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

JEFFERSON COVE

The bluffs of Jefferson Cove are a jumbled assortment of sandstone, conglomerate, and siltstone blocks intermixed in a matrix of smaller broken siltstone and claystone fragments. These rocks are similar to the landslide materials exposed near the mouth of the Hoh River. They constitute an excellent example of a TECTONIC MELANGE, a term now applied to many rock outcrops of Hoh rocks, both along the coast and inland. As the term implies, this jumbled array in both size and type of rock material forms a mixture or melange of rocks. Tectonic implies crustal deformation—thus, a mixture of rock material resulting from major crustal movements (see Tectonic Melange Rocks, Part I).

The exact nature of the forces or crustal movement that forms rock melanges is not fully known. However, geologists theorize that outcrops, such as those at Jefferson Cove, represent a part of a major fault zone between large segments of the earth's crust. The broken rock materials are believed to be debris developed from movement along these faults. Melange rock zones are structurally weak zones within the earth's crust. In some places these materials respond to pressures somewhat like plastic and are squeezed upward. The resulting structure of such movement of rock material is referred to as a PIERCEMENT STRUCTURE or DIAPIR (fig. 10). The rock melange of the Jefferson Cove bluffs may have undergone such migration.

On the earth's surface, melange rocks are more easily eroded than sedimentary rock sequences. Thus, along the coast outcrop areas of melange rocks are usually found in coves, such as Jefferson Cove, where the sea has eroded the coastline farther eastward than the headlands. Furthermore, because melange rock are structurally weak, numerous landslides develop where they are exposed.

HOH HEAD AREA

The headland between Jefferson Cove and Boulder Bay is a massive to thick-bedded sandstone and conglomerate sequence and is essentially the same rock type as the headland area to the south of Jefferson Cove. The structural trend or STRIKE is similar to the regional northwest strike with steep dips to the northeast. Local variations to this regional trend are common. For example, well-stratified sandstone at the northernmost edge of Jefferson Cove strikes in an east-west direction. This local deviation in strike is believed to have been established when the adjacent melange materials of Jefferson Cove were squeezed upward, thus dragging the resistant sandstone beds parallel to the contact between the two rock types.

At Boulder Bay (fig. 30) on the south side of Hoh Head (fig. 31), melange rocks again are exposed and form a low-lying, sharp saddle-like area between Boulder Bay and Secret Cove to the north. Because these melange rocks are easily eroded, the relatively resistant thick-bedded to massive sandstone of the main body of Hoh Head will eventually become an offshore rock. Hoh Head, although similar in composition to the sandstones of the other headlands of the area, structurally does not conform to the regional northwest trend, but instead strikes in an east-west direction. Strata seen on the south side of Hoh Head are steeply dipping to the north, whereas on the north side they dip steeply south. Thus, sandstone beds of Hoh Head have been folded into a small SYNCLINE or downwarp. Evidence of this fold can be seen from off Hoh Head where the actual end view of this folded structure can be visually traced in stratified beds exposed on the extreme western end of the head.

BOULDER BAY on the south side of Hoh Head is well named. Erosion of the massive sandstone of Hoh Head has produced this boulder and cobble strewn beach (fig. 30).

HOH HEAD and coastal area to the north (fig. 31).

SECRET COVE TO MOSQUITO CREEK

The melange rocks of Secret Cove exposed in the cliffs of the easternmost part of the cove are more extensive than at Boulder Bay. Nearby offshore stacks are also a part of this melange zone representing large resistant blocks of sandstone and conglomerate.

The contact of melange rocks with the sandstone of Hoh Head to the west is well exposed just above the high-tide level at a point a few hundred yards westward from the cove (fig. 32). From here westward, thick, well-bedded sandstone strata of Hoh Head form an extensive low-tide reef trending in a northwest direction. Westward from this outcrop, the bedding trend swings to a nearly east-west direction forming the north limb of the Hoh Head syncline.

MELANGE ROCKS OF A MAJOR FAULT ZONE can be seen in contact with Hoh Head sandstone (right) in the Secret Cove area (fig. 32).

Three small headlands lie immediately north of Secret Cove, consisting largely of massive sandstone but well-bedded in places with relatively thin siltstone strata. These headlands are a part of the regional anticline. Evidence from sedimentary features preserved in these rocks indicates that they constitute the west overturned limb of the anticline (fig. 25). Thus, a major geologic structure can be followed generally northward along the coast from the mouth of the Hoh River to these headlands.

