7

the writer during this period in the Guadalupe Mountains, northwest of the Marathon region. Most of then were accompanied by sharp changes in temperature some were associated with strong winds, but in other: the movement of air currents did not exceed 10 mile,. an hour. In all of them there was a persistent movement of air in some one direction, so that transportation of dust from one region to another apparently tool place on a grand scale.

Such dust storms are exceptional under present climatic conditions, but they suggest that a relatively slight decrease in annual rainfall might permit muck fine material to be carried from the region by deflation and that this process may have been active during dry times in the past.

The only features that can be definitely attributed to wind work are the "charcos" found in low places on the plains, generally in areas of fine-grained alluvium. They are steep-banked circular or oval depressions, 10 to 25 feet in diameter, and 2 or 3 feet deep. They are probably caused by cattle in search of water trampling a wet place after a rain. The vegetation is thus destroyed, leaving mud exposed, and during dry seasons this is carried away as dust by the wind. More conspicuous features ascribable to wind work, such as sand-blasted rocks and sand dunes, are entirely lacking.

Comparison of erosional agencies in and and humid regions -In regions of arid climate, as a general rule, mechanical weathering dominates over chemical weathering, and on the steeper slopes rill wash is more effective than soil creep. Because of the lack of creep, steep mountain slopes tend to endure in the unconsumed areas of such regions until well along in the cycle of erosion, whereas in regions of humid climate they have at this stage changed to subdued forms. In arid regions the streams of both mountains and plains areas are intermittent rather than permanent, but because of the lack of vegetation the subdivision of the more steeply sloping areas into watercourses is much more minute than in humid regions. When the streams flow the material carried by them is larger in amount and coarser in texture than that of streams in humid regions. The profile of such streams, even at grade, is therefore relatively steep.

The erosional agencies of arid regions, being unlike those of humid regions, produce unlike land forms. Davis notes that "the rocky and boulder-clad slopes of maturely dissected mountains in arid regions, together with the barren pediments below them", contrast strongly with "the soil-cloaked and forested slopes of maturely dissected mountains in humid regions, together with the fertile valley floors below them." He believes, however, that "their unlikeness is rather a matter of degree than of kind" and that "the unlike features are really homologous."

According to Davis, a very baffling problem is concerned with the relative rates of erosion and degradation in humid and arid regions. It seems as if humid stream erosion must * * * be more rapid than arid stream erosion in the early stages of an erosion cycle; also that, in a much later stage, degradation may be more rapid on the bare slopes of an arid region than on the plant-covered slopes of humid regions.

The Marathon Basin, on the crest of the Marathon dome, is 30 miles wide and 40 miles long and consists of plains, hilly lowlands, and low mountain ridges, carved from folded Paleozoic strata. The basin is surrounded by limestone escarpments, which stand higher than any of the ridges in the basin. On the east, south, and west sides the limestones of the escarpments are of Cretaceous age and are mostly gently tilted away from the uplift. On the north the Paleozoic rocks beneath the Cretaceous contain a resistant mass of limestone and form the broad cuesta- like upland of the Glass Mountains.

The Cretaceous rocks now found on the escarpments bordering the Marathon Basin at one time extended entirely over the crest of the Marathon dome. They have been stripped off the higher parts of the dome by recession of their cliffs and by the excavation of the weak underlying Paleozoic beds to form the Marathon Basin. These processes are still going on.

The northern part of the Marathon region slopes northward and northeastward toward the Pecos River, but the greater part slopes southward and is drained by Maravillas, San Francisco, and smaller creeks, which flow into the Rio Grande (fig. 9, B). The maximum relief in the Monument Spring and Marathon quadrangles is 2,700 feet. The lowest point, 3,450 feet above sea level, is on San Francisco Creek where it leaves the southeast corner of the Marathon quadrangle, and the highest summit is an unnamed peak in the Del Norte Mountains, 6,151 feet high, in the northwestern part of the Monument Spring quadrangle. Horse Mountain, the summit of one of the ridges of Paleozoic rock, is the highest peak in the Marathon Basin. Its crest, 5,010 feet high, is lower than the summits of any of the limestone escarpments on the rim. Most of the ridges in the basin are not more "

Blackwelder, Eliot, Yardangs: Geol. Soc. America Bull., vol. 45, pp. 164-165,1934.

This and other possible origins of charcos are discussed by Kirk Bryan (The Papago country, Arizona: 1". S. Geol. Survey Water-Supply Paper 499, pp 121-123, 1925).

This subject has been treated at some length by W. M. Davis (Rock floors in arid and humid climates: Jour. Geology, vol. 38, pp. 146-149, 1930). It is also discussed in less technical form in his Physiographic contrasts, east and west: Sci.

Davis, W. M., Rock floors in arid and in humid climates: Jour. Geology, vol. 38, p. 145, 1930.

Idem, p. 158.