The Pacific rim of the United States is the geologically active edge of the North American continental plate. It is and has been in a constant state of colliding and overriding the sea floor of the Juan de Fuca oceanic crustal plate. The Sanctuary is subject to tectonic forces caused by the combined movements of the large Pacific and North America Plates and the smaller Juan de Fuca Plate. The altered sedimentary rocks of the Olympic Mountains and the volcanoes of the Cascade Range (Mount Saint Helens, for example) are the result of the convergence of these plates composed of oceanic and continental crusts.

The coastal rim of the continental plate is marked by earthquakes associated with geological faulting and volcanism (McGregor and Offield, 1986). A continental shelf reaches out from Washington's coast from eight to forty miles (Washington State Dept. of Ecology, 1986). This shelf provides a relatively shallow coastal environment within the Sanctuary. Submarine canyons cut into the continental shelf and slope along the entire coastline of the sanctuary.

In the northern portion of the Sanctuary, the underwater continental shelf slope is steep with and jagged. The underlying sediments are largely glacial deposits. Modern sediments are brought in along the Strait of Juan de Fuca. The Columbia River is the dominant source of modern sediments for the southern Washington Shelf (Nittrouer, 1978 in Baker and Hickey, 1986). Year-round bottom currents and winter storms transport much of this sediment north to northwest. The sediment accumulates on the shelf as a band of sandy silt with the inner shelf sandy and the outer shelf comprised primarily of silt and clay (Carson, et al., 1986). Much of this sediment is transported to and deposited in the Quinault Canyon where it gradually works downhill into the Cascadia Basin (Cutshell, et al., 1986).

Overlying the bedrock along many areas of the coast are deposits of sand and gravel laid down by glacial streams during extensive glaciation of the Olympic Mountains during the Pleistocene Epoch some 17,000 to 70,000 years ago (Rau, 1973). Prominent gravel pockets lie off Cape Flattery, Grays Harbor, and the mouth of the Quinault River (Moore and Luken, 1979).

The uplifted broad coastal plain that forms the coast of Washington extends from Cape Flattery southward and includes two tidal inlets, Willapa Bay and Grays Harbor (Weissenborn and Snavely, 1968). Broad beaches, dunes, and ridges dominate the coastline from Cape Disappointment on the north side of the Columbia River mouth, to the Hoh River (Moore and Luken, 1979). The plain rises eastward and merges with the foothills of the Olympic Mountains. Wave action has eroded the plain through time and formed steep cliffs along the coast, except at river mouths.

For most of the coast between Cape Flattery and Point Grenville these cliffs rise abruptly 50 to 300 feet above a wave-cut platform. This wave-cut platform, which normally extends about half a mile from shore, is nearly two miles wide west of Ozette Lake. Small islands, sea stacks, and rocks dot the platform's surface.

Islands can be found in all stages of development from partially isolated promontories to true islands several acres in extent (op. cit.). The largest, Destruction Island, is 1.5 km long.