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North Cascades Geology

The Work of Running Water

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Skagit River
Skagit River near Newhalem
To understand the role of stream and river erosion in shaping the North Cascades, geologists look to the events creating the major drainages of the region such as the Columbia River, which cuts across the present day Cascade Range. The river’s immediate ancestor probably developed from streams running off an elevated block of crust in Washington and British Columbia dating from the uplift of land underlain by the thrust-thickened crust, probably sometime between 90 and 50 million years ago (middle Cretaceous to early Tertiary). Tributaries to the Columbia River and other lesser rivers to the north began etching out the ancestral North Cascade mountains, probably in a linear trellis pattern reflecting the major northwest-southeast alignment of the major rock units. (see How the Rivers Work)
spacer image Not long after (geologically speaking, of course) the ancestral Columbia River was established, probably about 60 million years ago (in the earliest Tertiary), tensional forces in the crust associated with the Eocene extensional event broke the region into fault blocks. Some blocks rose high enough to become mountains, some sunk sank low to become basins. Streams began to fill the basins with sediments mostly eroded from nearby highlands. Professor J. Hoover Mackin, an ardent fan of Pacific Northwest geology, imagined a scene not unlike the Basin and Range region of Nevada and Utah today, with upraised fault block mountains, exposing old rocks, surrounded by alluvial basins, slowly filling with sand and mud. Mackin called this collection of sunken basins and uplifted blocks the Calkins Range, after Frank Calkins, an early North Cascades geologist.
Chiwaukum Graben
View east across the Chiwaukum Graben, a down-dropped block of river-deposited sandstone and conglomerate, which first formed as a structure in the Calkins Range. Visible strata in the graben shows the folded beds of sandstone. The more easily eroded sedimentary rocks form the the low hills betweeen higher ridges of resistant gneiss and schist such as seen here in the higher Entiat Mountains. The west-bounding fault is hidden behind the foreground ridge of schist.

spacer image Although much of the material deposited in the basins came from nearby highlands, some of the sand grains in the sediment came from farther east, supporting the idea that rivers, like the Columbia, had already established their courses through the uplifted terrain. Not only were basins forming as the crust was stretching, but blocks of crust to the west were generally moving north relative to blocks to the east, along major faults such as the Straight Creek Fault. Volcanoes must have erupted above some of the plutons associated with the Eocene extensional event, and some of the stream patterns of the North Cascades may well be relicts from rivers draining radially off these volcanoes.
spacer image Whatever drainage pattern was established, it was profoundly altered about 35 million years ago (Oligocene) by the renewed volcanic activity of the Cascade Volcanic Arc. Although only remnants of these ancient volcanoes exist here and there in the North Cascades, the whole range during this period was probably blanketed by lavas and breccias, just as it still is today in southern Washington.

Preservation of volcanic cover in southern Washington.
Preservation of volcanic cover in southern Washington.


spacer image The growth of these volcanoes must have diverted numerous steams and established a mostly entirely new drainage system, but one reflected in the position of the major drainage divide today. This divide extends roughly north-south through North Cascades National Park, from Mt Redoubt on the north to Damnation Peak on the south, and separates streams that flow west into the Chilliwack, Baker, and lower Skagit Rivers from east-flowing streams that join the upper Skagit. This pattern may have been established as streams flowed down the slopes of the arc volcanoes that once erupted above the Chilliwack Batholith. However, as this volcanic cover was eroded off, differential erosion worked its magic in readjusting the streams and rivers to the hardness and softness of the rocks beneath the volcanic cover. Much of the drainage may have found itself back in the old patterns it had before the volcanic interruption. Creeks once again widened fault valleys, and hard metamorphic and granitic rocks were left as lofty crags.
Ruth Mountain hornfesed by nested plutons
Ruth Mountain underlain by erosionaly resistant contact metamorphosed volcanic rocks of Hannegan Caldera.
spacer image As the range continued to rise, the volcanic rocks were stripped away, except in places where they had been faulted down into the older rocks and are preserved still. Some are preserved because they were metamorphosed to hard rock by nearby intrusion of arc-root plutons. Mount Spickard and Ruth Mountain are examples of rugged peaks underlain by recrystallized (hornfelsic) volcanic rocks.
spacer image The overall drainage pattern we see today, with a few spectacular exceptions, was well-established by the time extensive glaciers began to form about 1.6 million years ago (Pleistocene). Pleistocene glaciers, however, put the final touches on almost every scene. The evidence of glacier erosion or deposition is everywhere.

Something extra: How Rivers Work

On to The Work of Moving Ice


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This page was last updated on 11/30/99
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Material in this site has been adapted from a new 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