Figure 9-2.-Geologic formation of earth layers.and sandstone, are huge reservoirs ofgroundwater. Sedimentary rock formations areamong the most common and productive ofall aquifers, and when found in sandy orgravel formations are easy to drill.Igneous andmetamorphic formations areboth viewed as a group of hard, denserocks. Unless highly fractured andoccurring close to the earth’s surface,they contain far less water thansedimentary rocks. The recovery of water fromsolid rock depends on the existence ofmany cracks, fissures, or crevices in therock. Extremely fine deposits ofsedimentary materials usually producelittle water. Althoughhighly porous, theyare relativelyimpermeable.Topographical features created bywater action offer an excellent chance forthe recovery of groundwater at relativelyshallow depths.Alluvial sedimentary depositsare the most productive formations forgroundwater. The wordalluvial meansdeposited by water. Such features includethe following:alluvial valleys that arerather extensive in area and are the sitesof ancient rivers or the flood plains ofactive rivers;alluvial fans that are anaccumulation of sediments at the base ofmountains, deposited where drainagestreams fan out; andalluvial basins that areessentially structural troughs created bya rim of mountains and glacial outwash.GROUNDWATER EXPLORATIONWhere extensive groundwater explorationhas not occurred, maps, official documents,unofficial documents, and native experiencemust be used to obtain a fairly reliableindication as to a groundwaterFigure 9-3.—Types of aquifers.9 -3
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