To augment the California Division of Mines State Geologic Map project, photogeologic mapping,

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groundchecked along main roads, was conducted in 1957-1958 on the new Army Map Service Alturas Sheet, scale 1:250,000, in the northeast corner of California (41°-42° North Latitude, 120°-122° West Longitude). Previous maps in the area, such as Anderson's in the Medicine Lake Highland, Peacock's and Powers' in the Modoc Lava Beds Quadrangle, and R. J. Russell's in the Warner Range were incorporated, with alterations to fit State Map units, newly available topographic detail, and the authors' geologic observations.

The oldest rocks in the area occupy about 100 square miles in the southwest corner of the area, and include Triassic sedimentary and metasedimentary rocks (Modin, Brock, Hosselkus, and Pit units); Jura-Trias metavolcanic rocks (Bagley andesite); Jurassic marine sediments (Potem formation); and Eocene arkosic sandstone (Montgomery Creek formation). Miocene to Recent basaltic flow rocks cover most of the remainder of the area, with a several thousand foot thick, uplifted section of Oligocene to Pliocene agglomerates, tuff-breccias, and pumiceous tuffs in the eastern part of the map (Warner Range); and a thousand feet or more of Miocene to Pleistocene diatomaceous, ash-rich, lake-laid sediments exposed beneath near-horizontal lava cover in large basins and river cuts in various parts of he sheet.

Some of the units, such as Cedarville series, Warner basalt, and Alturas formation, previously assigned and widely used to designate many of these rocks, appear to be subject to redefinition and subdivision, although the present authors have not completed this project.

Structurally, the eastern part of the area is dominated by the uplifted and tilted fault block of the Warner Range, flanked on the east by the Surprise Valley graben. The remainder of the area is mainly a dissected volcanic terrain of the Modoc Plateau, with many north-northwest-trending faults of slight displacement. Many basaltic shield volcanoes and cinder cones occur along the southern edge and in the northwestern corner of the area.

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