Agenda - 11-27-1995 - C-1-b - Laserfiche WebLink

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03 � Because these areas contribute to the public health and safety in the form of drinking water supplies, special land use protection measures are necessary for the areas' protection. Measures designed to insure high water quality are discussed in later sections of the Land Use Element. Geology and Ground water Yields Area geology was recorded in 1966 by the N.C. Department of Water Resources and the Geologic Survey of the U.S. Department of the Interior. While the bedrock geology of the county is fairly complex, it consists of three principal rock types: • Metavolcanic. Most of Orange County is underlain by low grade metamorphic rocks of predominantly volcanic-sedimentary origin. The formation is part of a large distinctive rock unit that extends from central Georgia to southeastern Virginia, known locally as the Carolina Slate Belt. As shown on the Geology Map, this formation consists-of Felsic Metavolcanic, Intermediate Metavolcanic, and Mafic Metavolcanic rock units. Interbedded with these units are Phylitte and Schist, Metamudstone and Meta-Argillite, Metamorphosed Gabbro and Diorite, and Metavolcanic-Epiclastic rock units. • Granite and grandiorite. In the northwestern corner of Orange County, intrusive rocks of granitic composition are found. The same formation also extends as a band through Carrboro and Chapel Hill to the west side of Durham. As shown on the Geology Map, this formation consists of the Metamorphosed Granitic rock unit. • Triassic. Found in the extreme southeastern corner of the county, this formation is comprised of rocks which are sedimentary in origin and formed by erosion of material from the upland Slate Belt. As shown on the Geology Map, this formation consists of the Chatham Group, Undivided, rock unit. These rock units are important because they govern, in part, the potential yield of ground water from wells. All ground water was once precipitation which fell in the form of rain or snow. The percentage of precipitation that becomes ground water depends on factors such as rainfall, slope, soil permeability, vegetation, and temperature. As an example, soil permeability differs considerably. The granites, grandiorites, and acid volcanic rocks weather to silt loans. Water percolates downward faster through the silt loans than through the dense, sticky clays developed on the gabbro, diorites, and more malfic volcanic rocks. Orange County receives about 43 inches of precipitation annually. Gravity causes water derived from precipitation to percolate down through the soil cover until it reaches a zone of saturation within which all openings or interstices are completely filled with water. The upper surface of the zone of saturation is referred to as the water table. It is not a flat surface, but one that generally conforms to the land surface. It also fluctuates in response to natural forces such as precipitation, temperature, evaporation, and transpiration. For example, ground water levels are highest in late Winter and early Spring, but decline through the Summer because of increasing transpiration of vegetation and the increased evaporative capacity of the air. In late Autumn, levels rise again as vegetation goes dormant and the evaporative capacity of the air drops.