This pink colored granitic rock is a glacial erratic. Because Illinois does not contain granite in its lithology, geologists have speculated that rocks such as this were left behind by receding glaciers. This small bolder may have been dropped off during any one of the frequent glacial invasions that occurred during the Cenozoic Era. Not only did the glaciers leave behind erratics, they also created the wind-blown silt that now covers most of LaSalle County. Geologists have named this the Richland Loess, and it covers most of the surface of the state as well. This was accomplished by the repeated scouring and scraping of the pre-glacial erosional surface during the Pre-Cambrian as the glaciers advanced and retreated. The last glacier retreated from Illinois about 10,000 years ago.