110

In the progress of the ocean's margin across the Texas region during the Cretaceous period the shore line moved in as the land subsided, and was not the same for any two subepochs of the Cretaceous period, as will be more fully set forth in the descriptions of the structure.

The traveler across the Black and Grand prairies along the line of the cross sections (Pl. LXVII), starting at the interior or Paleozoic borders and passing eastward to the Tertiary border, encounters a definite sequence of strata in ascending series, which is more or less similar in all sections, although presenting minor variations, which will be further amplified in descriptions of the formations to be given later.

In general the following sequence occurs, as exemplified on Pl. XVI and in a section from western Parker County to east of Corsicana (see Pl. LXVII, section DD). The lowest Cretaceous formation

• (1) resting on the Paleozoic rocks and outcropping at the surface in the belt of Western Cross Timbers consists of loose beds of friable sand (locally called pack sand), with a few pebbles at its base (the Trinity sands). These sands pass upward into
• (2) light-colored arenaceous clays and marls in which alternating layers or beds of firm limestone of varying texture and thickness gradually appear. The marls and limestones are the Glen Rose beds. Another thick bed
• (3) of pack sand (the Paluxy sands) succeeds the Glen Rose beds. The outcrop of the Paluxy sands is covered with timber. Above the Paluxy sands are
• (4) clays alternating with thin limestones, usually accompanied by vast numbers of fossil oysters. These, the Walnut beds, pass upward into
• (5) white chalky limestones (the Comanche Peak beds), which are very fossiliferous and which usually constitute lower slopes of the escarpment of flat-topped mesas or plains. The Comanche Peak beds are distinguishable from the succeeding bed
• (6) of white chalky limestone (the Edwards limestone) only by the superior hardness of the latter, which is the rock of the numerous flat-topped buttes of the western border of the Grand Prairie and in which, at least south of the Brazos, are numerous beds of flint. The limestone, which is not very thick along the section under consideration, is succeeded to the east by another group of beds making the surface formations of the dip plains of the Grand Prairie between the western scarp rock and the Eastern Cross Timbers and consisting of alternations of marls and indurated layers of limestone. The lowest beds of this group consists of
• (7) darker-colored clays (the Kiamitia clays) containing great quantities of another fossil oyster. Above these appear more bands of limestone strata
• (8) of a chalky white color, alternating with clays (the Duck Creek formation) in which