OCR Text |
Show probes, that permit tentative generalizations. Known conditions in local areas are projected across broad areas in which comparable conditions may be found, but where detailed information is lacking. In general, each of the ground-water basins shown on the map, occupies an intermontane trough, probablv of structural origin. Detailed drilling connected with mine development, geologic mapping along basin margins, and probing with electrical resistivity apparatus indicate that the mountains bounding the basins are uplifted relative to the basins themselves, on a series of faults essentially parallel to the axis of the individual basin. The alluvial material filling the basins is deepest near the axis, and is progressively shallower toward the sides. The extent to which this pattern is modified by local structures, now buried by basin fill, is unknown. The capacity of water-bearing materials to store and transmit water is imperfectly known. A general pattern infinitely varied in detail is suggested by pumping tests, reconnaissance study of the geology of the basins, and interpretation of well cuttings and drillers' logs. Two types of valley fill are distinguished, an older fill and a younger fill. The older fill occupies most of the basin from the hard-rock floor upward toward the present land surface. Total thickness of the older alluvium is in most places unknown, but in some basins wells have penetrated at least 2,000 feet of this fill. Typically the older fill is variable in grain size, and is almost invariably cemented. Fine-grained materials are common in all basins and dominant in those basins where ancient lakes or playas deposited clays, silts, fine sands, and locally limestone, gypsum, and salt. The younger fill overlies the older fill, wholly or in part, and ranges in thickness from a feather-edge at its outer margins to a maximum near the floodplain of contemporary through-draining streams. Maximum thicknesses of the younger fill |