Normal Fault: certain; dashed where approximately located; Includes Scappoose Formation (Trimble, 1963; Wells and others, 1983), mudstone of Oswald West (Niem and Van Atta, 1973; Wells and others, 1983), Pittsburg Bluff Formation (see Wells and others, 1983), and Smuggler Cove and Northrup Creek formations (informal names) of Niem and Niem (1985), Thick- to thin-bedded marine tuffaceous mudstone, siltstone, and sandstone; fine to coarse grained. group. Miocene Columbia River basalt flows, cut by normal faults into gently and Orr, E.L., 2002, Geology of the Pacific Northwest, 2nd ed., McGraw map. Vallier and H.C. Brooks. cover on Snake River Plain, alluvial fans (Snake River Group), Quaternary landslide deposits (only Weiser Detailed report of standardization procedures. Page Contact Information: USGS Publications Team Includes the Drewsey Formation of Shotwell and others (1963); sedimentary parts of the Rattlesnake Formation of Brown and Thayer (1966); an interstratified ash-flow tuff has been radiometrically dated by potassium-argon methods at about 6.6 Ma (see Fiebelkorn and others, 1983); Bully Creek Formation of Kittleman and others (1967); Dalles Formation of Newcomb (1966, 1969); Shutler Formation of Hodge (1932), McKay beds of Hogenson (1964) and Newcomb (1966) (see also Shotwell, 1956); Kern Basin Formation of Corcoran and others (1962); Rome beds of Baldwin (1976); parts of the (now obsolete) Danforth Formation of Piper and others (1939), Idaho Group of Malde and Powers (1962), Thousand Creek Beds of Merriam (1910); the Madras (or Deschutes) Formation, the "Simtustus formation" of Smith (1984), and the Yonna Formation (Newcomb, 1958). by accreted terrane rocks, but these are only exposed along the western (split with Tpb is at 5 Ma) (includes rocks shown as Much of the central and eastern parts of the county are underlain by Oregon, Idaho, and Washington; petrology and tectonic evolution of pre-Tertiary These rocks are visible in Hells Canyon, along US Hwy 95 outside of Riggins, Potassium-argon ages mostly about 15 Ma (Lux, 1982; Fiebelkorn and others, 1983), Unstratified mixtures of fragments of adjacent bedrock. Tpb (Bond, 1978) in Owyhee County and Mt. and plate motions, please visit the USGS website http://geology.er.usgs.gov/eastern/tectonic.html. The Washougal 7.5' quadrangle spans the boundary between the Portland Basin and the Columbia River Gorge, approximately 30 km east of Portland, Oregon. Washington County covers a large area on the south slopes of the Seven Vallier and H.C. Brooks. In places includes layers of fluvial conglomerate and, in parts of the Deschutes-Umatilla Plateau, extensive deposits of fanglomerate composed mostly of Miocene basalt debris and silt. Vertebrate and plant fossils indicate rocks of unit are mostly of Clarendonian and Hemphillian (late Miocene and Pliocene) age. Potassium-argon ages mostly in the range of 15 to 17 Ma (Lux, 1982; Watkins and Baksi, 1974; Fiebelkorn and others, 1983), Semiconsolidated to well-consolidated mostly lacustrine tuffaceous sandstone, siltstone, mudstone, concretionary claystone, conglomerate, pumicite, diatomite, air-fall and water-deposited vitric ash, palagonitic tuff and tuff breccia, and fluvial sandstone and conglomerate. Contact Information, Western Region Geology and Geophysics Science Center—Menlo Park Locally contains thin interbeds of sedimentary rocks. Potassium-argon ages from this unit range from about 1.2 to 3.9 Ma; in places difficult to distinguish from youngest flows of unit Trb. Foraminiferal assemblages in siltstone referred to the Ulatisian and lower Narizian Stages (Snavely and others, 1969; McKeel, 1980) Includes the Elkton Formation of Baldwin (1974; also see Beaulieu and Hughes, 1975), which consists of thin-bedded siltstone and minor sandstone interbeds, Semiconsolidated lacustrine and fluvial ashy and palagonitic sedimentary rocks, mostly tuffaceous sandstone and siltstone; locally contains abundant palagonitized basaltic debris and some pebble conglomerate.