INFLUENCE OF ALPINE GLACIAL EROSION ON THE HEIGHT AND MORPHOLOGY OF THE WASHINGTON CASCADES

The topographic development of the Washington Cascade Range has been profoundly influenced by the influence of alpine glacial erosion. Our analysis of climatic and topographic evidence from the Washington Cascades indicates that glacial erosion may limit the altitude of mountain peaks across the range. Glacial erosion linked to long-term gradients in the equilibrium line altitude (ELA) created a planar zone of 373 cirques across the central part of the range. The average relief of individual cirque basins is 380 ± 190 m, and in the range overall, peaks and ridges now rise ≤ 600 m above the Quaternary average ELA despite variation in precipitation, uplift rate, and lithology. The proportion of land area above the cirques drops sharply, and slope distribution analysis indicates that the landscape at and above the cirques may be at or near threshold steepness. Further evidence of the importance of glacial erosion on the high-precipitation western flank of the Cascades is the lack of a correspondence between stream power erosion models and measured exhumation rates from apatite (U-Th/He) thermochronometry, suggesting that non-fluvial processes (such as glacial erosion) influenced long-term exhumation. Hence, we propose that glacial erosion imposes a ~600 m envelope above which Cascade peaks rarely rise due to a combination of higher rates of erosion and enhanced hillslope processes resulting from the creation of steep glacial topography at and above the ELA.