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This rock is 2,200 – 2,450 million years old, from the middle Precambrian era- right around the same time that oxygen was being added to the atmosphere!ÌýIt's part of theÌýHuronianÌýsupergroup of rocks.ÌýThisÌýrockÌýformed in aÌýriverbed across a wide region as an ancient river channeled back and forth, rounding the pebbles in the rushing water.ÌýThe striking red pebbles in the rock are a type of cryptocrystalline quartz called jasper. The black pebbles are chert, another cryotocrystalline quartz. The white pebbles are translucent quartz.ÌýAll three of these wereÌýembedded in sand, which acted as a glue. The formation was then slightly metamorphosed, turning the sandy matrix toÌýquartzite.Ìý

This iconicÌýred jasper conglomerate is affectionately referred to as "Michigan puddingstone"Ìýor "Drummond Island puddingstone" by localsÌýand is used as a decorative stone. TheÌýformation itself liesÌýeast of Sault St. Marie, along the north shore of the Georgian Bay- butÌýwhen the glaciers came by, they scraped out chunks of this conglomerateÌýand carried itÌýwith them, as far away as Southwest Iowa!¹ÌýThe easily-recognizableÌýred pebbles of this formation meant that this stone was an excellent tool to track past glacial activity.ÌýBoulders that are carried far, far away byÌýglaciers are called glacial erratics. TheÌýconglomerate in our garden wasÌýactually a glacial erratic, too, found at a gravel pit near Sault Ste. Marie!

¹Chester B. Slawson. "The Jasper Conglomerate, an Index ofÌýDrift Dispersion."ÌýThe Journel of Geology, Jul.Ìý- Aug., 1933, Vol. 41, No. 5 (Jul. - Aug., 1933), pp. 546-552