Lower Fredericksburg Cretaceous of central Texas contains two shallow marine sandstone deposits of similar shape but contrasting lithologic character, genesis, and porosity trend. The lower sandstone (Paluxy Formation) is predominantly quartz sand and clay, a tongue-like projection southward from the main part of the formation, part of the transgressive phase, deposited by longshore currents, modified by wave and tidal action in a coastal nearshore environment. Porosity trends parallel the length of the body. The upper sandstone (Whitestone Limestone Member of the Walnut Formation) is made of carbonate grains (ooliths, pellets, shell fragments), an elongate mound-shaped body trending southeast, part of the regressive phase, deposited in an agitated, offshore, shallow environment by southeast-trending currents, modified locally by surge channels normal to this trend. . . .