Geology of Washington - Willapa Hills
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Geology of Washington - Willapa Hills 
 
 

The Willapa Hills, rising to 3,110 feet above sea level, are part of the Coast Range. They are situated between the Olympic Mountains to the north and the Columbia River to the south. The Willapa Hills physiographic 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, behind which there are major estuaries such as Grays Harbor and Willapa Bay.

The most complete section of Tertiary igneous and sedimentary rocks in the state is exposed in the Willapa Hills. Beginning with the Eocene Crescent Formation basalts, thick sequences of sedimentary and volcanic rocks of Eocene through Miocene age are present. A small Eocene quartz monzonite stock is exposed in the southern Willapa Hills. Embayments and shallow Tertiary seas surrounding Crescent-age basaltic islands were the sites of tuffaceous marine and nearshore deposition. Outcrops yield fossil pelecypods (clams), gastropods (snails), and crustaceans such as crabs. Thick lignite units and interbedded basalts are characteristic of the section in the eastern part of the Willapa Hills. There, the geological features demonstrate the presence of a marine shoreline during the Tertiary.

As noted in the section on the southern Cascades, Columbia River Basalt Group flows (Grand Ronde, Wanapum, and Saddle Mountains formations) flowed down the ancestral Columbia River and reached the Pacific Ocean at the mouth of the Columbia River. They also flowed into Willapa Bay and Grays Harbor. Along the way, these flows entered depositional basins containing semiconsolidated sediments. Instead of flowing along the surface, the flows tended to burrow into the sediments; such flows are called invasive. Uplands underlain by Columbia River basalts in Cowlitz County were exposed to lateritic weathering that produced ferruginous bauxite.

Unlike the Olympic Mountains to the north, the rocks of the Willapa Hills are not intensely deformed. They were not subject to subduction tectonism or the associated metamorphism. A current concept favored by geologists is that the subducting slab is arched under the Olympics, thus bringing deep subducted rocks to the surface. On the other hand, under Willapa Hills, the oceanic slab extends downward deep under the Earth's crust.

The Willapa Hills have rounded topography and deep weathering profiles. During the Pleistocene, a major river existed in the present-day valley of the Chehalis River. This glacial-age river channeled melt waters from the western foothills of the Cascades and the southernmost extent of the Puget lobe towards the Pacific Ocean.

Brian F. Atwater (1987) of the U.S. Geological Survey has 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. Recently, Yamaguchi, Woodhouse, and Reid (1989) announced their tree-ring research results. They 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).

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.

The above text is modified from the following article: Lasmanis, Raymond, 1991, The geology of Washington: Rocks and Minerals, v. 66, no. 4, p. 262-277. © Copyright  Heldref Publications (Helen Dwight Reid Educational Foundation). Used with permission.

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 Contacts

Geologic Hazards Group
Geology & Earth Resources Division
Washington State Department of Natural Resources
360-902-1450
Fax 360-902-1785
dnrgeologyhazards@dnr.wa.gov

Dave Norman
State Geologist and Oil & Gas Supervisor
360-902-1439
dave.norman@dnr.wa.gov

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