Weathering products

core stone in saprolite
This photograph was taken in the town of Rockville, Stearns County, along the south wall of a quarry developed in the Rockville Granite. The large round rock located just above the hammer is a corestone, a relict mass of unaltered bedrock, surrounded by saprolite. Corestones are very numerous in the thick saprolites typically found in Minnesota, and may account for the substantial number of "pre-rounded" glacial erratic boulders found in the state.

Saprolite is composed of clay minerals that result from in-situ weathering of bedrock by acidic groundwater. Weathering is concentrated along vertical and horizontal fracture planes that may intersect to leave rounded, relatively unweathered corestones such as the one in this photo. The texture of the original granite is visible in the saprolite as patches of green and white clay. Saprolite in Minnesota is locally overlain by thin deposits of upper Cretaceous sedimentary rocks and may have formed either before, or during Cretaceous time. Kaolin-rich saprolite is mined in the southwestern part of the state as a filler in the manufacture of cement.


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* Stearns Co. Geology map
* MGS home page
* Minnesota geology