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Chapter 2 Sector One Today The Natural Environment Geology Sector One, like much of San Marcos, is traversed by the Ba1cones Escarpment, an ancient fault line with broken and lifted sedimentary rock and exposed outcrops of Edwards limestone. This area, to the west of Hopkins and Hunter Roads, is hilly and characterized by rocky, shallow soil. Some of the best views in San Marcos are found from homes in the Heritage and Westover Neighborhoods, situated on the escarpment. The southeastern and southern portions of Sector One lie in the Blackland Prairie, an erodable chalk and clay-shale base generally covered by thick clay soils. These soils have a tendency to shrink and swell, a factor to be taken in to account when building. This area is characterized by gently rolling to flat land, with many large trees and significant amounts of open space. The Dunbar Neighborhood, the southeastern part of the Heritage Neighborhood, and the southern area of the City between Westover and the city limits fall within the Blackland Prairie. Topography Land elevations in Sector One vary from 570 feet above sea level near the intersection of Gravel Street and Jackman Street to 750 feet above sea level near Burt Street, making this sector one of the most picturesque areas of San Marcos. The Sector One Topography map is presented on the following page. Edwards Aquifer Zone The Edwards Aquifer is the water-bearing underground network of porous and honeycombed limestone formation of the Edwards Plateau. It is the source of water for San Marcos, as well as over 1.5 million other Texans, and is the source of the San Marcos River. The western portion of Sector One, on the escarpment, is located in the recharge zone, an area of fractures, caves and porous liIJ;lestone, which provides a path for water to run from the surface into the artesian area of the aquifer. The remaining portion of Sector One is located in the transition zone, where faults, fractures, and other geologic features present possible avenues for recharge of surface water into the aquifer. Watersheds A watershed is an area that drains into a river or creek. Portions of Sector One lie in the Purgatory Creek and Willow Springs Creek watersheds. Flood Plains There are floodways and flood plains along both the Purgatory and Willow Springs creeks. Approximately 41 percent of the land in Sector One is located in either the floodways or flood plains. There are 106 acres of Sector One in the lOO-year flood plain, an area which has a 1 % chance of being inundated by floodwaters in any given year. The 100-year flood plain has been adopted by the Federal Emergency Management Agency as the basis for flood 2-2