Relatively small and compositionally distinct plutons, known as the Roosevelt Gabbros, are found in the eastern and central Wichita Mountains. Like the layered complex, these are dark rocks. Unlike the layered complex, they contain appreciable amounts of biotite, a mineral that forms in magmas with elevated dissolved water. One of these bodies, the Mount Sheridan Gabbro, is exposed in roadcuts at Meers and underlies the Mount Scott Granite on the north side of the wildlife refuge. Weathered gabbroic soils are thicker and support more vegetation than those generated on weathered granite, so the tree line on the north side of Mount Sheridan approximates the contact between the Mount Sheridan Gabbro and the Mount Scott Granite. Geologic relationships suggest that the granites and gabbros intruded at depths no greater than half a kilometer in the crust. Minor uplift ensued, and magmatism concluded with the intrusive emplacement of subvolcanic features: rhyolite and diabase dikes. These are poorly exposed, with the exception of a large diabase dike in the Mount Scott Granite revealed during the recent widening of Oklahoma State Highway 49, just west of Interstate 44.