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by Bryan. It may have been brought about by heavy grazing of the country by stock, which has perhaps facilitated run-off and soil erosion.

-The lowlands of the Marathon Basin are drained by several streams with intermittent flow, the largest of which are Maravillas and San Francisco Creeks. In places these streams pass from one lowland area to another by crossing the ridges of hard rock in narrow water gaps. The broader features of the stream pattern have been acquired from the consequent drainage that was formed on the now removed cover of tale Marathon dome and that has been superimposed on the Paleozoic rocks beneath. The pattern has been considerably altered by later cutting along weak rock belts by subsequent streams, as may be seen by comparing on the topographic maps the dendritic stream pattern of the Cretaceous areas surrounding the Marathon Basin with the trellis pattern within the basin. Other modifications of the original pattern have been caused by the advantage possessed by streams flowing southward, off the dome, to the nearby, low-lying Rio Grande over those flowing northward to the higher, more distant Pecos River.

Maravillas Creek is apparently consequent to a post- Cretaceous syncline as far north as the latitude of Del Norte Gap. Subsequent streams, such as Woods Hollow Creek, enter it from the east in this part of its course (fig. 9, B). They are cut in weak-rock belts in the Paleozoic folds of the Marathon Basin. The water gaps by which they cross some of the hard-rock belts may have been inherited from a former southwardflowing consequent drainage system (fig. 9, A). The upper course of Maravillas Creek is apparently subsequent and is cut out along the disturbed zone at the west margin of the Marathon uplift. Its two northeastern tributaries, Peña Colorada and Dugout Creeks, have large headward extensions that drain the southern foothills of the Glass Mountains, where the strata dip north off the Marathon uplift. These creeks may possibly be the dismembered fragments of an original consequent stream that flowed southwestward through gaps in the Del Norte Mountains to join Chalk Draw, which is a consequent stream flowing in a Cretaceous syncline west of the Marathon uplift, in the Santiago Peak quadrangle (fig. 9). One such gap, Doubtful Canyon, is now followed by the eastward-flowing Maravillas Creek. Another, Del Norte Gap, is now a wind gap.

San Francisco Creek is a consequent stream which has acquired the original headwaters of the eastward- flowing Maxon and Dry Creeks by headward cutting along belts of weak rock (fig. 9, B). The two water gaps by which it crosses the limestone hogbacks near Haymond were probably inherited from the consequent headwaters of Maxon Creek. Maxon and Dry Creeks now head near the edge of the Marathon Basin, in broad valleys that are broken off to the west by dissected country draining into San Francisco Creek. The main western tributary of San Francisco Creek, Peña Blanca Creek, is a subsequent stream that flows in a weak-rock belt of northeastward-dipping sandstones and shales, between hogbacks of novaculite and of Carboniferous limestone.

Most of the bedrock that crops out in the Marathon region consists of consolidated sedimentary rocks. Such rocks also underlie the wide areas of Quaternary gravel and clay in the plains. Intrusive igneous rocks occupy only small areas, and extrusive rocks are found only along the west flank of the Marathon region.

The stratified rocks in the region are of many ages and represent nearly the whole span of geologic time from the Cambrian to the Tertiary. The base of the Paleozoic is not exposed, for the lowest beds raised in the anticlines are a part of the Upper Cambrian. These oldest rocks form the base of a thick Paleozoic succession that includes formations of Ordovician, Devonian (?), and Carboniferous age. The Carboniferous strata attain a great thickness and include in the Marathon Basin thick formations of Pennsylvanian age and in the Glass Mountains a series of Permian strata. Above the Paleozoic on the flanks of the Marathon uplift are rocks of Lower and Upper Cretaceous age. Above them on the west are Tertiary lavas and tuffs.

Nearly all the strata contain fossils, and in some places the fossils are very abundant. Marine invertebrate fossils are found in the rocks of Cambrian, Ordovician, upper Pennsylvanian, Permian, and Cretaceous age. In the lower part of the Pennsylvanian and in the Tertiary tuffs west of the Marathon region some of the strata contain plant remains.

The stratified rocks of the region were laid down under progressively changing geographic conditions. The Paleozoic rocks below the Permian were laid down in a subsiding area, the Llanoria geosyncline (fig. 16 and pl. 20). The geosyncline did not have the same form as the modern, nearly circular Marathon Basin. At the edges of the basin the rocks of geosynclinal facies strike northeast beneath the Cretaceous cover and are encountered again to the east in deep wells and to the southwest in the small uplift of the Solitario. "

Bryan, Kirk, Date of channel trenching (arroyo cutting) in the arid Southwest: Science, new ser., vol. 62, pp. 338-344, 1925.

Sellards, E. H., Pre-Paleozoic and Paleozoic systems, in The geology of Texas, vol. 1, Stratigraphy: Texas Univ. Bull. 3232, p. 23, 1933.