Southwest Washington, the Willapa Region: The Geologic Story:

The present unique geomorphology (land form) of the Willapa area is the result of the interaction of geologic processes over a vast amount of time. In general, the earths crustal movements (tectonic processes) and the erosional and depositional modifications by water (fluvial processes) play the major roles. Southwest Washington provides dramatic observable evidence of these geological events which are made even more interesting by their duration and magnitude.

To began with, we need to have some appreciation for the time involved with the geological evolution of our relatively young area. To aid in an understanding of the time aspect I would like to resort to a comparison of scale which equates time with distance. Thus, at the risk of over simplification the following time scale could be used here. If one year equals one inch (1 yr. = 1 in), a 72 year old person has lived 6 feet and Columbus made his arrival to the North American shore about 42 feet ago. Important to our area would be the following time/distant relationships; 100,000 years equals 8,333 feet (1.7 miles), 1.5 million years equals 125,000 feet or 23.7 miles, 6 million years equals 94.7 miles and 50 million years (our starting point for this article) equals 789 miles ago. Using the well known horse found in eastern Washington as a terrestrial index fossil, a Stratagraphic view of the sedimentary period can be shown. Now we will go back over the formation of our area.

We would need to start back in time about 50 million years to observe the geological events where portions of our present day land forms had their origin. This was called the Eocene Epoch which was part of the Tertiary period. At this time, around 50 million years ago, the landscape would hold no resemblance to the present. Instead of standing in the temperate forest of the Willapa Hills, far above sea level you are, if lucky, on a low tropical volcanic island surrounded by a warm shallow sea. A marine embayment during the Eocene time with an areal extent ranging from the present day Kalamath Mts. (Lat. 43 deg.) north to Vancouver Island. The east margin of this marine basin varied over the next 30 or so million years but generally was located inland where the western slopes of the present day Cascades are located. There were no Cascade mountains at this time. This marine embayment was a large subsiding (sinking) basin called a geosyncline in which volcanic materials, sediments and often mixtures of both were being deposited. As the crust was drawn down it would fracture thus allowing igneous materials to intrude into the sediments. If we had been able to fly over what will be western Washington about where Interstate 5 now runs we would see vast flat lying coal swamps (centered around Centralia) which extended north into Canada (previous geologic map). These marked the eastern margin along the marine embayment. Further east a low lying tropical landscape would be observed. The recently reported banana fossil and other tropical plants from the Clarno area in north central Oregon date back 43 million years. Also from this same time period fossils of many other unfamiliar mammals such as Brontotheres and small dog sized horses (Orohippus) with four toes have been recovered.

The subsidence of this marine embayment would continue for over 30 million years and in areas of maximum crustal down warping (future site of the present coastal range in Oregon and Willapa Hills) over 20,000 feet of deposits would accumulate below the ocean water level. Moving ahead in time to the Miocene epoch when the widespread deposits of the the flood basalts in the Columbia River downwarp spread south into the Willamette Valley and North toward Grays Harbor. These can be can be seen very well in the Columbia River canyon. They covered hundreds of square miles over eastern Washington and Oregon. It is known as the Columbia River Basalt.

Around 6 million years ago the subsidence stopped and uplift began. As these older deposits lifted above sea level they were constantly eroded down thus exposing older and older rocks over time. This process continues at present. The older rocks which were being deposited during the Eocene (50-40 million years ago) and buried ten to twenty thousand feet below sea level during the next 30 or so million years, are now exposed at various places throughout the Willapa Hills. For example, the upper forks of the Palix river carves through this age rock, Cape Disappointment is a large remnant and if you miss the turn traveling north at the end of the Astoria (Megler) bridge you would be in direct contact with these Eocene rocks.

As this uplift was in progress the ocean was also shaping the ancient coastline from these older rock formations. The next major event that would contribute to the land forms of the present was the late stage of the ice age (termed the Pleistocene which started 1.5 million years ago). Although our area did not experience direct glacial effects the secondary results of the sea level fluctuation are obvious today. The large continental glaciers, often over a mile thick, would advance and retreat by flowing from central and eastern Canada to the mid portion of the United States. Fluctuations in the glacial volumes caused the sea level to vary widely, often by as much as 300 feet above and below the sea level today. Many geological deposits during the later stages of the Pleistocene remain obvious around Willapa Bay today.

During the last 200,000 years of the Pleistocene at least three major sea level rises and corresponding lowering (relative to the present elevation) occurred. Evidence suggests these fluctuations would have been 40 feet above and below the level today. Sediments were being deposited, as they are today in Willapa Bay and as the sea level lowered (or the land raises) they would be left perched on the older rocks. Today the remnants of those marine and fresh water sedimentary deposits (terraces) are obvious features around Willapa Bay. They lap onto the older uplifted and eroded rocks which form the Willapa Hills and basically make up the eastern margin of the bay. For example, Long Island, Lynn Point, Bay Center, and Wilson Point are comprised of these deposits. The nearly horizontal terrace deposits often contain fossil layers such as the one at Goose Point in Bay Center (which has been dated at around 120,000 years old).

The various processes thus far still did not form the Willapa estuary that we see today. What was necessary was the formation of the Long Beach peninsula. This structure was the result of sand, much of which was carried to the ocean margin by the Columbia River. As the energy of this large river is decreased at its mouth some sediment transport is taken over by ocean wave (long shore) currents and transported mainly northward along the shore. The beach is like a river of sand. As a wind driven wave strikes the beach at a southerly angle the sand grains are driven to the north east. When the wave recedes gravity moves the water and sand directly down the beach slope but there has been a net movement north along the beach. These deposits that form the Long Beach peninsula remain in place because of the hard older (Eocene) rock on the north side of the Columbia River near the mouth including Cape Disappointment. This flow of sand is also interrupted when rivers draining the Willapa watershed combine to breach the sand barrier and thus form, in our case, the entrance to Willapa Bay. This interplay of flowing sand and tidal flows results in the ever changing patterns and channel characteristics at the Willapa Bay mouth.

Today the geological processes continue. The deposition of eroded materials (sand, silt and clay) from the Willapa watershed form the characteristic productive mud flats which floor Willapa Bay. These remain as deposits because of the protection from strong ocean waves and currents by the Long Beach peninsula. We can observe a brief episode in the geologic process. Willapa Bay is a very transitory occurrence in terms of geologic time and most likely would never be duplicated. The interplay of geologic forces will largely determine what form the land will take in the future. However, it is our privilege to coincide in time and space (geography) to this unique dynamic natural occurrence.

RLW