Less resistant melange rocks form the bluffs of each small cove between the three headlands as well as the cliffs for about a mile northward beyond these small headlands. Rock strata are completely disarranged and numerous blocks of sandstone are intermixed with a matrix of broken siltstone and claystone. Resistant blocks of sandstone, not yet worn to sea level by wave action, form many offshore rocks and low-tide reefs. Landslides are particularly prevalent and are made apparent by "drunken forests" and debris flows that extend onto the beach.

At a point about half way along this area of melange outcrop, erosion has exposed a particularly large sandstone block (fig. 33). This block will become a small headland, and then eventually an offshore rock as the less resistant part of the cliff recedes eastward due to coastal erosion. Close inspection will reveal that the top of the sandstone block is nearly flat and that this surface is covered with 5 or 6 feet of buff-colored sediments. The flat surface is a remnant of an ancient terrace that was carved by wave action sometime in the past when sea level stood higher than today. Evidence of this elevated sea-level surface can be seen in many places along the coast. Its height above sea-level varies somewhat from place to place because the earth's crust has warped slightly since Pleistocene time. The buff-colored sediments resting on this surface are unconsolidated sand and silt and are believed to be eolian (windblown) deposits. These sediments mark the last major geologic event of the coastal region, having been deposited some 8,000 years ago, some time after the uplift of the coastal area and the retreat of glacial ice from nearby valleys (see Deposits and Processes of the Ice Age, Part I).

EXCEPTIONALLY LARGE SANDSTONE BLOCK in a Hoh melange outcrop. Its flat top is an erosional remnant of an elevated wave-cut platform (fig. 33).

All rocks exposed in the bluffs, headlands, and in low-tide outcrops for the last mile or so northward to Mosquito Creek are a part of a very extensive zone of rock melange. The several small headlands in this area are especially large blocks of massive resistant sandstone; the less competent, badly sheared and broken sandstone and siltstone materials of the melange are exposed in the low-angled bluffs in the small coves that lie between. Broken sandstone constitutes a much greater part of the melange of this area than of that to the south, such as is exposed in Jefferson Cove.

MOSQUITO CREEK TO TOLEAK POINT

For 2 miles northward from Mosquito Creek, the coast can be easily traversed over wide, firm, sandy beaches during low tide; otherwise, during high tide the area is passable over loose sand, gravel and driftwood. The trail leaves the beach about a quarter of a mile east of Goodman Creek where a small sandstone and conglomerate point blocks further passage along the beach during medium and high tides. Furthermore, although during low tide Goodman Creek can be reached from along the beach, it cannot be crossed at its mouth as it flows into the ocean in a deep rock gorge (fig. 34). In addition, the coastline northward from Goodman Creek for about half a mile is mostly a sheer cliff extending into the sea with practically no beach exposed at any time (fig. 35).

GOODMAN CREEK enters the sea through a narrow channel carved in Hoh bedrock. This course was established in overlying Pleistocene deposits. During the slow downward cutting process of the stream, its course was maintained even into the underlying bedrock (fig. 34).

CONGLOMERATE, north of Goodman Creek, forms inaccessible cliffs nearly 200 feet in height (fig. 35).

The trail ascends from the beach over slumped melange rocks into salal brush, and then into timber for about a third of a mile where it descends to Goodman Creek. The stream level at this point, about half a mile upstream from its mouth, is still affected by the tides. Therefore, crossing during low tide would avoid wading in fairly deep water. The trail is marked on the north side of the gravel bar and from there it follows along the north side of the stream to Falls Creek. This stream receives its name from the picturesque falls located a few hundred feet upstream from its mouth where the water cascades some 50 feet over massive conglomerate rock.

From the mouth of Falls Creek during low tide, Goodman Creek can be easily traversed downstream practically to its mouth. However, the tide level should be watched carefully to avoid having to return in deep water. With luck, sea otter may be seen near the mouth of the stream as they were recently reintroduced to this area. Because the coast is impassable north of the mouth of Goodman Creek, it is necessary to return to Falls Creek to continue northward. The trail northward to the coast crosses Falls Creek near its mouth and continues steeply out of the valley, past several very large cedar trees, and onto a wooded ridge. The trail follows along this ridge for a quarter of a mile or so where it passes several excellent vantage points high above the beach. The descent to the beach is steep in places but should not pose any problem as ropes and steps have been placed in the steepest parts. Once back to sea level about a mile of open beach lies ahead to Toleak Point.

For the last 4 miles of its course to the coast, Goodman Creek flows in a relatively broad valley where it has shifted in places from time to time. A side trip up this valley, along its wandering stream and over the grass-covered valley floor, is a pleasant change from the sandy beaches of the coast. The going is fairly good. A trail may be followed most of the way. The nearest logging road access is about 3 miles upstream from the coast.