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than 700 feet above their surroundings, but the escarpments that encircle the basin rise 1,000 to 1,500 feet above the floor.

The physical features of the Marathon region have been formed by the differential erosion of resistant and nonresistant rocks by streams. Among the more resistant rocks are limestones, which, because of the semiarid climate, stand as hogbacks, steep-sided plateaus, and high mountains. The region as a whole has reached a mature stage of erosion. There are, however, wider areas of sloping plains and much steeper and more rugged unconsumed areas than would be present in similar areas at the same stage of erosion in a humid climate.

Superb views of the Marathon Basin are to be had from the high escarpments on its east and west sides (fig. 2). The panorama is particularly impressive from Housetop Mountain (fig. 2, A), a projecting tongue of the Cretaceous plateau on the east edge of the basin, whose western face rises as a bold cliff 1,500 feet above the basin floor. From this eminence, in the clear air of the desert, the whole basin and its surroundings appear spread out like a map.

On the sky line to the west and south rise the mountains of the Mexican highlands, which lie beyond the Marathon Basin. To the southwest are the rugged peaks of the Chisos Mountains, to the south the domelike mass of the Sierra del Carmen, gashed by the canyon of the Rio Grande. Farther east the Serrania del Burro and other ranges stretch far away into Mexico until they are lost in the bright haze of the horizon.

Between the observer and the mountains of the horizon are ridges and plateaus of lesser order. To the southeast are Cretaceous tablelands, sloping toward the east, intricately carved into canyons, whose sides are banded by limestone ledges, as straight as if drawn by a rule. Westward the tablelands rise and project in long promontories into the Marathon Basin. The limestone ledges at the ends of the promontories are broken into disconnected tables and conical buttes, perched on reddish rounded slopes and hillocks of Paleozoic rock. Here and there ledges are discernible in the lower beds, but these run at a steeper angle than those of the Cretaceous limestones.

In the middle distance, between the tablelands and the observer, are the hills and plains of the Marathon Basin. The flats, streaked in places by white gravel deposits, are covered by a lacy network of drainage channels, with fringes of dark vegetation. Between the flats are bare rocky ridges and miniature mountains, each of which assumes a color and form determined by the nature of the rock from which it has been carved. Near at hand are reddish hills and ragged ledges of sandstone and narrow hogbacks of limestone. Farther away, in the center of the basin, is a broad cluster of hills, streaked by white ledges of novaculite. From this distance, most of the hills have no evident plan or arrangement, and they are apparently turned and twisted in greatest confusion. Here and there, however, the eye can distinguish sharp ridges and chains of knobs in the white rock, and in places a spine of vertical strata projects above the rest.

Such a distant view, in a region of great complexity, can only reveal the outlines of the geography and geology and serve to arouse the imagination of the observer. If he should wish to untangle the geologic history of the land, he must descend into the plains, in order to analyze its many features.

- The Cretaceous limestone escarpments on the east and south sides of the basin stand 500 to 1,500 feet above the basin floor. These are parts of* the high western dissected margin of the Edwards Plateau. East of the basin the plateau surface inclines gently eastward on the stripped bedding planes of resistant layers (pl. 1, B, and fig. 3, A). To the south the inclination of the strata is steeper, and here two prominent parallel limestone cuestas, called the Maravillas scarp by Hill, face northward toward the basin (pl. 14, B, and fig. 3, B).

The rocks of the plateau consist of an alternation of resistant limestone layers with weaker beds of marl. Near the basin the cap rock of most of the escarpments is the Edwards limestone (pl. 13, D). The resistant beds of the Edwards and other limestones resemble "huge stone walls of so ancient a date that they have crumbled into ruins. The less resistant beds form less steep slopes covered with debris from the overlying more resistant layers, and the whole escarpment or canyon wall gives a buttress affect like that of ruined Gothic architecture."

Drainage in the plateau country adjacent to the Marathon region is prevailingly consequent; the streams follow the slope of the Cretaceous surface and radiate from the Marathon dome (fig. 9, B). Several of the consequent streams head in the Marathon dome and flow eastward or southward into the plateau country. These have broken the escarpments at the edge of the Marathon Basin into separate segments and in places have reduced the segments to narrow promontories. The streams in the plateau have carved innumerable canyons, which have broken the originally continuous plateau surface into small areas of tableland.

In one of the canyons east of the Marathon Basin there is evidence of a relatively recent drainage change. "

Hill, R. T., Physical geography of the Texas region: U. S. Geol. Survey Top. Atlas, folio 3, p. 4, 1900.

Baker. C. L., and Bowman, W. F., Geologic exploration of the southeastern front range of trans-Pecos Texas: Texas Univ. Bull. 1753, p. 133, 1917.