FIELD TRIP STOP 3 - North Cascade Highway (State Route 20) The Anomalous Skagit Gorge

Turning the River Around Leaving Newhalem, the highway begins a steeper, winding climb up the Skagit Gorge. The white rocks along the road are mostly tonalite orthogneiss; some are migmatites. Above Gorge Lake, outcrops are browner and darker where the road cuts through banded gneisses rich in layers of mica schist.

BEFORE GLACIATION - Ancient Skagit River flows north to Canada. The drainage divide is somewhere near present-day Newhalem. The gorge of the Skagit contrasts strongly with the broad glacial valley at Newhalem. Geologists have explained this anomalous topography in several ways, but the scenario most consistent with the general pattern in the North Cascades is that upper Skagit River, including its major tributaries such as Thunder Creek, Big Beaver Creek, and Stetattle Creek, once drained northward into Canada. The gorge is eroded where once there was a bedrock divide. The growth and retreat of successive Cordilleran Ice sheets brought on this reversal. DURING RETREAT OF THE CORDILLERAN ICE SHEET - Lake spills south over drainage divide to erode Skagit Gorge through the old divide. In this scene most of the alpine glaciers have retreated up the side tributaries. Each time the ice advanced south into the North Cascades, it dammed north-flowing rivers, forming lakes. Water from the lakes spilled over divides to the south and found new routes to the ocean. When the ice retreated, the lakes reappeared, and some of the sediments deposited in the lakes remain in the valleys to the east.

TODAY. As the ice melted back, lakes that rose high enough to find outlets, drained to the south, their rushing waters eroding deep gorges in the bedrock divides. The upper Skagit Lake did just that in the vicinity of Skagit Gorge. Eventually, the new canyon was so deep that even after the Cordilleran ice retreated, the river continued flowing to the south. At certain times in the year, travelers will be startled to see little or no water in the Skagit in the gorge above Newhalem. At such times, the entire Skagit River bypasses the gorge by travelings through tunnels from Gorge Dam above to the powerhouse at Newhalem, where turbines turn the steepness of the riverbed into electricity that lights, heats, and cools Seattle.

On to Diablo Dam

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| USGS Geology in the Parks home | NPS Park Geology Tour home | This site is a cooperative endeavor of the US Geological Survey Western Earth Surface Processes Team and the National Park Service. Please share your comments and suggestions with us! parkgeology@den.nps.gov http://www.nature.nps.gov/grd/usgsnps/noca/nocaft3.html This page was last updated on 12/1/99

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