Over 350 sandstone dikes and sills intrude the eastward-dipping Moreno Shale along the east flank of the Panoche Hills. The intrusions vary in width from several inches to 23 ft. Many crop out for several hundred yards and several extend continuously for 0.5 mi. The dikes and sills are composed of very well-sorted, fine-grained, poorly indurated sandstone.
The intruding sand was derived primarily from thick unconsolidated sand beds in the lower part of the Moreno and was injected upward as a water-charged, low-viscosity slurry into three principal sets of fractures. The sills intruded shear planes produced by flexural-slip folding. Two sets of dikes fill a system of conjugate shear fractures. The intrusions appear to be very late in the structural history and possibly are post-Miocene in age.