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USGS Geology in the Parks

Terranes of the North CascadesTerranes of the North Cascades: Nooksack Terrane

location of nooksack terrane rocks
Location of Nooksack Terrane rocks shown in green.

Summary: Conglomerate, sandstone, and shale, once deposited in a submarine fan flanking a volcanic island arc. From 70 to 120 million years old. Very little folded and faulted.

The rocks of the Nooksack terrane (also called the Nooksack Formation) are best seen in the vicinity of the North Fork of the Nooksack River. In the Wells Creek area the lower part of the terrane consists of volcanic rocks, probably deposited in the ocean at the edge of island arc volcanoes. These volcanic rocks are overlain by conglomerate, black shale, and sandstone, many thousands of feet thick.

Beds of argillite in the Nooksack Formation on Skyline Divide.
Beds of argillite in the Nooksack Formation on Skyline Divide. Mount Baker Volcano rises behind.

Hikers on the Skyline Divide Trail can view these rocks under their feet and spectacularly in the distance to the north across the North Fork of the Nooksack on Excelsior Ridge. Bedding characteristics indicate that some of these sedimentary rocks were deposited in submarine fans washing down off the volcanic island arc.

 

Belemnite fossils
Belemnite fossils

Many outcrops of the Nooksack sedimentary rocks contain fossils of belemnites, a cephalopod similar to a cuttlefish, with an internal tube-like shell. On Chowder Ridge and elsewhere abundant shells of clams may also be seen. From the fossils, geologists know the age of the rocks of the Nooksack terrane to be about 170 to 120 million years old (Jurassic and early Cretaceous).

Something extra: Sand in the Sea: How Does it Move?

Visit the Chilliwack River Terrane

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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