The Fredericksburg Group is one of three groups of rocks which comprise the outcropping Lower Cretaceous sediments in north central Texas. Four formations form this group; from the base upward these are the Paluxy, Walnut, Comanche Peak and Edwards (fig. 1). The Paluxy is made up of terrigenous elastics-red and gray sandstone plus some shale and conglomerate-that were deposited in subaerial and shallow nearshore marine environments. The Walnut consists of nodular chalk and microgranular limestone (micrite), marl, and pelecypod shell beds.