Rocks of Cretaceous age are widely distributed in Texas and represent
one of the more important rock systems of the State. Cretaceous outcrops
occur in central Texas, north Texas, the Edwards Plateau, parts of the
High Plains, the Gulf Coastal Plain, and Trans-Pecos Texas.
As mentioned earlier, the Texas Cretaceous has been divided into the
Lower Cretaceous (Comanche series) and Upper Cretaceous (Gulf series).
These rocks consist primarily of marls (a type of calcareous clay),
shales, chalks, and limestones, but sands and conglomerates also occur.
Cretaceous rocks occur on the surface of about 28 percent of Texas, and
many of the larger cities of the State are situated on Cretaceous
strata.
Many of the Gulf and Comanche formations contain fossils which are of
interest both to amateur and professional paleontologists. Because of
their wide distribution in and near large population centers, Cretaceous
outcrops can be conveniently visited by many amateur fossil collectors.
The fossils are usually abundant and varied, and some are well
preserved. Although numerous kinds of fossils may be collected, the more
common forms are cephalopods, pelecypods, gastropods, and echinoids.
Some of the more typical Cretaceous fossils are shown in Plates 16, 21,
25-28, 32, 33, 35, and 36.
Cretaceous fossils are more commonly found in shales and chalky
limestones. Fossiliferous outcrops of these rocks can be found along
many streams, roads, and highways of central Texas, north Texas, and the
Edwards Plateau. Outcrops which have been weathered are more likely to
provide good collecting. In general, collecting is poor in areas covered
with heavy vegetation or recent stream deposits. Good collecting
localities are outcrops which have a fairly steep slope with a covering
of weathered rock material and a minimum of vegetation. One should move
slowly from the base of the slope upward while searching the ground for
any evidence of fossils, and particular attention should be given to any
small gullies since these often contain fossils that have been washed
out of upper beds in the exposure.