Oligocene Formation refers to a time period of 25 to 35 million years ago. Current-Day mammals related to that time were small and evolved to a present, larger size. Most larger mammals of that time became extinct along with others that did not survive... Also See pictures of finds from this dig » Oligocene Dig A great view of what is called the White River Badlands. A geologic formation dated at 35 million years ago. These eroded formations contain Oligocene fossils dated at 35 millions years ago. More viewing of the White River Badlands which consists of a hard limestone deposit, but with rains becomes soft and erodes quickly. A great view of an eroding cliff possibly yielding a new find of fossils. Birds make use of a protected cliff area creating their hardend mud nests. A well preserved skull of an oreodont visible to the eye. Careful work to expose more of the oreodont skull reveals its skeletal remains trapped with it.