Geologic map of the Aldrich Mountain quadrangle, Grant County, Oregon
The Aldrich Mountain quadrangle extends part way across the transition zone between the Columbia plateau and Basin and Range provinces in east-centralOregon. The Aldrich Mountains fdrm the central part of a major east-west anticlinal uplift that includes the Ochoco Mountains to the west and Strawberry Mountains to the east (Brown and Thayer, in press), and for about 120 miles this uplift essentially marks the boundary between the Columbia Plateau and Basin and Range provinces (Cohee, 1961; Thayer, 1957). The John Day structural trough, the companion syncline on the north, extends about 35 miles east and 20 miles west of the quadrangle. The Aldrich and Ochoco Mountains are separated by a synclinal cross fold whose axis lies 2 miles west of the quadrangle and whose east limb extends to Aldrich Mountain. It is possible, therefore, to trace the later Tertiary rocks continuously over the uplift.