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Home Archived Aug 4, 2016

USGS Geology in the Parks

Field trip map 2; Baker River

FIELD TRIP STOP 11 - Anderson Creek Road overlook, Baker River

A Geologic Potpourri

Where the road to Anderson Butte and Watson Lakes trailhead branches left go straight about 0.5 mile to a bluffy road cut of highly deformed greenstone and shale of the Bell Pass mélange. The rocks are full of faults and fractures. From this spot--if the day is clear--visitors can admire a variety of geologic wonders. In the middle foreground to the south, Welker Peak rises above the Welker Peak thrust fault. The peak itself is mostly chert in the Bell Pass mélange. Below the fault are volcanic and sedimentary rocks of the Chilliwack River terrane. To the northwest, Mount Baker dominates the scene. The mountain and its old-rock foundation rise immense from the floor of the broad Baker River valley.

Baker valley
View to the southwest down Baker River valley from Anderson Creek Road.

On the near side of the volcano, glaciers point their tongues down valleys eroded between sharply sculpted old lava flows, called cleavers. These lava-flow caps on today's ridges were once hot lava floods in valley bottoms, since left high by erosion. Such topographic reversal is common on the flanks of volcanoes, where a valley bottom lava flow displaces creeks to the sides of the flow. In time, the newly routed creeks erode valleys that flank the original valley-bottom flow.

Baker River valley
Baker River valley from Anderson Creek Overlook; compare to previous sketch.

Looking southwest down the Baker River, conjure up a vision of a huge glacier filling the valley below. To view this scene from this vantage in the Late Pleistocene (about 17,000 years ago), an Ice-Age visitor would have had to stand on the ice surface some 800 feet above the road. A vast sea of ice would stretch out to the south. High peaks of the main Cascade Range would rise above the ice to the east, and on a very clear day, the dark peaks of the Olympic Mountains would show on the southwest horizon. The ice surface would have extended from 5,000 feet here to about 3,000 feet on the northeast flank of the Olympics. More likely, everything would have been obscured by blowing snow. At one stage, during the last glacial retreat, the outwash from Cordilleran ice in the lower Skagit River valley built a thick fan of gravel and sand at the margin of the melting glacier. The small hills in mid-distance toward the southwest are remnants of the outwash fan, which sloped down to the northeast, away from the ice.

On to Anderson Butte

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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