The Montesano Formation is known to occur over about 250 square miles of Grays Harbor basin. Eight stratigraphic sections were measured along the branches of the Wishkah and Satsop Rivers, the Wynoochee River, and the Canyon River. Exposures of the Montesano Formation along the Middle Fork of the Wishkah River are designated the type section. There it is 2,500 feet thick and is composed of 1,500 feet of fine-grained sandstone, with small amounts of pebble conglomerate and mudstone, overlain by 1,000 feet of tuffaceous mudstone and sandy siltstone. Toward the east the thickness of the formation averages only 1,800 feet, and it is composed principally of fine- to medium-grained sandstone, pebbly sandstone, and conglomerate. Along the West Fork of the Satsop River, an abnorm lly thick sequence of thin-bedded to laminated, tuffaceous mudstone and very fine-grained sandstone at least 1,100 feet thick contributes to a formation thickness that may exceed 3,000 feet.

Deposition took place in a sea which was transgressing eastward across a broad, east-west-trending embayment. Estimated water depths ranged from sea-level to more than 3,000 feet. The upper parts of the eastern sections apparently represent a regressive phase. Turbidite deposition in a partly closed basin was the principal cause of the abnormally thick accumulation of the thin-bedded sequence mentioned previously.

Foraminiferal evidence places the Montesano Formation mainly in the upper Miocene. It is unconformable on the lower Miocene Astoria and Oligocene Lincoln Formations. A unit sometimes referred to as the Satsop Formation, and questionably considered to be Plio-Pleistocene, overlies the Montesano unconformably.

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