Tillite is the name for the type of glacial erratic shown in this photo (below). Fairly common in the glacial drift of Illinois, these tillites were derived from Canadian Shield rocks (Precambrian Gowganda Formation) of Ontario. Tillite (also called diamictite and mixtite) is made up of sediment that was carried or deposited by a glacier and later cemented to form rock. It consists of a fairly fine-grained matrix that contains pebble to larger size pieces of distinctive rock types. Most tillite erratics have a dark green, gray, or grayish brown matrix that contains lighter-colored, fairly angular granite pebbles. The tillite extends over a larger area in Ontario than does the jasper conglomerate, which probably accounts for the greater abundance of tillite erratics in Illinois. Photo by J. Dexter.
The photo below shows tillite bedrock cropping out in a road cut north of Bruce Mines in Ontario. Photo by L. Joseph.