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The Upper Cretaceous deposits of the Gulf region are mainly of marine origin. Taking the east-central Texas section as a standard, the deposits present different sequences of units from place to place in both directions away from this section, both to the southwest and to the northeast. The sequence in east-central Texas resembles that in the Great Plains of the western interior. No other sequences in the two regions are similar to each other.
The Upper Cretaceous of the western interior may be roughly divided into belts representing three types of sequence: (1) the Great Plains sequence--mainly fine sediments, and of marine origin; (2) the Rocky Mountain sequence--continental and marine sediments, much sandstone, considerable coal, increasing in coarseness westward; (3) a western marginal belt dominantly nonmarine, coarse sandstones, conglomerates, and coals; sequence usually incomplete.
Fossils are abundant in both regions. Although many species are present in each region that are not found in the other, enough of them are common or closely analogous to permit a fairly close correlation of the deposits. Dependence for correlation is placed chiefly on the mollusks.
Although there is at present no physical connection between the Upper Cretaceous deposits of the Gulf region and those of the western interior, the presence of common and closely analogous species throughout the series in the two areas indicates that there was such a connection. Presumably the connecting deposits that once existed in some part of the area now separating the two regions have been removed by erosion.
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