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USGS Geology in the Parks

Domains of the North Cascades

FIELD TRIP STOP 1 - North Cascade Highway (State Route 20) West of Washington Pass

Limestone from quarry at Concrete
A drawing of crinoid fossils in a limestone block from the Chilliwack River terrane (from a photograph in Danner, 1970).

The town of Concrete

Concrete Limestone quarry

The town of Concrete was established in 1909 upon the merger of several earlier communities that manufactured cement. The large concrete silos at the west end of town are the most visible relics of this dusty enterprise, which shut down around 1968. The cement workers quarried limestone from large fossiliferous deposits just east of the Baker River dam, at the south end of Lake Shannon.


The limestone deposits, part of the Chilliwack River terrane, contain numerous fragments of large fossilized crinoid stems indicating an age of about 330 million years (Carboniferous). Crinoids, or sea lilies, evolved about 490 million years ago, and numerous species exist today in all the world's oceans. They belong to the sea urchin family, but, following a free swimming larval stage, attach themselves to the ocean bottom and grow into a plant-like animal.

(A) As the Puget Lobe of the Cordilleran ice sheet retreats, a lake forms between the Skagit River and the Darrington area. Skagit water spills out the future North Fork of the Stillaguamish River.

Concrete: Ice Age dam

Just east of the Baker River, on the north side of Highway 20, high cliffs of glacial gravels are the remnants of a gravel dam that, during the Ice Age, blocked the Skagit drainage completely. Glacial geologists say that this mass of outwash gravel formed as a tongue of ice from the Cordilleran ice sheet advanced eastward up the Skagit drainage from the Puget Lowland and blocked the Skagit River.

(B) Soon after ice sheet retreats, Skagit River breaches outwash plug of gravel and finds its old channel. Sauk and Suiattle Rivers continue to drain out the future channel of the Stillaguamish until the Sauk River builds an alluvial fan which diverts them north, as seen in the illustration to the right and below (C).

Outwash gravels from the the Cordilleran ice in the Puget Lowland began to fill the lake formed behind the dam. The rising lake waters, still fed my melting ice up valley, backed up the Sauk and Suiattle Rivers and eventually spilled out into what is now the North Fork of the Stillaguamish River near Darrington. When the Cordilleran Ice Sheet retreated from the Puget Lowland, the lake, followed by the Skagit River, eventually broke through the gravel dam (B). The Sauk and Suiattle Rivers continued to drain through the southern route for awhile.

(C) Today both the Sauk and the Suiattle Rivers drain into the Skagit.

Eruptions of Glacier Peak volcano at their headwaters eventually so choked the Sauk and Suiattle Rivers with volcanic debris that near Darrington they built an alluvial fan that diverted both rivers north to join the Skagit River.

On to Sauk 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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