Shadow Mountain is the northern-most cinder cone of the San Francisco volcanic field. It is probably Pleistocene in age. Chris Condit mapped the vent and lava flows in detail for his M.S. thesis at Northern Arizona University.
The surrounding sedimentary rock is the Shinarump Conglomerate Member, a basal conglomerate and pebbly sandstone, of the Chinle Formation that is relatively resistant to erosion and forms extensive benches in some parts of the Colorado Plateau.
Image courtesy of Google Earth.