INFLUENCE OF GLACIAL PROCESSES ON THE LARGE-SCALE MORPHOLOGY OF THE WASHINGTON CASCADE RANGE

We analyzed a 10-m grid digital elevation model (DEM) to constrain the influence of glaciers on large-scale topographic trends across the Washington Cascades. Despite a transition from alpine glaciation in the central Cascades to continental ice sheet glaciation in the northern Cascades, total relief and maximum peak elevations increase from south to north (from 650 to 1200 m and 1100 to 2000 m, respectively). A simple model to estimate the potential isostatic rebound of mountain peaks due to relief development (i.e., valley incision) allows the determination of the maximum possible amount of peak uplift due to this mechanism. Residual peak elevations after subtracting the isostatic component still increase from south to north, as do the mean elevations (from 700 to 1400 m). These trends demonstrate that the higher relief in the northern Cascades is not simply due to the excavation of deeper valleys. Therefore, a process other than glacial valley excavation, such as a long-term N-S gradient in tectonic forcing, causes the N-S variability in the maximum elevations and relief.

Conversely, E-W gradients in peak and mean elevations show a significant influence of glacial erosion. A surface connecting mountain summits on the west side of the central Cascades is nearly indistinguishable from the west-dipping equilibrium-line altitudes (ELAs) of modern glaciers. This correlation between maximum peak heights and the modern ELA suggests that peaks cannot be sustained above the elevation where glaciers have existed nearly continuously throughout the Quaternary. In addition, a surface constructed from mean elevations is roughly consistent with the mean altitude of cirque floors, which is parallel to and located about midway between the ELAs of the last glacial maximum and today. These correlations indicate that glacial erosion is a primary control on the topography of the western central Cascades, and that the Cascades may in fact sport a “glacial buzzcut.” Surprisingly, despite significant N-S variation in the style and extent of glaciation, the geomorphic signature of glacial processes appears to manifest itself more clearly in the maximum and mean elevation trends seen in E-W transects than in the maximum elevation and relief trends seen in N-S transects.