The depression includes the lowland regions of southwestern British Columbia and northwestern Washington along the shores of the Salish Sea.

During this period, the depression manifested as a broad valley with various rivers flowing down into it to reach the Pacific Ocean where the mouth of the Strait of Juan de Fuca lies.

[1][2] Much of the current topography was formed through the erosion of the depression by the Cordilleran Ice Sheet during the Vashon Glaciation, which lasted from about 19,000 – 16,000 BP.

The landscape features glacially striated tablelands and rolling hills underlain by sedimentary rocks.

[6] Human activity has greatly altered much of the natural environment here through industrialization, agriculture, forestry, urbanization, and suburban sprawl.