The intermingling of geologic units and regional structures in Culberson County creates a complex relationship between geology, regional ground-water flow, and ground-water yield to wells. Permian-age sedimentary rocks, Tertiary-age volcanic rocks, and Quaternary-age basin fill deposits make up the primary aquifer(s) in Culberson County, Texas. Permeability and ground-water flow in the Permian-age rocks are controlled by depositional locality within the Delaware Basin and regional tectonic structures (i.e. Victorio Flexure). Permeability and ground-water flow in the volcanic aquifer in southeastern Culberson County is locally controlled by volcanic stratigraphy of eruption centers (calderas). The direction of ground-water flow is from fractured sedimentary and volcanic rocks in the mountains and highlands to the basin-fill deposits, with the exception of ground-water flow from the Wild Horse Flat Basin to the Capitan Reef Aquifer in the adjacent Apache Mountains. Ground-water yield to wells can be over 3,785 liters per minute (1,000 gallons per minute) in the basin fill deposits and Permian-age Capitan Reef, where permeability is fairly uniform, but ground-water yield to wells can vary by several orders of magnitude in the volcanic rocks and in areas of fracture-controlled permeability.