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 Greg Seitz | 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.
St. Croix 360’s supporters keep the river stories flowing.
Please contribute, thank you!
St. Croix 360 builds support for river stewardship, and connects people and organizations. It is an independent news source produced by Greg Seitz, with past support from the St. Croix River Association.
Donate to St. Croix 360 today »
One of the biggest challenges facing the St. Croix River is harmful blooms of algae in in the lower river, due to excess nutrients in runoff. The goal is to restore the river by reducing phosphorus levels to 360 tons/year. Learn more »
St. Croix 360 is powered by a broad community of St. Croix River-loving people. Please visit our submission page to send tips, press releases, and other news.
Lower St. Croix: St. Croix 360 partners with online calendar St. Croix Splash.
For events elsewhere in the watershed, contact St. Croix 360 directly.
To help increase awareness of the St. Croix River, news organizations and other outlets are free to share St. Croix 360 content, as long as you follow a few simple rules.
Republish St. Croix 360 stories »