Toowa Valley, a broad east-west late mature valley astride the Sierra Nevada west of Olancha, California, contains flows and cones composed of olivine basalts, scorias and agglomerates in a volcanic field forumla miles long, including 4 eruptive centers, one of which is hitherto unreported. Each center shows several flow phases followed by cone building, breaching, and later partial reconstruction. Volcanic domes in and near Toowa Valley are also described. Contact relations of flows of Toowa volcanism, swamp sediments of meadows of pre-volcanic origin, volcanic domes of an adjacent area, and recognized physiographic surfaces of the Sierra Nevada, allow relative age determinations of several heretofore undated events.

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