Northern Cascade-Fraser River, Domain 7

Abstract

The Northern Cascade–Fraser River domain conforms to the Northern Cascade Mountains physiographic province in northwestern Washington and southern British Columbia, the San Juan Islands between the southern tip of Vancouver Island and the Northern Cascade Mountains, and much of the Interior Plateau province of British Columbia. The thread that connects these areas is the north–south Straight Creek–Fraser River fault system that runs through the Northern Cascade Mountains and northward along the Fraser River. The localities of domain 7 are along faults that branch off from this major fault system. The Northern Cascade Mountains are indeed mountainous, and the Interior Plateau of British Columbia is an area of dissected plateaus and scattered mountains. The Fraser River flows northwest in the Rocky Mountain Trench, which separates the North American craton on the northeast from accreted terranes on the southwest; then it turns around the northwest end of the Cariboo Mountains to the Interior Plateau. In the Interior Plateau, the Fraser River flows from Prince George south about 500 km to the Northern Cascade Mountains before turning westward toward the Pacific Coast. The northern part of domain 7 is in that part of the Fraser River basin, including tributaries northwest of Prince George, which is in the Interior Plateau province. Low, hilly terrain dominates the San Juan Islands. All of these areas in domain 7, except the Ingalls complex on southeast margin of the Northern Cascade Mountains, were covered by the Cordilleran ice sheet during the last stage of the Pleistocene glaciation, leaving <15 ka years for soil development on the current ground surfaces. Although alpine glaciers formed in the southeastern margin of the Northern Cascade Mountains, they did not cover all of the soils, allowing some of them longer time for development. Elevations in domain 7 range from sea level on San Juan Islands to mostly in the 600–1500 m range on the Interior Plateau of British Columbia, and up to 4392 m on Mt. Rainier in the Northern Cascade Mountains.