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Home Archived Aug 4, 2016

USGS Geology in the Parks

North Cascades Geology

The Eldorado Orthogneiss, a 90 million-year-old  stitching pluton in the Metamorphic Core Domain.
Looking south from the North Klawatti Glacier. In the right foreground, the glacier breaks into a heavily crevassed icefall where is descends steeply. The rock in the foreground and at a distance is the Eldorado Orthogneiss. Mount Buckner is on the left skyline and Mount Forbidden hidden in clouds.

Scenery Born of Ice

North Cascade valleys are U-shaped, with steep, cliffy walls and broad flat floors. The valley shape reflects erosion by long tongues of ice descending from peaks at the valley heads. Steep steps commonly occur at the junction of two or more major valleys where the combined action of the merged ice streams eroded faster and deeper.

Hikers nearing the heads of North Cascade valleys commonly encounter double cirques, making for two stiff climbs before reaching the high ridge tops. The lower cirque wall, a steep step in the valley, represents the head of an earlier great glacier, one that filled the valley during the Pleistocene. The upper cirque step, commonly still bearing a glacier, forms the head of the valley, and represents a new, higher, and smaller glacial bite into the ridge. Such multiple cirque steps are encountered on the east side of Cascade Pass, where the trail climbs from the lower valley to Pelton basin (one) and then to the pass itself (two), and also on Bacon Creek below Berdeen Lake and on Railroad Creek at Crown Point Falls.

During the latest Ice Age, about of 25,000 to 13,000 years ago (near the end of the Pleistocene), much of the North Cascades was covered by glacial ice. During that time of colder climate, the local peaks grew their own glaciers, but in the higher mountains of British Columbia a vast glacier, the Cordilleran Ice Sheet, was growing and advancing south into Washington State. Eventually, Cordilleran ice filled the Puget Lowland, and its surface rose to merge with ice originating in the North Cascades. Only the highest peaks, generally those above 6,000 to 7,000 feet, stuck up above the ice surface. This mass of slowly creeping ice smoothed and rounded off the lower peaks, but much of today’s scene was established, not by the scraping of the ice sheet, but by its retreat. Huge amounts of water from the melting ice poured down valleys. Lakes formed, deposits grew; lakes drained, and the same deposits were partially washed away. During glacial retreat, drainage reversal was widespread in the North Cascades. Some rivers that originally drained north to the Fraser River in British Columbia could no longer do so while the ice sheet remained. As a result, the water from the melting ice spilled south, cutting new and deeper canyons that held the rivers' courses even after all the ice was gone.

Glaciers in Marble Creek cirque
Glaciers and moraines in Marble Creek cirque. The high point is Eldorado Peak.

The Skagit River is the most prominent permanent reversal. Elsewhere, melt water carved deep notches, such as Chilliwack Pass at the head of the Chilliwack River, but failed to cut deeply enough to reverse much of the drainage once the ice had melted away. Actually, this story is more complex, for the drainage reversals could have developed in any of several earlier continental glaciations. Geologists find evidence of at least six major advances of the ice into the Puget Lowland over the past 2 million years. Evidence for these earlier glaciations and possible drainage reversals is sparse in the mountains, obliterated by subsequent glaciations. (Click here for more information on drainage reversal)

After the late Pleistocene retreat of the glaciers, the climate warmed to such an extent that most if not all the ice in the mountains disappeared. Then about 5,000 years ago, glaciers grew again. The glaciers we see today are remnants of this latest episode. Sometime between the 13th and 19th centuries, during the so-called Little Ice Age, small glaciers high in the Alps of Europe, the Himalayas, and many other mountain ranges, including the North Cascades, advanced again downvalley one or more times and left conspicuous terminal moraines. If there were other glacier advances in the last 13,000 years, their tracks have been covered by these latest advances.

On to The Work of Gravity

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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