In southern Oregon east of the Cascade Mountains a varied series of Tertiary lavas is cut by the northern continuation of the Basin Range faults, giving rise principally to relatively undissected horst and graben structure rather than to tilted fault-block mountains. The tensional origin of these faults is indicated by definite normal faulting, curving, and zigzag faults, step faults, circular fault basins, volcanic activity parallel to the faults, absence of folds, thrust faults, or other pressure effects, and by the even distribution of faults rather than their local concentration in zones of initial failure.