Thursday, February 5, 2015

Prehistoric Geomorphology of Bayocean Peninsula

While bushwhacking Bayocean's highlands looking for its highest point, I wondered how it had been formed. In researching that, I was surprised to discover that it had once been much larger and was in decline by the time the Potters discovered it in 1906. If they had known that, they might have had second thoughts on building a resort there. 

William Skinner Cooper is the source of this information. On pages 84 of his 1958 Coastal Sand Dunes of Oregon and Washington, he said the sandspit had been a "parabola complex (Pl. 2, Locality 7) is represented by a single short massive ridge just north of the bay outlet and adjacent to the pre-jetty shoreline. The ridge rises 75 m [246'] above sea level." Cooper sketched dotted lines on a 1939 Army Corps of Engineers topographic map inserted between pages 78 and 79 that show the dune passing over Barview and ending in the foothills behind it. He noted the obvious: the bay entrance would have been at the south end then. 

In an email exchange on February 17, 2015, Jonathan Allan (see Changes In Bayocean Beaches Studied By DOGAMI)  saw evidence of a prehistoric south inlet in Bayocean Road being "backed by marine cliffs, which were likely cut by the ocean." In 1955, Port of Tillamook Bay commissioner Margaret Coates told Howard Morgan (Richard Neuberger papers, UO Special Collections Ax078) that Tillamook Indians had a tradition of the bay's entrance having once been at the spit's south end. However, I have found no tribal histories, ethnological recordings of Tillamook legends and mythology, or any reference to such a tradition. 

Allan said I had found nothing other than Cooper's analysis because no extensive study of the spit's creation had ever been carried out. But he offered some general background and comments on the subject. At the peak of the last ice age, roughly 18,000 years ago, the sea level was "as much as 400 ft" lower than it is now. In effect, the current edge of the continental shelf was the Oregon Coast. As ice caps melted, and seas rose, sediments of the continental shelf, as well as new sand delivered by coastal rivers, were transported landward by waves and wind and moved up and down the coastline unhindered. 

When the Pacific Ocean approached the modern coastline, sands were entrained by rocky headlands in littoral cells. Bayocean is part of the Rockaway cell, bounded by Cape Meares and Neahkahnie Mountain. The Oregon Dunes National Recreation Area is part of a much larger littoral cell, fed by sands from a much larger river - the Umpqua. Those, of course, are not sandspits. Despite its shrinkage,  Bayocean remains the largest sandspit on the Oregon Coast, and one of the largest in the world.

Lloyd Ruff, a civilian geologist who wrote Preliminary Notes on the Geology of Bayocean Peninsula for the Corps of Engineers during its first study of erosion in 1939, said the five rivers that now drain into Tillamook Bay would have once merged into a single river before entering the ocean at the edge of the continental shelf. When the spit was at its largest extent and the ocean at its maximum submergence, the bay reached farther up each river than today. Allan agreed, adding that Oregon's climate was warmer and more humid then, resulting in greater river flows. 

Plate 2, # 1 and #2, Preliminary Notes on the Geography of Bayocean Peninsula. Explanations on pages 3-4. 

Where bay and ocean met, the movement of water of course slowed, and sediment from both dropped out of suspension to form a bar, much like at the mouth of the Columbia River. Allan said that, typically, discrete barrier islands emerge first and that later join to become a consolidated sand spit. He agreed with Ruff's conclusion that boulders falling from Cape Meares, pushed north by powerful winter storms, gave Tillamook Spit its original foundation.

Plate 2, # 6, Preliminary Notes on the Geography of Bayocean Peninsula
Geological studies indicate that the thawing of the ice caps was uneven. As the ocean receded a bit, sands of Bayocean spit would be exposed. Tectonics plate movement, slowly forcing the Oregon coastal area upwards, could have also factored into exposing the spit. Sands from both the ocean and rivers and/or bay would have continued adding to it at high water and during storms. Winds are predominantly from the west, so they would have blown the sand farther and higher inland, with the east slopes steeper than the west, as seen on topographic maps of Bayocean. 

Allan noted, "during the past 8,000 years there have been at least 19 great earthquakes (magnitude 9 or greater) on the Cascadia subduction zone and their associated tsunamis, all of which would have significantly influenced the evolution of the coast."  The 315th anniversary of the last great earthquake and tsunami was celebrated by the Cape Meares Community Association on January 26, 2015. 

This page was updated on February 19, 2022. Other posts about Bayocean's geology can be found on the Index page.