MINERAL OF THE MONTH
March 2013 - Jacobsville Sandstone
Jacobsville Sandstone is a red sandstone that has light-colored streaks and spots caused by chemical leaching across the bedding planes. This beautiful rock is found along the Lake Superior shoreline in northern Upper Michigan and Ontario. Scientists also think that Jacobsville lies under much of Lake Superior. There are also other sandstones in Wisconsin and Minnesota that may or may not be the same formation (Freda sandstone, Bayfield Group sandstone). Jacobsville Sandstone is composed of: 27.4% nonundulatory quartz, 27.0% undulatory quartz, 23.0% potassium feldspar, and 12.3% silicic volcanic clasts.
A hundred years ago this rock was mined and used as a building material. It was highly desired for its durability and aesthetics. Over 30 different quarries in the Upper Peninsula extracted Jacobsville sandstone between 1870 and 1915.
Below is a table showing the sandstone sales from 1899 to 1913.
There is some confusion about this rock since it has had several names including redstone, brownstone, Lake Superior Sandstone, and Eastern Sandstone. In 1907, however, the sedimentary formation was given its current classification and the name Jacobsville, in honor of the town in which some of the quarries were located. There is also disagreement about the age of the sandstone. Most believe that it formed around a billion years ago, but others feel that it is half that age. Since the sandstone is entirely devoid of fossils, the older age classification is more likely.
Today, the best place to see this rock is near Au Sable Point, located around eight miles west of Grand Marais.
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Gitche Gumee Museum.
E21739 Brazel Street
Grand Marais, Michigan 49839