fresh water
afterwards became so great that immense lakes spread over large portions
of the west, one of which occupied most of the region around the Black
Hills at the beginning of the Miocene, and animal life was more abundant
than ever before and of higher orders, many species being the same as
are now in existence. The weather became more and more inclement and as
the storms increased the erosion of the Hills also increased, and the
rivers changed to torrents with deep channels. Earthquakes are supposed
to have occurred and also volcanic eruptions.
The Black Hills were now rising steadily, and as the slope of the
streams increased, the channels cut deeper, and the fissures now known
as caves had long been filled with water.
The most important of the numerous animals of the Tertiary Age yet
discovered in the Hills and surrounding region, are the Titanotherium or
Brontotherium, similar to our Hippopotamus, the Oreodon, and a small
horse having three toes on each foot. A little later in the same Age
the horses were similar to those of the present time and of equal size,
which proves that the wild horses of the West were not descended from
the few lost by the Spanish Invaders. At this time the first lions,
camels, mastodons, and mammoths also appeared. The remains of these
animals are so abundant in places as to indicate that they perished in
herds that were overwhelmed suddenly by great floods, and many, no
doubt, huddled together and perished with cold; for with the beginning
of the present age the Hills had reached their highest elevation, the
inclement weather increased, and the tropical climate suddenly changed
to one extremely cold.
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