Before Illinoian ice sheet pushed south through Illinois, the Mississippi flowed along the eastern valley wall. The ice blocked the flow of the Mississippi between the eastern valley wall and Fountain Bluff.
The river eroded a new channel around the the west side of the bluff and along the western valley wall, leaving Fountain Bluff as a free-standing element, rising more than two hundred feet above the ancient channel. It’s named for the numerous springs that flow from it.
Where the Mississippi carved a new course into bedrock, it left behind a rocky channel that the Corps of Engineers cleared during the 1870s. The engineers left Tower Rock as a free-standing element in the river, thinking that one day it might serve as the foundation for a bridge. The rock, protected from quarrying, is now as the smallest national park in the country. If you dare, negotiate the currents that swirl between the rock and the eastern shore, and take a boat out to the rock for a picnic.
When the river runs low, at 0.1 on the Chester, Illinois gage, it is possible to hike out to Tower Rock.
Early nineteenth century geologists ignored the “broad belt of low, wet bottom, five miles in width, and mostly covered with ponds of water, except in the very dryest portions of the season, and over which for countless ages rolled the mighty currents that formed the valley in which the turpid waters of the Mississippi new find their way to the gulf.”
“From the fact that the waters of the Mississippi are restricted to an area much less than its average width, at what is called the Grand Tower, and are hemmed by precipitous limestone bluffs on either side, the theory has been entertained that at a former period these limestone cliffs extended quite across the river, forming an immense fall which has be gradually cut away by the current of the river.” —Amos Henry Worthen
[i] Wiggers, Raymond, Geology Underfoot in Illinois, Missoula, Montana, Mountain Press Publishing Company, 1997, 233; McDonald, Timothy A., “Illinoian Glacial Boundary,” Esling, Steven P. and Blum, Michael D., eds., Quaternary Sections in Southern Illinois and Southeast Missouri, Midwest Friends of the Pleistocene, 42nd Annual Meeting, 19-21 May 1995, 2.4; McDonald, Timothy A., “Quaternary Geology around Fountain Bluff,” Esling, Steven P. and Blum, Michael D., eds., Quaternary Sections in Southern Illinois and Southeast Missouri, Midwest Friends of the Pleistocene, 42nd Annual Meeting, 19-21 May 1995, 5.14; Hajic, Edwin R., Personal communication, July, 1999; Worthen, Amos Henry, Economical Geology of Illinois, Illinois State Geological Survey, 1882, 503