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Show PART V GROUND WATER Geologic Setting The rocks exposed range in age from Precambrian to Holocene (Recent). They consist mostly of consolidated and semiconsolidated continental and marine sedimentary rocks, but also include igneous and metamorphic rocks and unconsolidated alluvial and lacustrine deposits. The rocks are grouped into eight major geologic groups based on age and general hydrologic properties. The geologic formations that are represented in these groups are listed and the extent and general description are shown in U. S. Geological Survey Professional Paper 441, plates 1-3. Groups 2 (mostly shale, siltstone, and fine-grained sandstone) and 4 (mostly sandstone with interbeds of mudstone and con- glomerate) are the most widely exposed in the region. They crop out over about 62,000 square miles or about 56 percent of the region. Groups 1 (unconsolidated deposits) and 7 (volcanic rocks) are the least widely exposed, covering about 4,800 square miles or only about 4 percent of the region. The rocks have been complexly folded and faulted in the mountainous areas and in some subregions, but they have been relatively undisturbed in a major portion of the Colorado Plateau. In the Colorado Plateau the rocks have been deeply eroded by the Colorado River and its tributaries. Both the availability and chemical quality of ground water are greatly influenced by the geology. In general, the older consolidated rocks yield water slowly to wells and springs, whereas sand and gravel strata in the younger unconsolidated deposits and some igneous rocks yield water readily. However, even the oldest consolidated rocks, where they have been fractured by structural deformation or honey- combed by solution activity, yield water readily to wells and springs. Rocks that have their origin in brackish water or marine environments and have low permeability, such as shales and some lime- stones, siltstone, and sandstones, generally yield water of the poorest chemical quality. Alluvium generally yields water of the best chemical quality, but the ground water in some alluvial aquifers may be highly saline owing to hydrologic interconnections with rocks that normally contain saline water. 18 |