3
of the Comanche or Glass Mountains, he described the Caballos Ridges, to the south. These were said to be "low ridges * * * rising from the floor of the Marathon plain * * * composed of the degraded vertical edges of Paleozoic limestone, shales, and cherts * * * trending northeast and southwest. The cherts are often white in color and form the backbone of long ridges." On the south and east he found the region to be bordered by scarps and cuestas of "subhorizontal Cretaceous limestone unconformably resting on the subvertical edges of the Paleozoic rocks." He concluded that "the Caballos and Glass Mountains are exposures of ancient post-Paleozoic structure of Appalachian type and age, which have been revealed by the erosion of the Cretaceous sediments that probably once embedded them."
Observations made by Udden in 1905 in the course of a journey to the Chisos country amplified the results of Hill. Udden noted the discovery of Ordovician and Carboniferous fossils in the Marathon region. The comprehensive work in the region by Baker and Bowman in 1915, as a part of their exploration of the southern front ranges of trans-Pecos Texas, revealed the broad outlines of the physiography, stratigraphy, and structure. Rocks of Cambrian age, Ordovician strata with fossils at four horizons, a probable Devonian formation, and four thick Pennsylvanian formations were recognized.
During the same period Udden and Böse studied the Glass Mountains, to the north of the Marathon Basin, and described the upper part of the Paleozoic section exposed there, which includes a great thickness of Permian strata.
The writer began work in the area in 1925. During this and two succeeding summers, associated with R. E. King, he studied the Glass Mountains and the adjoining part of the Marathon Basin for the Texas Bureau of Economic Geology.
Field work:-The investigation that has provided the results for the present report was an extension of the earlier studies with R. E. King into an area adjoining on the south. The report is based on 8 months' field work in the region in 1929 and 1930 under the auspices of the United States Geological Survey. The season of 1929 was devoted to a reconnaissance of the entire Marathon Basin. In 1930 the Monument Spring and Marathon quadrangles were mapped in detail. Further observations were made during short visits in 1931.
At the beginning of the present investigation only the broader features of the structure and stratigraphy of the region were known. Because of the extreme complexity and small scale of the folds, the wide areas of valley fill, and the confusion that had arisen on some of the stratigraphic problems, many of the minor features of the region were poorly understood. The geologic mapping was therefore done with considerable care. Fortunately, excellent topographic maps were available for most of the area and could be used as a base for plotting geologic observations. The mapping was done partly by an elaborate system of pacing traverses, and partly by recording the observations on enlargements of the topographic sheets. Stratigraphic sections were measured mostly by Brunton compass and by pacing. Where there were exceptionally good exposures, however, a tape measure was used.
Acknowledgments-A small part of the field observations on which this report is based were taken from notes made by the writer when he was connected with the Texas Bureau of Economic Geology and from data furnished by E. H. Sellards, C. L. Baker, and others. Mr. Baker has also made available many recent fossil collections from the older rocks of the region, for study by paleontologists of the Geological Survey, and has joined the writer on several field conferences that made it possible to correlate the present work with that of Baker and Bowman in 1917. Much information on the fossil plants of the region has been afforded by collections sent to the Geological Survey by Sidney Powers. The report has also been materially improved by visits to the field with various members of the staff of the Survey, including David White, G. H. Girty, Edwin Kirk, G. R. Mansfield, and H. D. Miser.
After this report had been written the writer had the privilege of examining some excellent aerial mosaic maps of part of the Marathon Basin made by the Edgar Tobin Aerial Surveys, of San Antonio, Tex. Through the courtesy of this organization, he has been permitted to reproduce two of the single photographs from which the maps were made. (See pls. 17, 18.)
GEOGRAPHY
PHYSICAL FEATURES OF TRANS-PECOS TEXAS
The Marathon region is in trans-Pecos Texas, the westward-projecting part of the State that lies along the Rio Grande west of the Pecos River (fig. 1). Geographically, this arid and mountainous region is "
Udden, J. A., A sketch of the geology of the Chisos country, Brewster County, Tex.: Texas Univ. Bull. 93, pp. 18-21, 76-78, 1907.
Baker, C. L., and Bowman, W. F., Geologic exploration of the southeastern Front Range of trans-Pecos Texas: Texas Univ. Bull. 1753, pp. 67-172, 1917.
Udden, J. A., Notes on the geology of the Glass Mountains: Texas Univ. Bull. 1753, pp. 5-59, 1917.
Böse, Emil, The Permo-Carboniferous ammonoids of the Glass Mountains, West Texas, and their stratigraphical significance: Texas Univ. Bull. 1752, 1917.
King, P. B., The geology of the Glass Mountains, part 1, Descriptive geology: Texas Univ. Bull. 3038, 1931. Ping, R. E., The geology of the Glass Mountains, part 2, Faunal summary and correlation of the Permian formations, with description of the Brachiopoda: Texas Univ. Bull. 3042, 1931.
For a description and classification of the physical features of the trans-Pecos region, based on somewhat different criteria, see Carter, W. T., and others, Soil survey (reconnaissance) of the trans-Pecos area, Texas: U. S. Dept. Agr., Bur. Chemistry and Soils, ser. 1928, no. 35, pp. 1-7, 1928.