Conglomerate is a sedimentary rock, made up of fragments of other rocks, including both rhyolite and basalt.  Keweenawan conglomerates result from alluvial fans, which form along the base of mountains from boulders and cobbles that roll down steep river valleys and then spread out to form a fan shaped body that may be as wide as 10 miles or more.  Such fans form near mountains and along rift valleys where there are steep slopes.

Bill Rose

Professor of Petrology

15 November 2010