Geological Survey Professional Paper 705—A Inventory of Glaciers in the North Cascades, Washington

As defined by Fenneman (1931, p. 422) and others, the range comprises all the mountains between Snoqualmie Pass on the south and the Fraser River (in Canada) on the north. The North Cascades are bordered by the Puget Sound lowland on the west and the Columbia and Okanogan Rivers on the east. This report covers the glacierized American parts of the range and includes the Wenatchee Mountains west of Blewett Pass, the Entiat and Chelan Mountains, and the mountains west of the Methow and Chewack Rivers (pl. 1). This encompasses an area of about 20,000 km²—180 km in length from north to south and 140 km in maximum width.

The North Cascades are characterized by great vertical relief (pl. 1). Most ridges and peaks, typically sharp-crested and rugged, are separated by narrow, steep-walled valleys whose floors are generally less than 1,000 m (meters) above sea level, even near their headwaters. In many valleys, relief changes of 2,000 m occur in a horizontal distance of less than 5,000 m.

Two volcanoes, Mount Baker (3,285 m) and Glacier Peak (3,213 m), stand conspicuously above the general level of the surrounding peaks, which average about 2,500 m in altitude. Of the 20 nonvolcanic peaks that are above 2,700 m, Bonanza Peak (2,899 m) is the highest. Only a few of the higher peaks are on the Cascade divide, a hydrologic boundary that divides the State of Washington into two distinct parts. In much of the North Cascades the divide is so obscure that it is difficult to recognize among the seemingly random jumble of mountain peaks.

Present-day glaciers cover the slopes of higher peaks, cling to the sides of steep ridges, or occupy high-level cirques. The floors and lower slopes of valleys are heavily forested but, at altitudes near timberline, natural alpine parks contain some of the region's most magnificent scenery. Scenic, recreational, and scientific possibilities of the region have resulted in the creation of the North Cascades National Park, Lake Chelan and Ross Lake National Recreation Areas, and the Glacier Peak and Pasayten National Wilderness areas. Large hydroelectric developments include Ross, Diablo, and Gorge Dams on the Skagit River; Lake Shannon and Baker Lake on the Baker River; and Lake Chelan. Storage dams also impound Spada, Cle Elum, Kachess, and Keechelus Lakes.

Access to the region is provided by two cross-State highways—Interstate 90, which crosses Snoqualmie Pass, and U.S. 2, which crosses Stevens Pass—and by dead-end roads leading into the larger valleys. The North Cascades Highway, now under construction, will provide access to some of the most scenic areas. Commercial boats ply Lake Chelan, and several other large lakes are accessible to float planes.

The oldest rocks of the North Cascades are a metamorphic-plutonic complex that formed before Middle Devonian time. The predominant outcrops in the range are schists and migmatites, probably pre-Jurassic in age, and intrusive rocks which date from Cretaceous and Tertiary time. The present altitude of the range is largely a result of uparching that occurred during the late Cenozoic period. The arching, with a north-south trend, was associated with displacements on normal and thrust faults. During the Pleistocene, the volcanoes of Mount Baker and Glacier Peak were superimposed on terrain grossly similar to that of today (Grant, 1969).

All the high mountains have been repeatedly sculptured by glaciation during the Pleistocene, and their present ruggedness is due largely to ice modification of a highland already maturely dissected by streams and rivers. During each major glacial episode all high peaks and ridges were accumulation areas for glaciers that descended the major stream valleys. The northern part of the area was invaded by the Cordilleran Ice Sheet which covered most of the Skagit and Methow River drainages. Most of the westward-draining valley glaciers probably joined a lobe of this ice sheet in the Puget Sound lowland during early and middle Pleistocene time (Crandell, 1965). All the larger natural lakes, such as Chelan, Wenatchee, Cle Elum, Kachess, and Keechelus, are retained by moraine dams or are in basins that were overdeepened by ice erosion.