The Willapa Hills rise to 3,110 feet above sea level and are part of the Coast Range. They are bounded by the Olympic Mountains to the north and the Columbia River to the south. The Willapa Hills province includes the Black Hills, Doty Hills, and the adjacent broad valleys that open up to the Pacific Ocean. Barrier beaches characterize the low-lying coastline. Major estuaries include Grays Harbor and Willapa Bay. The hills have a rounded topography due to extensive weathering.
The Willapa Hills Province records several major events: the accretion of thick oceanic basalt onto western North America, a prolonged episode of flood basalt flows, and repeated catastrophic glacial flooding.
Most of the province is underlain by Crescent Formation basalts, which are a thick deposit of marine volcanics accreted to the continent by subduction processes formed during the Eocene about 56 million years ago.
Sedimentary rocks formed in shallow seas and deposited with and after the Crescent basalts yield many fossils. These include pelecypods (clams), gastropods (snails), and crustaceans such as crabs.
Thick lignite coal units and interbedded basalts are characteristic of the eastern part of the Willapa Hills, formed within a nearshore marine environment during the Paleogene, about 65 to 23 million years ago.
The Willapa Hills include exposures of Miocene-age Columbia River Basalt Group flows. These voluminous basalts flowed down the ancestral Columbia River from what is now eastern Washington and northeastern Oregon to reach the Pacific Ocean. Flows are also found in the Willapa Bay and Grays Harbor estuary. In many areas along their travels, the basalts burrowed into the underlying soft sedimentary rocks. These unique flow features are called invasive flows.
Columbia River basalt flows reached the Miocene Pacific Ocean and made their way into underlying formations, sometimes re-erupting! Modified from Marli Miller (University of Oregon) and Wells and others (2009).
During the last ice age in the Pleistocene, most of the northern half of Washington experienced several episodes of continental and alpine glacier advance and retreat. The Willapa Hills were never glaciated, though glacial deposits are found in river valleys. At this time, a major river existed in the present-day Chehalis River valley. This ancestral river channeled glacial meltwater from the Cascades and Puget lobe ice sheet toward the Pacific Ocean. This ancient river deposited vast quantities of sand and gravel. These deposits locally provide much of the road building materials for the province.
The southernmost border of the Willapa Hills Province in the Columbia Gorge also experienced a more extreme passage of glacial meltwater. Here, many cataclysmic glacial outburst floods originating from Glacial Lake Missoula swept through the Gorge to a height of 1,200 feet, as ice dams failed upstream. These floods are the largest known in the past couple million years globally. The effect of the floods was to widen the Columbia River Gorge and strip it of soil and talus.
Regional map of Pleistocene glaciers, glacial lakes, and outburst floods.
Exposures at 3,000 feet elevation of basalts and sandstones that were originally formed in a marine environment indicate extensive uplift. These uplifted rocks generally dip westward and are deformed by many folds and faults. All of the rocks show some amount of rotation due to the oblique subduction at the plate margin.
There are several potential geologic hazards associated with the Willapa Hills area. The most significant of these are earthquakes, tsunamis and landslides.
Earthquakes and Tsunamis
Coastal communities are at risk of significant earthquake damage and tsunami inundation from a large-magnitude Cascadia subduction zone earthquake.
Japanese historical records and regional tribal accounts suggest that a great earthquake occurred along the subduction zone in 1700. Brian F. Atwater (1987) of the U.S. Geological Survey further demonstrated that, during the Holocene, estuaries along the coast show evidence of repeated episodes of sudden submergence, killing cedar forests in the process. This downdropping has been attributed to great subduction-type earthquakes. Radiocarbon dating techniques have been used to develop a chronology for these events. Yamaguchi, Woodhouse, and Reid (1989) showed that the last major coastal subsidence event took place around the year A.D. 1670 when coastal trees were partially submerged in brackish Pacific Ocean water and died rapidly. More than 60 miles of coastline was affected by this event, corresponding to an earthquake registering at least a magnitude 8.0 on the Richter scale, based on analogues such as the earthquakes from Chile (1960) and Alaska (1964).
Landslide hazards are a recognized problem in the Willapa Hills since the 1960s. Landslide activity in this region generally correlates with precipitation. This is most evident following the wet year of 1996, where numerous landslide complexes were reactivated by high groundwater. In February of 1998, a deep-seated earth flow reactivated in the Aldercrest neighborhood of Kelso. This slide ultimately destroyed or condemned 138 homes. This event represents the 3rd most devastating landslide in Washington State.
Large scarp within the Aldercrest landslide. Note the back-tilted trees below the scarp with the remains of houses. Photo credit: Karl Wegmann.
Except for sand, gravel, and rocks, there are no mineral resources being exploited from the Willapa Hills. On the coast, near Ocean Shores, the state's most productive petroleum well was drilled in 1957. It produced 12,000 barrels of crude oil from Eocene Ozette melange rocks. Offshore bars, in particular at the mouth of the Columbia River, contain ilmenite-rich sands.