Six ways of seeing the St. Croix: Bedrock geology

The rock underlying the river region spans Earth’s history — and affects the present day.




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:

  • Intrusive Igneous Rocks form when magma is trapped deep inside the Earth, where it cools and solidifies over thousands of millions of years. Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture. (More)
  • Sedimentary Rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth’s surface. Sedimentary rocks often have distinctive layering or bedding. These rocks often start as sediments carried in rivers and deposited in lakes and oceans. When buried, the sediments lose water and become cemented to form rock. Common sedimentary rocks include sandstone, limestone, and shale. (More)
  • Metamorphic Rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Common metamorphic rocks include phyllite, schist, gneiss, quartzite and marble. (More)

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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Six ways of seeing the St. Croix: Bedrock geology