ABSTRACTComparison of the hydrologic and geologic characteristics of the Gallup Sandstone (Late Cretaceous) has shown that its aquifer properties, the movement of its ground water, and the variation in its water quality are subject to considerable geologic control. This control is imposed mainly by the distribution of seven progradational sandstone bodies which make up the Gallup. These fluvial and marine sandstone bodies were deposited along the shore of a northeast‐regressing sea. The two transmissivity zones recognized have the same trend as that of the upper marine sandstone bodies (northwest‐southeast), and transmissivity decreases northeast of the pinch‐out of the lower marine sandstone bodies. Ground‐water flow is variously controlled by geologic structure, the geometry and orientation of the marine sandstones, and the position of the northeast limit of the Gallup Sandstone. Concentration of dissolved solids, as reflected by specific conductance, is controlled in part by distance from recharge area and in part by the distribution of transmissivity. The types of geologic control determined for the Gallup should apply to other Cretaceous, marine‐sandstone aquifers in the San Juan Basin, and, in a general way, to ancient shorezone deposits