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USGS Geology in the Parks

Domains of the North Cascades

SIDE TRIP A - Sauk Mountain Road, off the North Cascade Highway (State Route 20)

Sauk Mountain

Sauk Mountain
On the trail to Sauk Mountain

Views and remnants of old volcanoes

For a rewarding side trip east of Concrete, take the Sauk Mountain Road. The road climbs seemingly endless switchbacks up a very large landslide, eventually reaching a parking area and trailhead for Sauk Mountain. From this spot, hang-glider pilots launch themselves for a wonderful ride down to fields along Highway 20, 4,000 feet below. The views from the parking lot are spectacular. For even better views, climb the switchback trail to the summit of Sauk Mountain.

The switchback trail climbs up through cliffs of volcanic rocks of the Chilliwack River terrane. Along the lower switchbacks the metamorphosed volcanic rock is highly foliated, but along the summit ridge, look for volcanic breccia of mostly green, grey, and black angular fragments of volcanic rocks surrounded by dark green or grey matrix. This breccia was deposited in a submarine fan at the toe of a volcano in the ancient Chilliwack volcanic arc, perhaps 250 million years ago (Permian).

Sketch of volcanic breccia of the Chilliwack River terrane on the summit ridge of Sauk Mountain.
Sketch of volcanic breccia of the Chilliwack River terrane on the summit ridge of Sauk Mountain.

On the top, look south across the broad valley floor at the confluence of the Skagit and Sauk Rivers. Here is a good place to contemplate the lake that filled these valleys when the Skagit drainage was dammed by glacial debris at Concrete. The lake drained via the North Fork of the Stillaguamish near Darrington. To the east much of the mountainous interior of the North Cascades National Park and Ross Lake National Recreation area is in view on a clear day, as are rocks of all three geologic domains of the North Cascades.

Looking northeast from Sauk Mountain.

Looking northeast from Sauk Mountain at high peaks, mostly in North Cascades National Park. All three Domains of the North Cascades are visible, but the faults bounding the Domains go from left to right in this drawing and are mostly hidden behind the ridges.

Bald Mountain, Cloudcap Peak, Mt Watson. Mt Blum, Bacon Peak, Diopsud Buttes and Helen Buttes are in the Western Domain. All the other named peaks and ranges are in the Metamorphic Core Domain, with the exception of Jack Mountain which is in the Methow Domain.

On to Newhalem
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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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