A conglomerate is a rock consisting of individual rounded fragments within a finer-grained groundmass of rock that have become cemented together.

The rocks in the photo below are a distinctive type of glacial erratic called jasper conglomerates. They were collected from glacial drift in Illinois. These conglomerates were derived from the Canadian Shield rocks (Precambrian Lorrain Formation) that crop out north and northwest of Bruce Mines, which is located in Ontario along the north shore of Lake Huron. The Lorrain Formation contains rocks associated with Precambrian glaciation, and the jasper conglomerates are attributed to sand and pebbles derived by erosion from older rocks and redeposited as gravity flows in water. Later these materials were lithified to form conglomerates and transformed by heat and pressure of later volcanic activity to form quartzite conglomerates. Early English settlers in the Bruce Mines area called the jasper conglomerate "puddingstone" because it looked like boiled suet pudding with cherries and currants. Though somewhat rare in Illinois, jasper conglomerate erratics are found in northeastern-source drift. Photo at left taken by A.K. Hansel. Photos below taken by L. Joseph.

Glacial drift over bedrock photo

ConglomorateConglomorate to scale