Washington Department of Natural Resources Geology and Earth Resources Division Bulletin No. 66 Geology of the Washington Coast between Point Grenville and the Hoh River Weldon W. Rau

THE HOGSBACKS AREA

JUMBLED HOH ROCKS

One of the largest continuous outcrops of jumbled Hoh rocks to be seen along the Washington coast is exposed from the contact of the Quinault Formation, about three-quarters of a mile north of Pratt Cliff to within about one-half of a mile south of Raft River, a distance of some 2-1/2 miles (cover photo and fig. 45). This outcrop is a classic example of the extreme effects of crustal deformation; the results are referred to as a "tectonic melange" (see Part I, Tectonic melange of Hoh rock assemblage). Here, much like the outcrop area of Hoh rocks exposed south of Duck Creek, the materials forming the cliff are a chaotic mixture of completely broken siltstone in which are set a variety of more resistant blocks (fig. 4). Such intensely jumbled rocks are generally believed to have been deformed into their present chaotic condition by major forces generated by the continued movement of large segments of the earth's crust. These forces have caused much folding, faulting, and jumbling, possibly, in part at least, by mass slumping of large segments of the sediments on the sea floor. These forces have continued periodically since the rock materials were deposited, and although not as active today as in the past, movement that is not always perceptible still goes on. Earthquakes locally felt on occasion are the result of sudden movements of the earth's crust, and serve to remind us that the earth's crust is indeed active.

Paleontologists have determined from microscopic fossils found in siltstones of these deposits that some of the sediments were deposited in a deep marine basin during a middle to early part of the Miocene Epoch, some 15 to 22 million years ago. Because this formation is so intensely broken and many of the clay minerals it contains tend to swell when they are moistened, it is particularly vulnerable to erosion by ocean waves during high tides and by winter storms. Much precipitation in this area further contributes to the erosion process.

Although resistant blocks are scattered throughout this 2-1/2 miles of outcrop, a concentration of particularly large boulders have been eroded out of the melange and now rests on the beach at Boulder Point (fig. 10). A variety of rock types are present among these boulders, representing a number of different environments of origin. Many are altered sandstone and other sedimentary rocks and a few are volcanic in origin.

Willoughby Rock (fig. 34) and Split Rock (fig. 48), the two small islands a mile or so offshore from this area, are also very large resistant blocks that were once in the jumbled deposits of Hoh rocks. As the coastline receded slowly eastward, they were left behind, protruding above the general surface of the Continental Shelf, as well as present-day sea level.

LITTLE HOGSBACK

Northward to Little Hogsback, jumbled Hoh rocks are continuously exposed in the cliffs. At a point about one-quarter of a mile south of Little Hogsback, bedded sandstone and siltstone crop out that are more nearly intact than the deposits to the south, but nevertheless they are tightly folded and extremely contorted.

Like Willoughby Rock and Split Rock, Little Hogsback (fig. 49) is a particularly large block of relatively hard material that has been differentially eroded from the softer, less resistant clays and siltstones of the "melange" deposits. It, too, will soon be an isolated island. The rock of Little Hogsback is volcanic in origin, quite unlike the marine sedimentary rocks in which it was incorporated. A contact between well-bedded sedimentary rocks and volcanic rocks can be seen on the southeast side of Little Hogsback.

Immediately north of Little Hogsback, steeply dipping well-stratified sandstone, siltstone, and conglomerate are exposed on the beach and in the cliff (fig. 46).

WELL-STRATIFIED SANDSTONE AND SILTSTONE beds dip steeply on the beach immediately north of Hogsback. Sedimentary structures, which were formed in these rocks when they were first deposited in a nearly horizontal position as sediments millions of years ago, indicate that the top of this sequence now faces east (right). Therefore, because they now dip steeply west (to the left), they have been rotated beyond vertical to a slightly overturned position. (Fig. 46)

Although very little unconsolidated sand and gravel overlies Hoh rocks immediately south of Little Hogsback, northward to the Raft River valley these deposits thicken to as much as 25 feet.

HOGSBACK

Jumbled Hoh rocks extend northward to the Hogsback, forming most of the cliffs of that area. A particularly large accumulation of resistant blocks eroded from the Hoh melange rests on the beach a short distance south of the Hogsback. These are also volcanic in origin and are largely breccia or angular fragments of volcanic materials that have been welded together.

Hogsback (fig. 5) also is welded brecciated volcanic fragments but, in addition, it contains some fragments of siltstone and other sedimentary rock types. A steeply dipping fault surface can be seen on the south side between the softer parts of the formation and the volcanic rock of Hogsback.

