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USGS Geology in the Parks

Domains of the North Cascades

FIELD TRIP STOP 6 - North Cascade Highway (State Route 20)

Western Ross Lake Overlook

Views of faulted terranes

Ross Lake is a hydroelectric reservoir behind Ross Dam, mainly constructed between 1937 and 1949. The lake is 22 miles long and ends just beyond the United States border at the 49th parallel. From the Western Ross Lake overlook (about 3.3 miles east of the Diablo Lake Overlook), look north up Ross Lake hopefully, to view (on a clear day) the Ross Lake Fault, which separates the Metamorphic Core Domain of the North Cascades from the Methow Domain to the east.

 

View north from Ross Lake Overlook on Highway 20.
View north from Ross Lake Overlook on Highway 20. The main strand of the Ross Lake Fault Zone is under Ross Lake near and parallel to the East Bank Trail..

On the west side of the lake, high ridges carved from the Skagit Gneiss Complex lead up to Mount Prophet (7,660 feet). On the east, Jack Mountain (9,066 feet), carved from basalt of the Hozomeen Terrane, towers some 7,400 feet above the lake. The lower slopes of Jack Mountain are underlain by mica schist and other rocks of the enigmatic Little Jack terrane, which is separated from the overlying Hozomeen terrane above by a thrust fault.

The metamorphic minerals in the Skagit Gneiss Complex west of the Ross Lake Fault formed at much hotter temperatures (about 720° C) than those in the Hozomeen greenstones (about 300° C) east of the fault. This differential indicates that the Skagit Gneiss has been uplifted from much deeper in the crust than the Hozomeen rocks.

On to Crater Mountain

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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