Geologic Observations and Interpretations Along Segments of the Coast
Immediately north of the mouth of Duck Creek, siltstone beds of the Quinault Formation crop out again and are continuously exposed in the cliffs nearly to Camp Creek (fig. 29). These strata all dip generally to the southeast some 35° and represent a thickness of over 1,000 feet of marine deposition that took place during the Pliocene Epoch, some 1.5 to 7 million years ago. This interval of sedimentary beds constitutes an upper part of a nearly 2-mile-long, almost continuously exposed section of the Quinault Formation, a total thickness that is estimated to be over 4,500 feet. With the exception of a covered interval at the mouth of Camp Creek, the formation is continually exposed along the coast to a point about three-quarters of a mile north of Pratt Cliff (fig. 45).
North of Camp Creek and extending to the south end of Pratt Cliff, siltstone beds are steeply dipping in a continuous sequence that measures more than 500 feet in thickness. These strata are similar to those exposed immediately south of Duck Creek. Most are massive to faintly bedded. Fossils are generally common and scattered throughout. In many places individual bedding has been disrupted by organisms as is apparent south of Duck Creek.
For a distance of nearly one-half mile the beach is inaccessible in the Pratt Cliff area (fig. 36). Even during low tide the surf pounds directly on these cliffs. Therefore, it is practically impossible to inspect these rocks closely. However, offshore observations from the air have verified that here too the rocks are largely siltstone and crop out to the top of the cliffs, some 100 feet in height. Furthermore, the strata uniformly dip rather steeply to the southeast in a similar direction as those exposed immediately to the south. Calculations indicate that some 1,400 feet of additional thickness of the Quinault Formation is represented in those tilted beds exposed in the Pratt Cliff area.
UPLIFTED WAVE-CUT TERRACE
From a distance, the Pratt Cliff area appears nearly flat. This surface is a remnant of an old wave-cut sea level surface formed during the Pleistocene Epoch at a time when sea level stood higher relatively than it does today. It may well be the same wave-cut surface that is visible in many other places along the coast, varying in height from nearly sea level at Browns Point to over 150 feet above present-day sea level in the Cape Elizabeth area. In most places, such as between Cape Elizabeth and Point Grenville, this surface is buried beneath sand and gravel deposits and only its trace can be seen in the cliffs at the contact of the older formations with the overlying sand and gravel. However, at Pratt Cliff, very little, if any, sand and gravel cover remains; and therefore the present-day surface is essentially the old Pleistocene wave-cut surface that was formed over 70,000 years ago. Variations in its height above the present-day sea level is thought to be due to slight warping of the earth's crust since the wave-cut surface was formed.
North of the inaccessible part of Pratt Cliff, the Quinault Formation is exposed in the sea cliffs for a distance of about three-quarters of a mile (fig. 45). This section of beach can be approached only from the north because of impassable Pratt Cliff and then only during low tide because of several headlands. In this interval the Quinault Formation also dips generally in a southeast direction. A thickness of some 900 feet of the formation is exposed in these cliffs and in tidal areas. Here, many of the beds are sandstone, varying from fine grained and thin bedded to medium grained and thick bedded. Sandstone is particularly prevalent in the middle and upper (southern) part of the section, whereas the lower or northern part is largely siltstone. Fossils are scattered throughout this sequence of beds and are particularly common in the lower or siltstone part.
Concretions or resistant nodules and irregular resistant layers are common in the upper or southern part of this sequence. A greater amount of calcium carbonate or limy material cements these structures and, therefore, they are more resistant to erosion than the surrounding sandstone.
Numerous pear-shaped resistant nodules can be seen in the sandstone beds of the central part of this traverse (fig. 19). These peculiar structures range from 4 to 8 inches in length and are always oriented with their long axis perpendicular to the bedding plane. All appear to have a central core of slightly harder and different colored material. They may have been cavities originally formed in the sand before consolidation, and perhaps were made by the burrowing of marine organisms. Subsequently, these cavities were filled with additional sediment before the formation became lithified. The greater resistance to weathering of these nodules is due to a cementing material left behind, possibly by the organism that formed the hole. This material has since penetrated outward to serve as a cementing agent in the surrounding areas.
Many of the individual beds, particularly in the siltstone part near the northern end of the outcrop area, as in the Duck Creek area, are disrupted or appear to have been churned by burrowing marine organisms (fig. 20).
The base of this major sequence of the Quinault Formation is well exposed in a low-lying cliff about three-quarters of a mile north of Pratt Cliff (fig. 18). Here the Quinault Formation is in contact with dark-gray colored chaotically mixed Hoh rocks. The actual contact is generally delineated by color contrast between the two rock units.
Last Updated: 28-Mar-2006