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The aquifers older than the Mancos may yield small to mod- erate supplies of water to wells. The consolidated formations younger than the Mancos contain water generally adequate only for domestic and stock supplies, although, in some areas where these younger rocks are thick, supplies large enough to furnish small towns might be obtained. More information is needed, however, before the water-yielding properties of the younger rocks, especially those of Tertiary age, can be appraised. In the western one-fourth of the state, rocks older than the Mancos predominate at or near the surface south of the Grand valley: whereas, rocks younger than the Mancos predominate at or near the surface north of the Grand valley. In general, supplies of ground water adequate for domestic and stock uses are avail- able south of the Grand valley. North of the Grand valley, sup- plies of ground water are rather meager except in some places in southern Moffat county where moderate quantities are available. Between the western one-fourth of the state and the Front Range, the complex geologic structure of the Rocky Mountains has exposed rocks of many ages, and the outcrops of anyone formation or group of formations within a particular age group are rel- atively small in areal extent. Again using the Mancos as a reference plane, most of this area either has surface rocks older than the Mancos or is covered with igneous rocks of Tertiary age that overlie rocks much older than the Mancos. None of these are known to be good aquifers except locally. scattered through- out the area, however, are small areas underlain by water-bearing beds ranging in age from Triassic to Quaternary. Also, small amounts of water can be obtained locally from fractures in dense rocks of sedimentary, metamorphic, or igneous origin. Present Development The withdrawal of ground water for municipal use is small compared to that for irrigation. Of the more than 2 million acre-feet of ground water withdrawn annually, only about 50,000 acre-feet (about 2 percent of the total) is used by municipalities. About 96 percent of the withdrawal is for irrigation, and the remaining 2 percent is used chiefly by industries. Prospects for Future Development The rate of increase in use has been substantially less for ground water than surface water because of the population trend. The large towns, which are supplied largely from surface-water sources, have had the largest increase in population. The small towns, most of which are supplied from ground-water sources, have had little increase in population, and the small increase in water use can be attributed chiefly to the increase in per capita con- sumption. Because the surface-water resources are rapidly nearing full development, however, the less fully developed ground-water resources should become an increasingly important source of supply. 20