Interstate Park, Polk County, Wisconsin, USAi

Regional Level Types
Interstate Park	Park
Polk County	County
Wisconsin	State
USA	Country

Interstate Park (Wisconsin) contains a variety of fascinating geological features and is worth a visit even though collecting in the park is prohibited. The bedrock is dominantly the Keweenawan age (1.1 billion year old) Chengwatana basalt flows of the Mid Continent Rift System. These are technically greenstones, having been raised to greenschist facies metamorphism. The matrix is converted largely to a mixture of chlorite, epidote, albite and actinolite. However igneous textures still dominate. Both augite and plagioclase pyric flows occur. Flow tops are amygdaloidal and contain white quartz, pink adularia feldspar, green epidote and nearly black chlorite. The flows were dissected into a complex landscape prior to the incursion of the Cambrian sea. High areas stood out as islands, with bouldery talus slopes now preserved as a unit called the Mill Street Conglomerate. Between these, clays, now shales, accumulated in calmer areas. These coarsen upward into sandstone as the water got deeper. The sandstones contain phosphatic shell fragments and locally large elongated ironstone concretions. Erosion by a glacial St. Croix River excavated deeply into the bedrock, wearing gorges, leaving abandoned waterfalls and carving spectacular potholes into the hard basalt. Near the Silver Brook Mansion site, pits and shafts of an old copper mine occur. The copper mineralization was as veins of chalcocite and malachite in the Cambrian sandstone. The copper was perhaps remobilized and reprecipitaed from basaltic bedrock. The western terminus of Wisconsin’s Ice Age trail is in the park, at a spot overlooking the St. Croix River near the rock formation carved in basalt known as the “Old Man of the Dalles”. The park has a visitor center with excellent displays, largely devoted to Ice Age geology.

Fossils

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Other Databases

Wikipedia:	https://en.wikipedia.org/wiki/Wisconsin_Department_of_Natural_Resources
Wikidata ID:	Q1467508

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