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.

This field guide is designed to provide an opportunity to observe a variety of facies that are the fundamental blocks with which principal depositional systems have been fabricated. Available data is provided and a genetic interpretation is proposed. Although the validity of the interpretation may be questioned, it is anticipated that the interpretation will focus attention on the problems and limitations of facies interpretation in basin analysis.

The Texas Coastal Zone is marked by diversity in geography, resources, climate, and industry. It is richly andowed with extensive petroleum reserves, sulfur and salt, deep-water ports, intracoastal waterways, mild climate, good water supplies, abundant wildlife, commercial fishing resources, unusual recreational potential, and large tracts of uncrowded land in close proximity to major population centers.

The Allamoore district of Hudspeth and Culberson counties, Texas, has become one of the most significant talc-producing areas of the United States. Exploitation of talc deposits in the district began in 1952 with a cumulative production of 120,000 tons through 1957 (Flawn, 1958). With continued growth, annual production exceeded 160,000 tons in 1968, making the district second only to New York State in national output, Moderate- to large-sized deposits have been developed with near-surface parts inexpensively extracted. Talc reserves are estimated in the tens of millions of tons.

Mountain-bounded basins or bolsons are the most important aquifer systems throughout most of southwestern and western United States and northern Mexico. The dissection of the Presidio Bolson by the Rio Grande and its tributaries has afforded an ideal situation for detailed study of the stratigraphy of one of these important aquifer systems. The results of this investigation provide a framework for understanding the relationships between the types and distribution of bolson-fill sediments and the occurrence of ground water in desert basins in the United States and Mexico.