to the higher elevation of the recharge zone, extends 10–20 miles (16–32 km) south on the west end to only a few miles south on the east end. Across the eastern half of the aquifer, the recharge and artesian zones occupy common area. Geology Approximately 70 million years ago, activity of tectonic plates caused a revival of the Rocky Mountains. As these tectonic processes were occurring, millions of tons of sediments were deposited by alluvial and fluvial processes across Texas. The tremendous weight of these sediments resulted in faulting between the Edwards Plateau and the Gulf. The main geologic unit, known as the Edwards Limestone, is tilted downward toward the south and east and is overlain by younger limestone layers as well as several thousand feet of sediments. The Edwards Aquifer is a group of limestones and is considered a highly heterogenic aquifer. Three stratigraphic columns across the San Antonio area represent the Edwards Aquifer. These stratigraphic units are known as the Maverick Basin. the Devils River Trend, and the San Marcos Platform. The Maverick Basin portion of the Edwards Aquifer consists of the West Nueces, McKnight, and Salmon Peak Formations. The Devils River Trend unit of the Edwards Aquifer is composed mostly of Devils River Limestone with a thickness of approximately 550 feet. The third unit of the Edwards Aquifer, the San Marco Platform, consists of the Kainer, Person, and Georgetown Formations. Hydrogeology The Edwards Aquifer is a highly productive karst aquifer made up of Edwards Group limestones. The Edwards limestone is variable in hydrologic character, but is generally highly porous and permeable, which makes it able to hold and move a lot of water. The limestone is broken by faults and joints. Water flows through these fractures and continues to dissolve the limestone, creating larger and larger pore spaces over time. Some units also store water in eroded fossil burrows that formed through the burrowing action of worms and crustaceans at the seafloor. The effective porosity, or the amount of water that is capable of being recovered, of the Edwards aquifer is estimated to be about 5%. The aquifer ranges in thickness from about 300 to 700 feet (100 – 200 meters).Unlike sand and gravel aquifers that store water in very small pore spaces, karst aquifers store water in large pockets or caverns, forming underground "rivers" and "lakes". The rate at which groundwater moves through these conduits can