Glen Mills Quarry, Glen Mills, Thornbury Township, Delaware Co., Pennsylvania, USA
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Extremely complex geology of Precambrian age. Series of interbedded greywackes and carbonates, along with graphitic gneiss and intermediate to dark igneous rock, folded and metamorphosed. The resulting rock types were then deformed again, and pegmatites were intruded, fracturing the total mass.
The quarry was first opened in 1884. In 1905 the General Crushed Stone Co. started operations and by 1976 was the largest quarry in the area at a depth of 300 feet. Although well below the elevation of nearby Chester Creek, water seepage is minimal.
10 valid minerals.
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Geyer, Smith, Barnes, Min. Col. Pennsylvania: 114-116.