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USGS Geology in the Parks

Domains of the North Cascades

FIELD TRIP STOP 5 - North Cascade Highway (State Route 20)

Diablo Lake Overlook

Rocks and glaciers behind the scene

From the parking area at the Diablo Lake Overlook (1.5 miles along Hwy 20, traveling east from the bridge over Thunder Arm of Diablo Lake), look southwest to views of glaciers on Colonial, Snowfield and Pyramid Peaks. These mountains are carved from banded gneiss in the Skagit Gneiss Complex, also described in the display at this overlook. Look to the west across Diablo Lake to see the long, straight-but-small, valley of Sourdough Creek, climbing steeply up to Sourdough Mountain. This erosional trench is carved along the crushed rock of the Thunder Lake Fault.

View of Colonial Group
View of Colonial Group from Diablo Lake Overlook

Prior to the Pleistocene Ice Age, the upper Skagit River and major tributaries such as Thunder Creek drained north to the Fraser River in Canada. Cordilleran ice blocked the river's northward course, and it's dammed waters spilled south to erode the Skagit Gorge. The level benches and terraces prominent east of Diablo Resort and where the viewer now stands were probably part of the old valley bottom when the waters all ran to the north. The deep gorge of upper Diablo Lake was eroded by the voluminous meltwaters coming off of Cordilleran ice as it advanced and retreated. During the peak of glaciation the valley was filled with ice over a mile thick, which also rasped away at the rock and rounded many of the landforms.

On to Ross Lake Boat Landing

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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