GEOLOGY OF THE REGION 421 



has been fractured by faults and by volcanic intrusions and extrusions 

 and largely eroded. 



Eegional Geology 



The Big Bend country and the area to the northward comprise the 

 volcanic plateau of western Texas. A vast region is covered by lava flows 

 and ash beds or by bolson deposits which conceal the bedrock. Cretaceous 

 rocks are exposed at the edges of, and occasionally within, the volcanic 

 area. 



That volcanic activity began during the Upper Cretaceous and con- 

 tinued into the Tertiary is shown by plant remains.^. The lavas are ande- 

 sitic and rhyolitic in character and they are described by Dr. Whitman 

 Cross in an unpublished manuscript of the United States Geological Sur- 

 vey by Mr. R. T. Hill on the Big Bend country. Both flows and ash 

 beds can be traced for many miles in mesas, and the good exposures invite 

 careful mapping and study. 



Structure of the Big Bend region must be considered for the Paleozoic 

 and Cretaceous terranes separately. The younger, as shown to the writer 

 by Mr. Hill and later observed in the field, forms a high plateau 100 

 miles in width, broken on either side by fault blocks tilted toward the 

 plateau.* The faults on the east are in the Boquillas (Carmen) Moun- 

 tains ; those on the west are in the Sierra Vieja Mountains. Many other 

 faults with profound escarpments, like those south of Lajitas and Ter- 

 lingua, add complexity to the structure. In spite of these fractures, the 

 Rio Grande has maintained its course developed during the igneous 

 activity, and the only sedimentary records of the canyon-cutting stage, 

 as interpreted by the writer, are part of the very thick bolson deposits 

 between Presidio and El Paso. In Mexico, the Burro Mountains south- 

 east of the Big Bend represent block-faulted Cretaceous mountains cut 

 by intrusions. 



Paleozoic rocks outcrop at only four localities in the Big Bend region — 

 Marathon, the Solitario, Shaffer, and west of the Chinati Mountains, in 

 Pinto Canyon (undescribed). The nearest Paleozoic areas are the 

 Guadalupe and Delaware Mountains to the north (figure 1) and the 

 Llano-Burnet (Central Mineral) region to the east. The former is 



3 E. W. Berry : An Eocene flora from trans-Pecos, Texas. U. S. Geol. Survey, Trofes- 

 sional Paper 125A, 1919. 



*C. L. Baker described this plateau as a block relatively less elevated than any other 

 part of the trans-Pecos (Review of the Geology of Texas, Universifv of Texas, Bulletin 

 44, 1916, p. 15). 



