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Agencies observed in the Marathon region.-The sedimentary rocks of the Marathon region, especially the greatly deformed strata of Paleozoic age, have been prepared for weathering by previous jointing. Where they are least fractured and have the fewest bedding planes they stand as bold cliffs and hogbacks. The dominant rocks of the region, which are limestones of various sorts, weather chiefly by solution, despite the low humidity. Solution widens joints and fractures, makes channels, pits, and shallow depressions on the exposed surfaces, and undermines ledges. In places, some of the granular limestones of Pennsylvanian age and crystalline dolomites of Permian age show a well- developed exfoliation of undetermined origin. The older Paleozoic cherts and novaculites are not affected greatly by either solution or exfoliation, but in most places they break down readily along closely spaced joints. Rock breakage by diurnal temperature change does not appear to be an important agent of weathering in any of the rocks of the region.

The fractured blocks of limestone and chert are loosened from their parent ledges by frost action. Gravity and rain wash help carry them down the steep slopes below the outcrops. Many rock masses may also be broken from the cliffs by lightning, for scars apparently produced by its impact may be seen on the faces of the steeper bluffs at many places in the area.

Because of the dry climate, there is but a thin cover of vegetation and, in comparison with humid regions, a small amount of rock decay. The soils on the hillsides and mountain slopes are therefore thin and are full of angular rock fragments. Rock ledges are abundant on the slopes, except in the shaly formations, and even here gullies only a few feet deep lay bare the underlying strata. Talus is generally lacking. In the plains the surficial material forms a thicker cover over the bedrock, but most of this has been washed in from the surrounding hillsides.

The surfaces of all the mountains, hills, and plains are covered with a network of watercourses, ranging from small gullies to broad, gravel-covered creek channels. This strongly suggests that the dominant erosional agent of the region is running water. In the semiarid climate, however, the work of water is spasmodic, and the drainage channels are dry most of the year. After rains the water runs rapidly down the mountain slopes, discharging into rocky gorges in the mountains or directly onto the plains. There is so little vegetation and soil that not much rain water is absorbed where it falls. The drainage channels leading away from the storm area become rushing turbid rivers, with the flood waters at times advancing down the hitherto dry channel like a wall. Sometimes the writer has heard these torrents emit a rumbling sound, doubtless from the impact of boulders against each other while in movement. Nearly all the erosion accomplished by the streams of the region takes place during the flood periods. Banks are deeply undercut at the stream bends, depressions are hollowed out in the channels, and gravel bars are shifted downstream. The undercutting is a phase of lateral corrasion, which, according to Blackwelder, is "geologically * * * rapid, apparently more so than most other processes in the desert."

On the level plains the flood waters may spread far beyond the insignificant swales on the surface and flow down the slope as a mass of interlacing rivulets, or even as sheet floods a few inches to a few feet deep and several miles in width. In the path of sheet floods the writer has seen on the steeper slopes closely spaced shallow gullies and on the gentler slopes small heaps of sticks, rubbish, and fine mud. Erosion and deposition of this sort, accomplished by sheet floods, appears to be of minor consequence, and in the Marathon region at least sheet floods are not the important agent of erosion that McGee suggested.

The flood waters eventually disappear into the gravel channels of the streams or in the alluvium of the plains. Very little of the run-off reaches the Pecos River or the Rio Grande by surface flow. However, there is much continuous underflow within the gravel beds of the channels. In the larger creeks there are stretches of permanently flowing water where the underflow is raised to the surface by sills of bedrock.

During years of normal climate the wind is not an important agent in the erosion of the Marathon region. The most striking wind storms are the great gusts that precede summer thundershowers. These carry great quantities of dust into the air and sometimes even across the lower mountain ridges, but they are local in extent and erratic in direction. In dry years wind storms may occupy several weeks of the spring and may carry much suspended matter into the air. During the exceptionally dry winter and spring of 1933-34 such dust storms were more prominent than usual in trans-Pecos Texas. Many storms were observed by "

Udden, J. A., Etched potholes: Texas Univ. Bull. 2509, 1925.

Blackwelder, Eliot, Exfoliation as a phase of rock weathering: Jour. Geology, vol. 33, pp. 793-806, 1925; Insolation hypothesis of rock weathering: Am. Jour. Sci., 5th ser., vol. 26, pp 97-113, 1933.

Blackwelder, Eliot, Talus slopes in the Basin Range province [abstract]: Geol. Soc. America Proc., 1934, p. 317, 1934.

The same criterion has been used by Bryan, Kirk, Wind erosion near Lees Ferry, Ariz.: Am. Jour. Sci., 5th ser., vol. 6, pp. 303-305, 1923.

Shuler, E. W., A rise down canyon [Davis Mountains]: Sci. Monthly, vol. 31, pp. 129-133, 1930.

Blackwelder, Eliot, Desert plains: Jour. Geology, vol. 39, p. 138, 1931.

McGee, W J, Sheetflood erosion: Geol. Soc. America Bull., vol. 8, pp. 87-112, 1897.

Baker, C. L., and Bowman, W. F., Geologic exploration of the southeastern front range of trans-Pecos Texas: Texas Univ. Bull. 1753, p. 163, 1917.