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! It's part of the Huronian supergroup of rocks. This rock formed in a riverbed across a wide region as an ancient river channeled back and forth, rounding the pebbles in the rushing water. The 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. All three of these were embedded in sand, which acted as a glue. The formation was then slightly metamorphosed, turning the sandy matrix to quartzite.
This iconic red jasper conglomerate is affectionately referred to as "Michigan puddingstone" or "Drummond Island puddingstone" by locals and is used as a decorative stone. The formation itself lies east of Sault St. Marie, along the north shore of the Georgian Bay- but when the glaciers came by, they scraped out chunks of this conglomerate and carried it with them, as far away as Southwest Iowa!1 The easily-recognizable red pebbles of this formation meant that this stone was an excellent tool to track past glacial activity. Boulders that are carried far, far away by glaciers are called glacial erratics. The conglomerate in our garden was actually a glacial erratic, too, found at a gravel pit near Sault Ste. Marie!
1Chester B. Slawson. "The Jasper Conglomerate, an Index of Drift Dispersion." The Journel of Geology, Jul. - Aug., 1933, Vol. 41, No. 5 (Jul. - Aug., 1933), pp. 546-552
Jasper Conglomerate in the Peter Russell Rock Garden
Crane assisting with lifting the Jasper conglomerate out of the pit.
A close-up of the Jasper Conglomerate showing striking red (jasper), black (chert), and white (translucent quartz) pebbles.
