Back-arc basins are depressions on the land side of the continents behind volcanic rims. During the Eocene and Oligocene epochs volcanoes erupted depositing several cubic meters of ash and lava over the earth. Some of these are visible today in places like the Big Bend National Park . Chinati Mountain south of Marfa and Paisano Peak west of Alpine are also volcanic remnants. Some of the mountains are the eroded remnants of lava flows and ash deposits which were extruded from the earth, while some have taken shape due to weathering and erosion of intrusive bodies.
Intrusions are bodies of magma which squeeze into, around or displace native or country rock.
After cooling below the surface, the intrusions may become exposed by natural weathering processes over extensive periods of time. A few of the laccoliths found in the Big Bend area are Maverick Mountain just north of Study Butte, Rosillas Mountains and Grapevine Hills north of Panther Junction, Big Bend National Park Headquarters . The Big Bend of the Rio Grande , 1968. Ross A. Maxwell. Bureau of Economic Geology, University of Texas Austin, Guidebook 7.