During part of the late Pleistocene (at least 14,000 years B.P.), the Strait of Juan de Fuca was occupied by a lobe of the continental ice sheet that extended from the Cascade Mountains of Washington and British Columbia on the east, to the nearshore waters of the Pacific Ocean on the west.

As the front retreated, marine waters were able to re-invade the lower region, changing the type and character of sedimentation, and permitting benthic Foraminifera to migrate into the strait. The sediments of this time appear to be glacio-marine, and indicate that the fauna was able to adapt to a sedimentary environment of cold brackish waters containing vast amounts of sediment, with a few coarser sediments and cobbles supplied by berg or shelf ice. As the ice front retreated farther toward the east and north, the Strait of Juan de Fuca waters became progressively less brackish and supported a more marine fauna.

Later, almost catastrophically, the environment changed from one of primary deposition to one of non-deposition or erosion. The upper part of the section and the surface sediments today are primarily coarse sand and gravel. This upper sand layer appears to lie disconformably on the underlying glacio-marine section. Measurements of the currents in the strait suggest that it is presently an area of non-deposition for fine sediments. Whether the upper sand layer in the strait represents a lag deposit after the removal of the finer fraction, or sediments added from coastal erosion, has not been determined. There is some seismic evidence, however, that Recent sediments are being shed into the strait from both sides.

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