Evolution of Glaciated Topography of Upper Skagit Drainage Basin, Washington

Abstract

A typical tributary entering the upper Skagit Valley from the east has a broad, gently arcuate, U-shaped headward segment that passes downvalley into a narrow, crooked, V-shaped canyon. Striations and barely weathered glacial drift, however, embellish not only the valley heads but the apparently unglaciated segments and intervening divides as well. Lengthy or repeated episodes of alpine glaciation apparently shaped the upvalley segments, whereas drift and striations throughout the region evince a brief, southward excursion of the Cordilleran Ice Sheet. The Skagit gorge probably lies at or near the downvalley limit of early Fraser and older alpine glaciers in Skagit Valley, rather than at an ice divide during ice-sheet glaciation as was formerly supposed.