Allouez (#15) Conglomerate



This Conglomerate Boulder is probably originally from the Allouez Conglomerate, a geologic unit that can be traced from Lost Lake in the Keweenaw to near Painesdale.  Three mines (Delaware,  Allouez and Franklin Jr) mined this conglomerate for copper.  This boulder was preserved as glacial outwash from near the Bluffs in Houghton, about a mile SW of its current location.  It was torn loose from the bedrock by glaciers and deposited on the surface in moraine, then redistributed by water from the melting glacier, about 10,000 years ago.  It was moved to its present site in August 2011.  Conglomerate rock layers form part of the Portage Lake Lavas, a 1. 1 billion year old sequence of 400 lava flows that mark the great Keweenaw rift zone, the first great continental rift lava sequence in the world.  The Keweenaw rift split an ancient continent called Rodinia, and extended from Kansas through Lake Superior all the way to Ohio.  Within the 400 lava flows there were also a few conglomerates. Conglomerate is a sedimentary rock with boulders and cobbles that formed along the edge of the rift. It requires high energy to form in great fan-shaped deposits called alluvial fans. You can find rocks like these forming today from energetic floods down the steep slopes of mountain ranges, in places like Death Valley, California.  When Houghton was founded in the 1850s, boulders like these were littered around the surface.



EarthCache Site                                Alluvial Fans


What kinds of rocks do you find in this conglomerate?

Why isn’t basalt the main rock type?