In the Pacific Coast/western mountain region (fig. 1), geologic units that contain clay with high swelling potential are widespread in the Coast Ranges of California and southern Oregon, in northern and northeastern Arizona, and in scattered localities in other States of the region. These units range in age from Triassic to Quaternary, though the Triassic units occur only in Arizona and southern Nevada. Clays with moderate to slight swelling potentials are in geologic units of the same age range and have a distribution pattern similar to the coastal and inland areas of Oregon and Washington.

Geologic units containing little or no swelling clay range in age from Precambrian to Quaternary. With the exception of the Coast Ranges and some parts of Nevada, Oregon, and Arizona, the units are generally in the mountains of the region, where fresh igneous and metamorphic rocks prevail. These units include most of the mountain ranges and their bordering erosional debris in the Basin and Range Province; the Peninsular Ranges of southern California; the Sierra Nevada and Klamath Mountains of eastern and northwestern California and southwestern Oregon; much of the Cascade volcanic belt of northern California, Oregon, and Washington; and much of the mountainous areas of Idaho, eastern Oregon, and eastern and northern Washington. Some valleys and plateaus are underlain by unaltered Quaternary and Tertiary volcanic rocks that do not contain swelling clays.

Mesozoic marine and continental deposits containing clays of high- to- moderate swelling potential are present at sites of active or potential damage to engineered structures in northern and northeastern Arizona, in Nevada, and in the Coast Ranges and off-shore islands of California, Oregon, and Washington. Swelling clays are found in shale beds of the following rock units in Arizona and Nevada: the Chinle Formation of Triassic age, especially its Petrified Forest Member; the Morrison Formation of Jurassic age; the Burro Canyon Formation and Tropic Shale of Cretaceous age; and the Mancos, Mowry, Lewis, and Kirtland Shales of Cretaceous age (Keller, 1962; O'Sullivan and others, 1972). In California, Mesozoic and (or) lower Cenozoic shales of the Moreno, Panoche, Chico, Knoxville, and Franciscan Formations, and the Yager Formation of Ogle (1953) and Berryessa Formation of Crittenden (195 1) contain swelling clays, although in some of these, the swelling potential is low (Manning and Ogle, 1950; Ogle, 1953; Briggs, 1953; Burnett, 1965; Schlocker, 1971, 1974).

Damage to homes and other structures has occurred in areas underlain by the melange parts of the Franciscan Formation of western California. Similar melanges are in the Mesozoic geologic units of Oregon and the Olympic Peninsula of Washington (Stewart, 1969). The matrices of the melanges generally contain large amounts of clay minerals with moderate to high swelling potential, such as montmorillonite, chlorite, illite, and regular and random mixed-layer clay minerals of these components (Schlocker, 1971, 1974),

Tertiary marine and continental sedimentary and volcanic rock units contain swelling clays that have damaged man-made structures. The swelling clays are in bentonitic beds throughout the Tertiary formations, but are most abundant in Miocene and Pliocene units. Damage is greatest in the urban areas of southern California and San Francisco Bay.

In southern California, the following are some of the Tertiary units that contain troublesome swelling clays: Rose Canyon Shale of Howell (1975), Sespe Formation, Monterey Shale, Puente Formation, Topanga Formation, Modelo Formation, Capistrano Formation (Merriam, 1960; Blanc and Cleveland, 1968: Keff and Drew, 1969; Jahns and Vonder Linden, 1973), and Pico Formation Quaide, 1957).

In the San Francisco Bay area, the following are examples of Tertiary units that contain swelling clays: Butano (?) Sandstone (Pampeyan, 1970; Meehan and others, 1975), Monterey Shale, Claremont Shale, Contra Costa Group, Orinda Formation, and Siesta Formation (Kachadoorian, 1956; Radbruch and Case, 1967; Radbruch, 1969).

