"The Bluffs were formed from drumlins, which in turn were created by glaciers in the last ice age. The erosive power of wind, rain, snow, and waves—both from above and below—has formed the landscape into razor-sharp pinnacles. Although the impressive pinnacles and cliffs, some of which rise up to 150 feet from the lake shore, have existed throughout recorded history, they present a changing panorama: the average erosion of bluffs is 1 to 5 feet per year."
http://en.wikipedia.org/wiki/Chimney_Bluffs_State_Park