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Polk Co., Wisconsin, USA

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Polk County includes the southernmost exposures of the volcanic rock of the 1.1 billion year old Midcontinent Rift. These are masked to the south and west end of the county by Cambrian sandstones and Early Ordovician carbonate rocks. They are well exposed from Dresser north to St. Croix Falls. They are particularly visible in Interstate Park along the Dells of the St. Croix River and numerous hiking trails in the park. Outcrops in the park allow reconstruction of a chain of volcanic highlands progressively covered by the advancing Cambrian sea. Basalt outcrops are also sporadically exposed outside the park from Dresser north to the county line. Minor native copper was produced from the rocks, which are have been subjected to the greenschist facies during burial metamorphism. Epidote, chlorite, adularia and quartz are common amygdule fillings, vein minerals and groundmass replacements. The basalts are quarried at Dresser, which supplies crushed stone, rip rap and other materials. Thick glacial drift covers much of the county, and can yield interesting materials such as float copper, jasper and "Lake Superior" agates to the determined collector. The Ice Age Trail traverses the county, with its western termination in Interstate Park. Interstate Park has excellent displays in its Visitor Center on Ice Age geology. The public library and historical museum in Luck, Wisconsin also features fine geology displays, with artwork illustrating the geological history of the region.

Mineral List

Mineral list contains entries from the region specified including sub-localities


22 entries listed. 18 valid minerals.

Localities in this Region


The above list contains all mineral locality references listed on mindat.org. This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in mindat.org without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.

References

Wirth, K., J. Vervoort & Z. Naiman (1997) The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift: Canadian Journal of Earth Sciences: 34: 536-548.

Wirth, K, W.S. Cordua, W. Keen, M Middleton & Z. Naiman (1998) The geology of the southeastern portion of the Mid-Continental Rift system, eastern Minnesota and western Wisconsin: 44th Institute on Lake Superior Geology part 2: 33-78.

Dott, Robert H. & J.W. Attig, (2004) Roadside Geology of Wisconsin, Mountain Press, 345 p.

External Links

Wisconsin Mineral index - Wisconsin Geological and Natural History Survey http://wgnhs.uwex.edu/wisconsin-geology/minerals-wisconsin/

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