Some of the Tertiary rhyolitic to andesitic volcanic rocks and related intrusive feeder dikes in the Pacific Coast/western mountain region have been altered to swelling clays. The detrital sediments eroded from these rocks and washed into adjoining basins are also expansive (Coats, 1964, p. 10; Papke, 1970; Albers and Stewart, 1972; Johnson, 1977).

The upper Tertiary and Pleistocene Idaho Formation, near the eastern border of Oregon, is more than 4,000 ft thick and consists mostly of freshwater deposits of shale, claystone, mudstone, clay, loose sand, and clayey and tuffaceous sandstone. Swelling of bentonitic shale and claystone in the Idaho Formation caused severe distress of supports of an 80-in.-diameter steel-pipe siphon crossing the Malheur River in eastern Oregon (Mielenz and Okenson, 1946).

In Idaho, the tuffaceous facies of the Challis Volcanics, of Eocene age, are bentonitic. Bentonite, mined near Salmon in Lemhi County, is believed to have been derived from the Challis Volcanics (Hosterman and Prater, 1964).

Mudstones and claystones containing swelling clays are common in the Pliocene, and Pliocene and Pleistocene formations of the Eel River area of Humboldt County, California (Ogle, 1953).

Swelling clays are abundant in Pleistocene and Holocene sediments in lakes and valleys in the Pacific Coast/Western Mountain Region; these clays complicate design and construction of engineering projects, In Seattle, Washington, swelling clays are present in glacial lake deposits (Mullineaux and others, 1964).

Estuarine sediments, such as those in San Francisco Bay, can contain large amounts of smectitic clay and silt. In San Francisco Bay, vast areas underlain by these sediments have had fill added and are being used for urban development. Although the swelling potential of these smectitic sediments is low to moderate, their use for foundation purposes demands careful engineering design to avoid shear failures and differential-settlement problems (Langston and others, 1958; Radbruch and Schlocker, 1958; Mitchell, 1963; Meade, 1967; Schlocker, 1974, p. 83-84).

Sediments in modem playas within basins in the Basin and Range Province generally contain smectitic beds. The smectite is derived from volcanic ash washed into the basins from surrounding mountain ranges. In some basins, the playas have been more extensive in the past than they are today; drilling has revealed swelling clay beds hundreds of feet below the present surface and beyond the borders of the modern playas (Droste, 1961; Hay and Moiola, 1963; Morrison, 1964, p. 116; Wiliden and Speed, 1974, p. 48; Deike and Jones, 1980).

Some hot spring deposits and deposits of sulfide mineralization have wide areas of bedrock-alteration aureoles that contain swelling clays. The swelling-clay minerals are generally montmorillonite, chlorite, and halloysite. Most of these are not shown on the map because of the small map scale and lack of detailed data on the aureoles. Some are shown where the clay has been described as a separately mined mineral commodity (Schwartz, 1947; Ames and others, 1958; Papke, 1970, 1971; Whitebread, 1976).

Some Tertiary basalts of Oregon, Washington, and Idaho are partly altered to swelling-clay minerals, especially nontronite, an iron-rich smectite. The swelling potential is generally low to moderate, but it is sufficient to be deleterious if the basalt is used for such construction materials as concrete aggregate, bituminous aggregate, or base and pavement courses of roads and streets. Rock containing 10-15 percent swelling clay may appear sound, but concrete made with it may disintegrate in as little as 2 yrs. Basalts from widely scattered quarries in the Coast Ranges of western Oregon were used as base and paving courses for highway sections that failed in as little as 1 yr. Petrographic examination of the rock used in the failed sections of highway showed substantial alteration to swelling clay (Scott, 1954; Higgs, 1976). Basalts from other localities in these States yield construction materials of high quality for use as concrete and bituminous aggregates and road metal. Petrographic examination of basalt, including polarizing microscopy and x-ray diffraction, should be used to detect deleterious amounts of swelling clay (Hosterman, 1960; Waters, 1961; Peck and others, 1964, p. 17-18, 29, 40; Hosterman and Livingston, 1966; Snavely and others 1968; Schlicker and others, 1973. p. 56-57; Van Atta, 1976).