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Polk Co., Wisconsin, USA

Polk County includes the southernmost exposures of the volcanic rock of the 1.1 billion year old Midcontinent Rift. These are asked 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
Actinolite
'Albite-Anorthite Series'
Augite
Bornite
Calcite
Chalcocite
Chalcopyrite
'Chlorite Group'
Chrysotile
Copper
Cuprite
Epidote
Hematite
Ilmenite
'K Feldspar'
'var: Adularia'
Magnetite
Malachite
Quartz
Tremolite
Zoisite


21 entries listed. 17 valid minerals.

Localities in this Region

USA
USA

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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.

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Copyright © Jolyon Ralph and Ida Chau 1993-2014. Site Map. Locality, mineral & photograph data are the copyright of the individuals who submitted them. Site hosted & developed by Jolyon Ralph. Mindat.org is an online information resource dedicated to providing free mineralogical information to all. Mindat relies on the contributions of thousands of members and supporters. Mindat does not offer minerals for sale. If you would like to add information to improve the quality of our database, then click here to register.
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