Six ways of seeing the St. Croix: Bedrock geology – St. Croix 360

St. Croix 360
River stories to inspire stewardship.
The rock underlying the river region spans Earth’s history — and affects the present day.
By | March 4, 2022 | 2 minute read
Part two of St. Croix 360’s special map series takes us to the foundation of the river and its watershed. The bedrock underlying the region is varied, from billion-year-old basalt formed by magma to 500-million-year-old sandstone formed by shallow seas.
Bedrock geology differs from surface geology, which in this region, often features deposits of sand, gravel, stone, and other material left by glaciers. Sometimes, bedrock contacts the surface: like the igneous basalt of the Dalles or the sedimentary sandstone bluffs of the valley.
Here are some helpful definitions, courtesy the U.S. Geological Survey:
In addition to forming the valley and affecting the ecosystem, the region’s bedrock is an economic force. Igneous trap rock is mined at the Dresser Quarry. Sandstone and other sedimentary rock is excavated at the Rivard Quarry near Somerset.
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