Extensive exposures of Tertiary rocks trend northeast-southwest in a
broad band across the Gulf Coastal Plain area. These strata, consisting
of sands, clays, and poorly consolidated limestones, are underlain by
Cretaceous rocks.
Invertebrate fossils are common in certain Tertiary formations and
pelecypods, gastropods, and corals are the predominant forms. In
general, however, fossiliferous exposures are of local occurrence and
most of the Tertiary formations are unfossiliferous. Those Tertiary
invertebrates that are present, however, are often well preserved and
represent many interesting types (Pls. 16, 22, 23, 29, 30, 31).
Tertiary invertebrate fossils are commonly found in sands, clays, and
marls. Many of these sands and marls have a green color which is due to
the presence of glauconite (a green mineral containing iron and closely
related to the micas). At certain localities on the Gulf Coastal Plain
the glauconite marls and sands of the Weches and Crockett formations
contain large numbers of well-preserved clams, snails, and corals.
Fossiliferous exposures of Tertiary rocks are sometimes found in road
cuts, but better exposures may be found along the banks of rivers and
creeks. Certain bluffs along the Brazos, Sabine, and Trinity rivers are
well-known Tertiary fossil collecting localities. Many of these better
localities are listed in some of the Bureau of Economic Geology
bulletins included in the bibliography of this publication (pp.
109-110).