Vol. V] DUMBLE— TERTIARY DEPOSITS IN NORTHEASTERN MEXICO \^y 



PHYSIOGRAPHY 



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 and 



