The Bad Lands of the White river are also noted for their wealth of animal fossils, which have been found in such quantities as to cause geologists to believe that the vertebrates perished there in droves during a severe storm or flood.
This area extends from the mist meridian up the White river for about 120 m.
The superficial Miocene and Pliocene deposits in the west, above referred to, are underlaid by the White river groups of the Oligocene, whose outcrops of Brule clay and Chadron formation also have been mentioned.
This included, roughly speaking, all of the land between the Missouri River and the Black Hills and between the White River and the Big Cheyenne and a strip extending north from the Black Hills to the North Dakota line between the 102nd and 103rd meridians.
The White river and Hat Creek have carved canyons in deep lacustrine deposits, creating fantastic cliffs and buttes, bare of vegetation, gashed with drainage channels, and baked by the sun.
By way of the White river cut-off the Arkansas finds an additional outlet through the valley of that river in times of high water, and the White, when the current in its natural channel is deadened by the backwaters of the Mississippi, finds an outlet by the same cut-off through the valley of the Arkansas.
Fossil insects referable to the order have been found in Tertiary beds as old as the White River Oligocene of North America, and the Baltic amber, but nothing is known as to the previous history of the group.
In the south-west the results of this erosion are seen in an accentuated form in the region between the White river and the South Fork of the Cheyenne river, known as the Bad Lands or terres mauvaises.