Excerpt from Tertiary Deposits of Northeastern Mexico
The structure of this coastal area is largely that of a mono clinal plain with local foldings which are, however, of very slight extent, except in close proximity to the western bound ary. Within it there are comparatively... few exposures of igneous rocks and these are confined to its southern portion, and, as a whole, it corresponds closely in its topographic features with the southwestern Texas region. The gentle slope of the land immediately adjacent to the Gulf shore, which has such a broad development in the Texas coast. Prairies, here rapidly narrows toward the south. In the more elevated region directly west of it are exposed the calcareous materials and conglomerates which were first described from Reynosa on the Rio Grande and named for that place. These beds, which may be correlated with the Lafayette formation of the Pliocene, not only form the surface rocks of the area lying east of that in which the lower Tertiaries are now ex posed, but judging from the numerous residuals occurring over the entire area and even in the Tamaulipas Range itself, seem, when originally laid down, to have formed a mantle over it all. Indeed, this formation, either by its actual pres ence or by the wide distribution of the detritus from it in the territory from which it has been denuded, masks and covers the underlying beds to such an extent as to render it difficult, if not impossible at many places, to determine their age or even their character.
Taken as a whole, the area occupied by the lower Terti aries is one of rather low relief as compared with the terri tory west of it and hills of any considerable height above the general level of the country are rather few and principally confined to the western portion. They are practically all the result of erosive action. In the northwestern portion of the area there are long ranges of these hills extending from Amole Creek to the Salado River which are known as the Ceja del Macha and Ceja Madre and are made up of clays and hard Tertiary sandstones. To the south similar but lesser ranges and isolated hills occur which owe their exist ence to other sandstones and clays of the same or later horizons. To this class also belong the hills east of Cerralvo, the Sierra Colorado northeast of Ramones, Loma Alta and168 california academy OF sciences [ploc.4'rn 833.
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