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The Butte Valley region includes an area of about 600 square miles, between long 121°37' and 122°10' W. and lat 41°38' and 42° N., in northern Siskiyou County, Calif. U.S. Highway 97, connecting Weed with Klamath Falls, traverses the region in a northeasterly direction, and the Southern Pacific railroad serves several small farming communities in Butte Valley. The region is near the west edge of the Modoc plateau. It includes along its western border a part of the Cascade Range, Butte and Red Rock Valleys, the Oklahoma district, and a prominent northwestward-trending fault block (the Mahogany Mountain ridge) which separates Butte Valley fro-n the Oklahoma district and the Lower Klamath Lake marshland. Geologic units have been divided into two groups: Volcanic rocks which range in age from Eocene to Recent; and sedimentary rocks whi^h range in age from Pliocene to Recent. From oldest to youngest the volcanic rocks include: (a) Predominantly andesitic lavas and pyroclastic rocks comprising the volcanic r?cks of the "Western Cascades"; (b) older volcanic rocks of the "High Cascades"; (c) basaltic flows and pyroclastic rocks east of the Cascade Rang^; and (d) younger volcanic rocks of the "High Cascades." Volcanic rocks of the "Western Cascades" are the oldest rocks in the region. They range in age from late Eocene to late Miocene. These rocks are chiefly pyroxene andesite, and andesitic tuff-breccia, but include lesser amounts of basalt, rhyolite, and associated pyroclastic rocks. In most places they are badly decomposed and less permeable than the younger volcanic rocks. They are best exposed in the Klamath River canyon, wh^re the prevailing dips are to the east and northeast. The angle of dip d'minishes in these directions from about 15° near the base of the series to nearly zero where the rocks disappear beneath a younger series of volcanic rocks designated on plate 1 as the older volcanic rocks of the "High Cascades." The volcanic rocks of the "Western Cascades" are at least 12,000 feet thick. The older volcanic rocks of the "High Cascades," which UE conformably overlie the volcanic rocks of the "Western Cascades," are Pliocene and Pleistocene (?) in age. They consist chiefly of basalt and basaltic andesite that spread out in successive sheets from a chain of northward-trendinr shield volcanoes built along the crest of the Cascade Range. Here the topography is almost wholly constructional, and even the oldest cones retain irnch of their original shape. East of the Cascade Range several large dome-shaped lava cones and most of the northwestward-trending fault block called Mahogany 2 GEOLOGY AND GROUND WATER, BUTTE VALLEY Mountain ridge are composed of volcanic rocks of similar litholojry. In most places the older volcanic rocks of the "High Cascades" are highly fractured and moderately permeable; they serve as a large intake area and groundwater reservoir. East of the Cascade Range, basalt of Pleistocene and Recent age issued from vents and fissures and spread out over alluvial deposits and laVe sediments in the southern parts of Butte and Red Rock Valleys and the Lover Klamath Lake marshland. One of these flows the Butte Valley basalt forms the most productive water-bearing formation in the region. In the southwestern part of Butte Valley, where it is overlain by about 20 to 60 fee'; of alluvial materials and lake deposits, this basaltic flow is an excellent aquifer. Yields of more than 100 gpm (gallons per minute) for each foot of drawdown are common and yields of 1,000 gpm for each foot of drawdown have been recorded. Late Pleistocene and Recent lava flows and cinder cones in the Cascade Range and extensive basaltic extrusions near Sharp Mountain are important chiefly as recharge (intake) areas for ground water. Sedimentary deposits range in age from Pliocene to Recent. The oldest of these deposits is a massive fresh-water diatomite which under1 ies a large part of the Oklahoma district. The diatomite is impermeable, but wells penetrating interbedded sand or cindery lapilli lenses in the diftomite may yield moderate quantities of water. Glacial moraines and fluvioglacial outwash deposits of late Pleistocene age occur near the mouth of Butte Creek canyon. These deposits are unstratified or poorly sorted and commonly are only slightly permeable. Semiconsolidated lake deposits, ranging in age from Pleistocene to Recent, underlie most of the Butte Valley plain. West of U.S. Highway 97 these deposits are composed principally of impermeable layers of clay, tfiatomaceous clay, and volcanic ash. However, east of U.S. Highway 97, and especially near the eastern border of the valley, the lake deposits contain a larger percentage of sand, and permeabilities range from about 50 to 230 gpd per square foot as determined from tests made in 3 pumped wells. Alluvial-fan deposits on the west side of Butte Valley range from Pleistocene to Recent in age. They are composed of poorly sorted rock debris derived from the Cascade Range and are only slightly permeable. Areas mapped as alluvium include thin beds of gravel, sand, dry, and peat covering older lake deposits in Butte Valley and the area around Lower Klamath Lake. They include also small playa deposits, poorly sorted alluvium collected in broad, shallow basins and depressions, and alluvluir in present intermittent stream channels. In most places alluvium forms a thin cover resting on lava flows or lake deposits. In general the alluviuir is slightly permeable and of little hydrologic importance except in the southwestern part of Butte Valley, where it consists of sand and gravel, ranging in thickness from 20 to 60 feet and rests on the Butte Valley basalt. In tl is area the alluvium probably yields moderate quantities of water to wells. Elsewhere the alluvium is largely above the saturated zone and is important chiefly because of its ability to absorb precipitation and surface runoff which percolate through it into underlying rocks. Linear wedge-shaped talus strips, formed at the foot of precipitous fault scarps, are partly concealed beneath and probably interfinger with alluvium and lake deposits. The blocky talus debris is very permeable and, where saturated, yields water readily to wells.

Geology and ground-water features of the Butte Valley region, Siskiyou County, California