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o ;j :i't 9 " 19. and additional measurements are planned to give more widespread coverage. This program will become more and more valuable as development increases; it will show the status and trends of depletion. ., Ground-water investigations by the U. S. Geological Survey, appraising the ground-water situation in the High Plains of Colorado, are nearing completion. Field studies are complete except for one area--eastern Cheyenne and Kiowa Counties; this study was started in 1959 and is scheduled for completion in 1962. " Consolidated formations ("Artesian aquifers "). - -The con- solidated formations in Colorado generally are capable of yielding much less water per well than the unconsolidated formations. Very few areas are underlain by rocks capable of yielding more than 300 gpm (gallons per minute) per well; a few local areas have wells that yield more than 1, 000 gpm; but throughout most of the area well yields generally range from 5 to 50 gpm. At least three-fourths of the State is underlain by materials capable of supplying water for domestic and stock use although in some places the water is of poor quality. Where thick alluvial deposits overlie the consolidated formations, the alluvium generally is the most productive aquifer. '.. The principal consolidated water-bearing material is sand- stone; however, some conglomerate, limestone, and volcanic de- posits are known to yield water locally. The mo st widespread sandstone deposits are of Cretaceous age although older water-bearing deposits of Jurassic and Triassic age underlie considerable areas in the western part of the State, and important water-bearing deposits of Tertiary age underlie fairly large areas in the eastern part of the State. Rocks older than Triassic generally do not contain fresh water except in a few small areas where they occur near the land surface. " In eastern Colorado, the oldest important water-bearing unit is the Cheyenne sandstone member of the Purgatoire formation of Cretaceous age. The next younger important waterbearing unit is the Dakota sandstone which overlies the Purgatoire formation. Two other younger formations of Cretaceous age, the Fox Hills sandstone and the Laramie formation are separated from the older aquifers by thick deposits of dense material which include the Pierre shale. Younger consolidated formations that are considered important aquifers are the Dawson arkose and Denver formation of Tertiary age. - 3 -