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lin murphy
Feb 11, 2009
Needles, SD. These pinnacles are pegmatites, possibly coarse grained off-shoots of the 1.7+ billion year old Harney Peak Granite, which were injected into basement rock during the Trans-Hudson Orogeny. The basement rock has been eroded (away),leaving the pegmatites as pinnacles. One of the pegmatite minerals is mica, which appears as a squarish "book," of many sheets. I recall that the entire mica foothold bent as I stepped on it. See for comparison, the pegmatite spires of the Monastery, CO.  


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