Geologic Observations and Interpretations Along Segments of the Coast
Destruction Island, about 3-1/2 miles offshore, is the largest island off the coast of Oregon and Washington and the first one north of the Farallon Islands of the San Francisco area. Its distinctive flat-top shape served as an unmistakable reference to the early coastal explorers. It was first named "Isla de Dolores" by Bodega y Cuadra. Others referred to it as "Green Island," because of its dense cover of green foliage. Later Meares transferred the name of "Destruction" to the island from the river, so named by Barkley, the presently named Hoh River. Aside from its fascinating historical background. Destruction Island is also one of the more interesting places geologically along the Washington coast. It represents the western most major bedrock outcrop exposed above sea level in this local area.
The foundation or bedrock of the island and surrounding low-tide reefs is very similar to the bedrock exposed in the Browns Point-Starfish Point area. Most strata are massive to thick-bedded graywacke sandstone and, like the Browns Point-Starfish Point strata, represent typical turbidite deposition of the Hoh rock assemblage (fig. 54).
Structurally the beds are dipping steeply to nearly vertical and in places are even slightly overturned. Because the less resistant thin siltstone layers have been more deeply eroded, the thick more resistant sandstone beds stand out prominently and can be easily traced along the island and to the nearby reefs. From the air, these beds can be seen to form a horseshoe-shaped pattern. Large and well-formed sedimentary structures (fig. 61) indicate that the original upper surfaces of all strata face toward the center of this pattern. Therefore, these beds were not only downfolded to form a "syncline" but also rotated up on end so that today a beveled end of this nearly vertically plunging structure forms the bedrock foundation of the island.
SAND AND GRAVEL DEPOSITS
In much the same way as at Browns Point and Starfish Point, nearly horizontal, relatively unconsolidated sand and gravel deposits overlie the hard sandstone Hoh beds and form the upper 50 feet or so of Destruction Island. The contact between the two units forms a generally horizontal plane only a few feet above the present-day sea level. This surface can be discerned slightly above the high-tide level from the mainland, particularly from the Kalaloch area where it may be seen prominently on the southwest end of the island. It represents the same wave-cut surface visible in the Browns Point-Starfish Point area just a few feet above the high-tide level. As in the latter area, this surface was carved by the sea during the Pleistocene Epoch prior to the deposition of the overlying sand and gravel. The flat-top surface so prominent on Destruction Island is a westernmost remnant of the old piedmont surface that was essentially completed some 17,000 years ago during the final stages of outwash deposition from a late period of glaciation. On the mainland, this surface constitutes the present-day land surface adjacent to much of the coast of this area. From places along the coast the upper surface of Destruction Island can be visually projected to a similar relatively level surface on parts of the mainland. This projected line slopes very gently seaward and, somewhere to the west, intersects present-day sea level. Wherever the sea level stood at the close of the Pleistocene Epoch (possibly even lower than today), the sea has eroded the land from there eastward to our present-day coastline. Destruction Island, having a particularly resistant foundation, has not yet given way to the forces of the sea.
COASTAL EROSION RATE
The rate at which the coastline migrates eastward due to erosion, leaving Destruction Island behind, can be estimated from a comparison of old land surveys with those made in recent years. In the Hogsbacks area about 15 miles to the south, records indicate an eastward recession of the cliffs of some 225 feet in a 60-year period (see Part II, Hogsbacks area; Coastal erosion). This is equal to a rate of 375 feet every 100 years. Although the rocks of that area are particularly susceptible to erosion, nevertheless the unconsolidated sands and gravels of Destruction Island and the adjacent mainland area also are easily cut by the sea. Except for local resistant headlands and islands, such as Browns Point and Destruction Island, the gross configuration of the shoreline is generally straight. Therefore, erosion must have been reasonably uniform along most of the coastline of this area. Assuming a conservative rate of 300-feet eastward recession of the cliffs every 100 years, Destruction Island, now about 3-1/2 miles offshore, would have been a part of the mainland some 6,000 years ago.
Last Updated: 28-Mar-2006