Tertiary marine sediments and volcanic rocks ranging in age from early Eocene to late Miocene, or possible Pliocene, are present in southwestern Washington State. Total thickness of these rocks is more than 50,000 feet.

The Eocene history of the area is characterized by widespread vulcanism. Sedimentary rocks definitely dated as early Eocene have not been found in southwestern Washington. Middle Eocene rocks are represented by volcanics and interbedded sediments of the Crescent Formation and typically contain Amphistegina california and Asterigerina crassiformis. Upper Eocene rocks are represented by the marine Raging River Formation and the non-marine Puget Group in the eastern part of the area. To the southwest, equivalents of these formations range from marine to non-marine facies and are mapped as McIntosh, Northcraft, Skookumchuck, and Cowlitz Formations. Further west, upper Eocene rocks are wholly marine and are mapped as the Cowlitz Formation. Total thickness of middle and upper Eocene rocks i at least 15,000 feet. Foraminiferal zones equivalent to Laiming's B Zones and B-1-A, A-2, and A-1 Zones of the California Eocene can be identified.

In early Oligocene time, regional subsidence, accompanied by notable pyroclastic vulcanism, took place and Oligocene rocks are characteristically tuffaceous.

Rocks assigned an Oligocene age have a maximum thickness of 9,000 feet in southwestern Washington and comprise the Lincoln Formation. Two lithologic members usually are distinguished: a lower basaltic sandstone member and an upper tuffaceous siltstone member. Three foraminiferal zones are recognized and are correlated with the Refugian and Zemorrian stages of California.

In early Miocene time, deposition continued uninterrupted in the same basins that were receiving Oligocene sediments. In middle Miocene time widespread diastrophic activity was accompanied by withdrawal of the seas from southwestern Washington, except along the present Pacific Ocean front and a deep embayment in the Grays Harbor-Montesano area.

In the Chehalis basin, Miocene sediments consist of 2,000 feet of wholly non-marine rocks comprising the Astoria (?) Formation (early Miocene) and Wilkes Formation (late Miocene).

In the Grays Harbor-Montesano basin, lower and middle Miocene rocks consist of an estimated 6,000 feet of marine sediments mapped as the Astoria Formation. The Astoria Formation is subdivided informally into three lithologic units which are assigned an early and middle Miocene age, equivalent to the Saucesian, Relizian, and possibly Luisian stages of California.

At Ocean City, sediments notably different from the typical Astoria Formation are correlated tentatively with the lower portion of the Astoria Formation.

Marine sediments of late Miocene to questionably Pliocene age occur only in the Grays Harbor-Montesano basin and the Pacific Ocean coastal strip. In the Grays Harbor-Montesano basin, these rocks are mapped as Montesano Formation and consist of 2,500 feet of marine conglomerate, sandstone, and siltstone containing Foraminifera which indicate a late Miocene age equivalent to the Mohnian and Delmontian stages of California. Along the Pacific Coast, in the vicinity of Ocean City, sediments of equivalent age are represented by up to 6,000 feet of sediments informally denominated

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the Ocean City and Quillayute Formations. A late Miocene age for these rocks is supported by the presence of Rotalia garvyensis, Uvigerina hootsi, and Pulvinulinella gyroidinaformis.

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