In the town of Gilbert, Minnesota is an outcrop of 2,700 million year old greenstone. Lava initially erupted into an ocean was basaltic, after undergoing a weak metamorphosism involving higher pressures and temperatures. Plagioclase and pyroxene were changed to green minerals like chlorite and epidote. Deposited on top of the greenstone is the 1,900 million year old Biwabik Iron Formation partially consisting of red hematic chert. The difference between the two units are 800 million years of time.
Glaciers originating from the Rainy Provenance during Late Wisconsin glaciation removed most of the Biwabik Iron Formation from this location. While these glaciers came through the area they also left numerous glacial striations seen in these two pictures as well.
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