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USGS Geology in the Parks

field trip map 3

FIELD TRIP SIDETRIP L - Off of Twin Lakes Road; Mount Baker Highway (State Route 542)

Lone Jack Mine

Big Bosom Buttes
Looking east at Big Bosom Buttes from Lone Jack Mine. Volcanic breccia, forming rugged peaks, overlies volcanic tuff which in turn overlies older, river-deposited sandstone..

Gold and an ancient caldera

Miners developed the Lone Jack Mine in the 1890s, and by 1924, when the mine closed, they had removed several hundred tons of ore. Over the years, avalanches and fire have destroyed several concentrating mills and other buildings on this steep valley wall. Quartz veins with gold in the black phyllite here have attracted miners right up to the present day. The source of the gold-bearing veins may be granitic root plutons of the Cascade Arc, which are exposed in Silesia Creek, below to the east. Across the valley to the east, Big Bosom Buttes reveal the remains of a volcanic caldera that erupted about 25-30 million years ago (Oligocene), during the early years of the Cascade Volcanic Arc. The volcanic rocks overlie older stream deposits of sandstone. These older rocks are remnants of extensional deposits that filled fault-bounded valleys before the birth of the volcanic arc.
On to Nooksack Cirque

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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