Tertiary sedimentary strata are well exposed in the upper Nehalem River basin. They have been known for many years to geologists interested in West Coast Tertiary marine deposits but until recently no systematic study of the stratigraphy of the region has been attempted.

The rock sequence of the upper Nehalem River basin is here divided into three district groups separated by unconformities. The oldest group, the Tillamook volcanic series (Eocene), forms rugged highlands in the southwestern part of the area and represents the core of the Coast Range geanticline. Bordering the Tillamook volcanic series is a belt of Tertiary sedimentary strata ranging in age from Upper Eocene to possibly lower Miocene, constituting the second group. The third and youngest group is the Columbia River basalt, a sequence of lava flows of middle Miocene age. These flows cap hills along the east side of the area and form extensive uplands.

The sequence of Tertiary rocks lying between the Tillamook volcanic series and the Columbia River basalt consists of about 5,000 feet of gently warped marine and brackish-water clastic sedimentary beds which are of interest as a possible source of petroleum and natural gas. These beds, beginning with the oldest, are divided into four formations; Cowlitz, Keasey, Pittsburg Bluff, and Scappoose, and each is characterized by a distinct assemblage of fossils. The Cowlitz and Keasey formations, together, represent at least 2,700 feet of marine strata, variable in composition but dominantly shaly and tuffaceous. Mollusks and foraminifers characteristic of shallow to moderately deep water are fairly abundant. Overlying the Cowlitz and Keasey formations are approximately 2,300 feet of dominan ly arenaceous, marine to brackish-water beds of the Pittsburg Bluff and Scappoose formations which contain near-shore megafaunas.

Each of the sedimentary formations represents an advance of the sea into embayments covering the area of the present upper Nehalem River basin. Into these embayments streams from the east brought the products of normal erosion as well as varying contributions of contemporaneous volcanic ash and lapilli.