Washington Department of Natural Resources Geology and Earth Resources Division Bulletin No. 72 Washington Coastal Geology between the Hoh and Quillayute Rivers

DEPOSITS AND PROCESSES OF THE ICE AGE (PLEISTOCENE EPOCH)

Thick, widespread deposits of unconsolidated sand and gravel blanket much of the Hoh bedrock of inland areas, particularly in and adjacent to the major stream valleys of the Hoh, Quillayute, and Bogachiel Rivers and Goodman Creek. Along the immediate coastal area, such deposits are thinner, less extensive, and are confined largely to areas near the mouth of the Hoh River, Goodman Creek, and the Quillayute River. These deposits are thickest and may best be seen along the coast overlying bedrock sea cliffs and sea stacks between the Quillayute Valley and Taylor Point (fig. 12). Sand and gravel deposits are also present at the mouth of Goodman Creek and, although not very apparent, are also present near the mouth of the Hoh River. These unconsolidated deposits record the activities of the PLEISTOCENE EPOCH, generally known as the ice age. Rock debris carved during extensive glaciation of the Olympic Mountains in the past have been transported to the present-day coastal area largely by melt water from glaciers. Some of these rock materials in nearby inland areas are believed to have been deposited directly by ice. These deposits indicate that at one time glaciers stood very near, and perhaps even beyond, the present-day coastline.

IDEALIZED CROSS SECTION showing relations between major Pleistocene deposits and the land surfaces that were formed by periods of either erosion or deposition during the Pleistocene Epoch. Older deposits were thick and extended many miles seaward. Erosion by wave action followed, removing much of these materials from the present-day Continental Shelf. A second and thinner series of sand and gravel was brought down major stream valleys and deposited on the now uplifted wave-cut platform. These younger deposits also extended seaward, but less so than the other deposits. They have been eroded together with bedrock by wave action back to the present-day coastline, leaving Alexander Island as a remnant of the mainland that existed farther west a few thousand years ago. Following the deposition of the younger sand and gravel deposits and probably before much vegetation had developed, windblown silt and sand covered much of the coastal area to a depth of as much as 15 feet (fig. 11). (click on image for an enlargement in a new window)

UNCONSOLIDATED PLEISTOCENE SEDIMENTS rest on the bedrock of Hoh rocks at Quateata. The boundary between the two is the trace of an ancient, now-elevated wave-cut platform (fig. 12).

The Pleistocene Epoch is confined to approximately the last 1-1/2 million years of geologic time (fig. 1). During the Pleistocene, a relatively short period geologically, the land surface was sculptured into its present-day form. In the Olympic coastal area, glacial ice played an important role in forming the surface of the land, either by direct ice erosion or by deposition of rock debris.

Glaciers advanced and retreated several times to carve the landscape and transport rock debris. Two major events of deposition from glaciers are apparent in the rock record along the coast (fig. 11). Evidence for a major period of erosion by the sea divides these two major deposits. This erosional surface is apparent in many places along the coast. It is manifested by the essentially horizontal trace of an elevated wave-cut terrace at the top of many bedrock outcrops in the sea cliffs (frontispiece). Although usually buried today by younger deposits of sand and gravel, the erosional surface extends inland various distances up to a mile, indicating that the coastline was, at one time during the Pleistocene Epoch, generally farther inland than it is today. Bare, flat-top rocks and sea stacks standing at an approximate elevation of 100 feet are remnants of the old erosional surface (fig. 13). This surface represents an ancient stand of sea level when a nearly flat surface was carved by the sea, in the same manner that the sea is carving a somewhat horizontal surface at the lower present day sea level (fig. 14). In other areas along the coast the ancient erosional surface was carved on older Pleistocene deposits rather than Hoh bedrock. These deposits constitute the older of the two major Pleistocene periods of deposition. The older deposits of sand and gravel, laid down prior to the forming of the elevated wave-cut surface, are thick and widespread in adjacent inland areas, particularly along the valleys of the major drainages (fig. 11). However, in coastal areas they were largely removed by wave action of the Pleistocene sea. The younger deposits are the sand and gravels that rest on top of the elevated wave-cut surface. They are thinner and less extensive than the older deposits (fig. 12).

THE FLAT TOP of several bare rock islets in the "Quillayute Needles Group" represents an old elevated sea-level surface (fig. 13).

MODERN WAVE-CUT TERRACE forming south of Jefferson Cove. (fig. 14).

A third and the youngest of all Pleistocene deposits can be seen as a 3- to 15-foot thickness of light, buff-colored silt and sand at the uppermost part of nearly all sea cliffs and on many sea stacks along the coast (fig. 16). They are thought to be WINDBLOWN (EOLIAN) deposits that were laid down during a low stand of sea level at the close of the Pleistocene Epoch. During this time a wide expanse of coastal plain lay bare of vegetation and served as a source area of sediments for transport by the prevailing westerly winds (Rau, 1979). Although these light-colored deposits are very noticeable at the top of most cliffs, a closeup view is accessible along the trail at the top of the descent to Third Beach from Taylor Point (fig. 15).

A CLOSEUP VIEW OF WINDBLOWN DEPOSITS on top of Taylor Point (fig. 15).

WINDBLOW DEPOSITS (loess) form the uppermost buff-colored layer on many coastal cliffs as shown here on the north side of Taylor Point (fig. 16).

AGE DATING OF ICE AGE EVENTS

The general sequence of events of erosion and deposition are documented in the rock record; but the actual time when they occurred is not as well established. Based on carbon-14 isotope dating methods, dates can now be calculated to nearly 75,000 years B.P. (before present). On this basis, the oldest date that has been established in coastal Pleistocene deposits is approximately 71,000 years B.P. (Stuiver and others, 1978), a date essentially at the limit of the carbon-14 dating capability. This date was established for materials from 25 feet above the wave-cut terrace level. The actual times when all materials below this level were deposited, as well as when the wave-cut terrace was carved, are not known. It is speculated, however, that the elevated wave-cut terrace may have been formed some 125,000 years ago (Heusser, 1977). Therefore, the maximum age of the extensive Pleistocene deposit that pre-dates this erosional event is not known other than that they were deposited at some earlier date, probably during the 1-1/2 million years of Pleistocene time.

Dates younger than 71,000 years ago have been established for younger Pleistocene coastal and nearby inland deposits. Some of these dates are approximately 59,000, 30,000, and 16,000 years B.P. (Heusser, 1972, 1978). The youngest date based on pollen correlation is about 8,000 years B.P. (Florer, 1978, personal communication). The age was estimated for materials from the uppermost windblown deposits exposed near La Push.