The geologic history of the region began with the deposition of late Precambrian to early Cambrian sandstones collectively known as the Tillman metasedimentary group. These sediments were intruded by a coarse mafic layered complex as the region began to rift apart during breakup of the Neoproterozoic continent, Pannotia. The exposed portion of this unit, named the Glen Mountains Layered Complex, consists of gabbro, anorthosite and troctolite. Recent high-precision dating yields an age of 532.49 ± 0.15 Ma. These are the dark gray rocks found throughout the Raggedy Mountains, the central lowlands region of the wildlife refuge, and to the immediate north of the refuge.