No record of the next geological stage, which was the Eocene, or earlier part of the Tertiary Age, has been found in the Hills, because they were at that time dry land with gently flowing, shallow streams, and consequently no strata were laid down; but they are supposed, through later evidences, to have had a tropical climate and vegetation, enjoyed by large animals of strange new forms.  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.  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 crystal, are particularly attractive