Mineralized Veins

Large veins of calcite are present in the volcanic rocks of Hogsback. In most cases, the mineral has crystallized in a form known as "nailhead spar." In addition, very small quartz crystals have formed on the surfaces of many of the calcite crystals. These quartz crystals can be identified by their six-sided elongated shape. Formation of these mineralized veins took place sometime after the volcanic "host" rock was fractured. Then, by the slow precipitation of calcium carbonate, these fractures were filled. In areas where well-formed crystals remain, the fractures never became completely filled with calcite. Apparently the last ground water to percolate through the crystal-coated cavities was rich in silica, some of which precipitated out of solution adding the coating of small quartz crystals.

Jumbled Hoh rocks extend for about three-quarters of a mile northward from Hogsback, forming the lower part of the cliffs and tidal outcrops. Large sandstone blocks are common immediately north of Hogsback, and another large block of volcanic rock forms a stacklike feature a short distance beyond. Cliff outcrops of Hoh rocks end at the south edge of the Raft River valley, about one-quarter of a mile south of Tunnel Island.

COASTAL EROSION

Natural processes of erosion have been active on this shoreline for many thousands of years and most likely will continue to be so for some time to come. Sea cliffs of jumbled Hoh rocks, like those in the Hogsbacks area, are particularly susceptible to wave action of the sea. Just how much and at what rate land is being eroded can be calculated by comparing original land surveys of 1902 with those made in recent years. A few hundred feet north of Hogsback (fig. 45), an east-west line between an established corner to the east and the edge of the cliff to the west measured 2,053 feet in length in 1902. That same line was surveyed in 1962 and found to be only 1,828 feet in length, a loss of 225 feet in 60 years. At that rate, the cliff recedes eastward about 375 feet in 100 years. Such a general rate of erosion is further indicated by the actual loss in acreage along the coast during the same period of time. Surveys show that one particular government lot contained 30.3 acres in 1902, whereas only 17.3 acres were left in 1962. Another adjacent lot was reduced from 33 acres to just a little over 20 acres. Based on the same rate of coastal erosion, it is therefore possible to estimate that a little over 200 years ago the coastline was probably about even with the western edge of Hogsback; and some 1,200 years ago the present-day offshore Willoughby Rock may have been part of the mainland.

RED BEACH SANDS

Although isolated patches of red-colored beach sands can be seen in many places along the coast, they are probably most prevalent along the 2-1/2-mile-long Hogsbacks area (fig. 23). "Ruby Beach" to the north no doubt was so named because red sands are also common in that area. This reddish color is caused by the concentration of small "almandite" crystals, a type of red garnet, which is an entirely different mineral than the precious stone known as ruby. The garnet sand grains were originally individual dodecahedron (12-sided) crystals, but most of them have been abraded to a nearly spherical shape. This mineral is relatively heavy and tends to become concentrated in patches by wave action. Several other heavy minerals are frequently found in association with garnet sand. Most common is magnetite, a black-colored mineral that is readily attracted to a magnet. Other associated dark-colored minerals are ilmenite and hematite. In addition, grains of zircon, a heavy and very hard non-magnetic mineral, are found in substantial amounts with the garnet. Zircon grains are very small, well-formed, clear, glassy crystals.

For many years it was thought that the only source of these garnet-bearing sands was sand and gravel deposits brought south from Canada by continental ice during the Pleistocene Epoch. However, many of the better concentrations of garnet along the coast are confined to areas where continental deposits have never been found, such as the Point Grenville-Hoh River area. Nevertheless, local glaciers originating in the Olympic Mountains did bring large amounts of sand and gravel to this area. These deposits are primarily sand and sandstone (graywacke) pebbles, much of which were removed from bedrock outcrops of Hoh rocks and other rocks of the Olympic Mountains. Recent studies have shown that these rocks do indeed contain garnets and are therefore now thought to be a major source for garnet and associated minerals found in this beach area.

It is interesting to note that at one time a considerable thickness of sand and gravel covered the Hoh rocks of the Hogsback area (see Part I, Late Cenozoic deposits). Presently, however, very little remains at the top of the cliffs. Most of the sand and gravel was removed when sea level stood at a relatively higher elevation and a flat wave-cut surface was carved on the Hoh rocks, the trace of which can be seen at the top of the cliffs of this area. During that period of erosion, heavy minerals were concentrated and deposited on that wave-cut platform. Today, the sea, at a lower level, is eroding the cliffs; and the heavy mineral concentrates are being lowered to present-day beach level and reconcentrated.