These Bad Lands were once a fairly level plain, but intricate stream erosion produced the labyrinth of ravines and ridges for which the region is noted.
High winds and seams of burning lignite coal have aided the rains in giving the Bad Lands their peculiar configuration.
Travel across the bad lands is very fatiguing because of the many small ascents and descents; and it is from this that their name, mauvaises terres pour traverser, was given by the early French voyageurs.
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.
In the foothills there are typical canyons, as along the Platte forks, and in the northern edge of the sand-hills Those of the upper Republican are the largest, those of the Bad Lands are the most peculiar; and the Niobrara tributary system is the most developed.
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.