The Cascade Range is an active north-trending volcanic arc at the western edge of North America (Figure 1). At the northern end of the range, between 47°N and 49°N, the average elevation increases, peaks become sharper, numerous small glaciers survive on the higher slopes, and volcanic rocks of the Cascade arc are scarce. This region is the North Cascades Range. The North Cascades are bounded on the west by the fore-arc basin of the Puget Lowland, on the south by the arc volcanic rocks of the Central Cascades, and on the southeast by the back-arc flood basalts of the Columbia Plateau. The geologic identity of the range is not so clearly defined to the north, but it is geographically bounded on the northeast by the Okanogan Ranges and on the northwest by the Fraser River, which separates the Cascades from the Coast Mountains. |