Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement.
Ironside Mountain is a 6- by 10-km block of folded rhyolite, andesite, and basalt flows of the Strawberry Volcanics bounded by a horse-shoe-shaped reverse fault and surrounded by Mesozoic rocks. The toe of the horseshoe points northeast; the fault dips outward at angles of 45° to 90°, has a minimum displacement of 600 m at the toe, and dies out or is buried at the open southwest end. Flows are folded to vertical in a crosswise anticline at the northeast end, and in a synclinal structure along the southwest edge. Local unconformities and thickening of flows in a small basin show that some deformation accompanied volcanism, which has been dated as late Miocene and early Pliocene. No centers of eruption have been identified within the Horseshoe fault.
The Ironside Mountain block seems to be a unique expression of cross folding in the region. The northwest-striking Mine Ridge–Ironside anticline follows the main regional trend, and is faulted where two large down-warps strike into it from the northeast. The Ironside Mountain structure is interpreted as being a transverse compressional feature in which opposing reverse faults took the place of anticlinal folding, directly on strike with a syncline to the northeast. A horst of pre-Tertiary rocks, which has been raised at least 600 m, crosses the axes of the abutting structures; the nose of the Horseshoe fault bounds one side of it, and a range-border fault bounds the other. Cross faults, of which we found only one, are believed to have facilitated arching and longitudinal shortening within the horst.