Thursday, February 5, 2015

Pre-historic Geomorphology of Bayocean Peninsula

The forces of nature that tore away much of the Bayocean Peninsula during the middle third of the Twentieth Century have been analyzed at length. But the explanation of how a spit of sand grows to such great size in the first place is much harder to come by.

The most recent and comprehensive analysis I found was Beaches and Dunes of the Oregon Coast, produced by the Oregon Coastal Conservation and Development Commission (OCCDC) in 1975. It turns out that when Bayocean Park was built the spit had already shrunk: it was just a remnant of a much larger dune that had once reached Barview, the community across the current bay inlet to the north. OCCDC said Bayocean reached its maximum size 3000 to 6000 years ago, after the last maximum submergence when the global ice melting had reached its peak. They cite William Cooper, who describes Bayocean, on page 84 of his 1958 analysis Coastal Sand Dunes of Oregon and Washington, as a "parabolic complex...represented by a single short massive ridge [that] rises 75 m [246'] above sea level." Since the inlet to Tillamook Bay could not then have been where it is now, Cooper suggests the south end as the most likely location.

In an email exchange, Jonathan Allan, mentioned in Changes In Bayocean Beaches Studied By DOGAMIthought Cooper's south inlet a reasonable possibility, because Bayocean Road is "backed by marine cliffs, which were likely cut by the ocean."

Allan explained that I was having a hard time finding a comprehensive study of the geomorphology of Bayocean because none had ever been done. But he offered some general background and comments on what I had read. He said that at the peak of the last ice age, some 18,000 years ago, the sea level was 400' lower than it is now. In effect, 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. As 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. When I asked, Allan answered that the dunes of the Oregon Dunes National Recreation Area are larger than  Bayocean because they are part of a much larger littoral cell, fed by sands from a much larger river - the Umpqua.

Lloyd Ruff, who wrote Preliminary Notes on the Geology of Bayocean Peninsula for the Corps of Engineers in 1939, in response to erosion on the spit, illustrated how five rivers that drain into Tillamook Bay would have then joined into one river before entering the ocean; and that at maximum submergence the bay would have reached far up into each river. Allan thought this made sense, 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

Where bay and ocean meet, the movement of water of course slows, and sediment from both drops out of suspension to form a bar, much like at the mouth of the Columbia River. Allan said that spits typically form from discrete barrier islands joining. He agreed with Huff's conclusion that boulders falling from Cape Meares, pushed north by powerful winter storms, gave Bayocean Spit a solid 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 also pointed out that "during the past 8000 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. Perhaps such an event explains the sand gaps on Bayocean, or how it shrank from 246' to 152'.