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. It was the beginning of the Glacial Period or Ice
Age, when a large portion of the United States is supposed to have been
covered by a sheet of ice. The ice is believed to have entered South
Dakota from the northeast and its drift across the state limited by a
line so closely following the present course of the Missouri River that
many of us would be inclined to consider it the western bluff. Beyond
this line the ice failed to push its way, but the Hills were subject to
heavy rain storms that filled the streams and carried large quantities
of bowlders and other eroded material, both coarse and fine, down into
the valleys and over the lower hills, where much of the moderately
coarse can now be seen exposed on the surface, and fine specimens
collected without the use of a hammer. The brilliantly colored, striped
and mottled agates, and the bright, delicate tints of the quartz