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The nomenclature used in describing these rock sheets is as follows: The geologic unit is the bed or stratum. Series of lithologically and paleontologically related strata are grouped into formations, as the "Glen Rose formation." When the strata are all of one kind of rock the name of the rock is sometimes substituted for the word formation, thus: "Edwards limestone" or "Edwards formation." Groups of this class may be embraced in still larger groups called divisions, as the "Fredericksburg division," and these may be grouped into series, as the "Comanche series," and these compose the still larger geologic groups, such as the Cretaceous or the Eocene.

As the work of geologic investigation. has progressed new facts have made necessary the revision of classification, and new names have from time to time been substituted for those first employed. In the classification herein presented the previous nomenclature has been refined by substituting appropriate geographic names (mononyms where possible) for all formations and abandoning the use of paleontologic and mineralogic names. Thus the terms "Dinosaur sands","Caprina limestone," "Exogyra texana beds","Exogyra arietina clays","Fish beds","Exogyra ponderosa marls","Hippurites limestone," etc., have all been replaced by appropriate geographic names. Even some geographic names originated in earlier writings have been abandoned because found to duplicate names previously used for different formations elsewhere.

The sequence and systematic classification of the formations of the region tinder discussion are presented in the table below, and in the following pages the several formations will be individually described. They will be taken up in the order in which they were deposited, the oldest and lowest first.