The Cascade Mountains divide the state into two major climatic provinces. These mountains, while topographically a north-south range, are structurally a series of major anticlinal folds with N. 40° W. to E.-W. trends developed in Paleozoic and Mesozoic rocks which have been metamorphosed by large batholithic intrusions of granodiorite. This general pattern of folding is reflected in the Tertiary rocks throughout western Washington with the exception of the Olympic Mountains and the coastal belt.

The Tertiary rocks ranging from upper Eocene to Pliocene probably offer the best possibilities for the development of future oil and gas reserves. The Cowlitz (upper Eocene) formation is best developed in the Cowlitz-Puget Sound trough. The Oligocene is best developed in southwestern Washington and again along the Straits of Juan de Fuca and adjacent part of the Puget Sound. Miocene and Pliocene rocks are best developed in the vicinity of Gray's Harbor.

Volcanics in the form of tuffs, flows, and breccias of basic or medio-basic character are widespread

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and well developed in the lower and middle Eocene. Upper Eocene and younger volcanics are largely confined to the southern part of the state bordering the Columbia River.

Seepages of oil and gas are most abundant in the coastal belt but evidence of petroleum is found in Eocene and Oligocene rocks inland, ordinarily from water wells.

The most important wells, as far as evidence of oil and gas, are the Union State No. 1 and No. 3, located on the coast west of Gray's Harbor.

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