For many years, the town has been threatened by the erosion of the river banks.
The granite forms the prevailing rock in valleys of erosion.
The helicopter ride was demonstrating the widespread erosion of the surrounding plains.
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.
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.