ADOBE FLATS

The broad flats that are formed by deposition from sheet floods. are floored with sandy clay or, in local parlance, adobe. Hence they are called adobe flats.

The channels in the flats are of two types-shallow sandy channels ordinarily 2 to 5 feet wide and 3 inches to a foot deep, and deep channels from 5 to 6 feet deep and 15 to 30 feet wide. Parts of these larger channels may have a clay bottom and hold water for a time after rains or floods. Other shallow basins or pans, apparently produced largely by the trampling of animals, also hold water, and these two types of pools are called charcos. (See pp. 121–123.) Orchard-like forests of mesquite are common on adobe flats. Usually the trees grow along the channels, and not uncommonly these lines of trees are paralleled by similar lines of dead or dying trees that mark the position of channels which have now disappeared. The adobe flats of Vekol Valley, however, are bare of trees and are covered by hummocks of "galleta" grass. Scraggly bushes of mesquite and unusually large creosote bushes separate the grassy adobe flat from the alluvial slope covered by vegetation of the normal type, dominated by the creosote bush.

Adobe flats are of all sizes. Many small ones 30 to 40 feet wide and a few hundred feet long occur along streams otherwise marked by channels. Others cover square miles of country and when flooded are temporary gently moving sheets of water. The surface when dry is normally hard and smooth and somewhat streaked in appearance owing to the presence of fine crenulations in the direction of flow. There are also bits of rubbish collected on the upstream side of every obstruction.

PLAYAS

The only temporary lake in the region lies on the international boundary and receives the drainage of the southern part of Tule Desert. The locality is called Las Playas, and the bed of the temporary lake is a good example of a playa-a flat area occasionally flooded by water which stands and evaporates. The surface is crossed by mud cracks, which break the dark clay soil into large blocks. Many of the cracks are 3 to 4 inches wide at the surface and extend downward at least 3 feet. The edge of the barren flat is marked by scrubby mesquite bushes clustered about the charcos and channels that occur at the places of entrance of tributary streams.

WIND DEPOSITS

In the area west of Ajo, on the whole the driest part of the region, wind-blown sand is fairly common. The Yuma Desert is almost

completely mantled with sand from 1 to 10 feet deep. The presence of grass and bushes and the general shapelessness of the dunes indicate that the movement of sand is not rapid. The eastern margin of the Yuma Desert and the west side of the Gila Mountains are drained by a stream flowing northward to Gila River, and along this stream erosion is sufficiently active to prevent the deposition of sand.

In the flood plain of Gila River there are many groups of low dunes composed of sand derived from the broad sandy river channels. These dunes are commonly fixed in position by clumps of mesquite.

Along the eastern margin of the Lechuguilla Desert, Tule Desert, and Mohawk Valley are belts of sand dunes, which appear to be the residue of sand accumulated by the winds after sweeping the entire width of the valleys. The belt of dunes is particularly conspicuous at the south end of the Pinta Mountains. In this locality the dunes are invading the mouths of the mountain canyons and impeding stream erosion. A belt of wind-blown sand from a quarter of a mile to a mile wide surrounds the Pinacate plain. Growler Valley and the Valley of the Ajo are almost free of wind-blown sand, but patches of drifted sand occur on the Sentinel Plain and around its margin.

In the Quijotoa Valley, The Great Plain, and the plain near Copeka there are areas of wind-blown sand. The sand is usually only from a few inches to 4 feet thick and lies on the clayey surface of extensive adobe flats. The irregular distribution of the sand impedes the drainage and helps to break up and spread the floods. In making the artificial ponds of Tonukvo, Copeka, Comovo, and Camote the Papago Indians have taken advantage of the partial damming of channels in the adobe flats by wind-blown sand.

Interference with stream channels is perhaps the most notable effect of wind work in this region. In many localities channels in which water ran six months or a year before are blocked by low dams of wind-blown sand from 6 inches to a foot high. The next flood will probably take a new course, which will in turn be blocked. Thus the wind assists in the further diversion of streams which are already. likely to be diverted under the laws of stream action.

TERRACES

Many of the streams of the Papago country now flow below the level of deposits which they have laid down in past time. They flow in trenches bordered by bluffs leading to a broad terrace which, con

forming to the grade of the alluvial slope, extends to the mountains. Some streams have deposited material at more than one level and thus are bordered by several terraces.

TERRACES OF GILA RIVER

95

96

The valley of Gila River contains many terraces which parallel the river. The highest terrace is represented only in a few localities by small benches or by the tops of isolated hills and is composed of the material already described as older alluvium (p. 66). The lower terrace is well developed along the whole length of the river and is bounded on the lower side by bluffs about 75 feet high. It is composed of alluvium deposited in part by the river itself and in part by side wash from the ephemeral tributaries of the river. This material is referred to as the younger alluvium (p. 67). Below the terrace is the flood plain of the river, with its recent deposits. These relations were recognized by Lee, who correlated the alluvium of the higher terrace with similar material on Colorado River, which he called the Temple Bar conglomerate, and the lower terrace would thus be the equivalent of the Chemehuevis gravel of the same river. The older alluvium, which corresponds perhaps to only a part of the Temple Bar conglomerate, lies at various altitudes and is tilted in a number of localities, as at the north end of the Gila Mountains, 5 miles north of Blaisdell, south of Wellton, and in Tonto Basin. Near Mesa a well 1,305 feet deep penetrates only alluvium and shows that the valley fill extends more than 100 feet below sea level.98 As Gila River could not excavate a valley below sea level, earth movements must have followed the deposition of the older alluvium. Doubtless to these movements and the subsequent extensive erosion may be attributed the small size of the outcrops of the older alluvium and their inequality in height. Field work in San Pedro Valley in 1920 and 1921 indicates that a large part of the deeper valley fill is of Tertiary age and older than the older alluvium.

97

The younger alluvium, of Chemehuevis age, forms a well-defined terrace and indicates that in Pleistocene time the river filled its valley to a level of about 75 feet above its prevent elevation. The data for the age of the younger alluvium are presented on page 68.

95 Lee, W. T., Underground waters of the Salt River valley, Ariz. U. S. Geol. Survey Water-Supply Paper 136, p. 112, 1905.

De Lee, W. T., Geologic reconnaissance of a part of western Arizona: U. S. Geol. Survey Bull. 352, p. 18, 1908.

7 Lee, W. T., Underground waters of Salt River valley, Ariz.: U. S. Geol. Survey Water-Supply Paper 136, p. 112, 1905.

98 Idem, p. 115. Additional evidence from well records is given by Ross, C. P., The lower Gila region, Arizona: U. S. Geol. Survey Water-Supply Paper 498, pp. 77-84. pl. 12, 1923.

Since that time the river has cut a valley through the deposits of Chemehuevis age and is again depositing material on its flood plain. Early reports indicate that up to about 1880 the Gila flowed in a relatively deep channel through its flood plain, overflowing it only in times of flood. There was also a considerable low-water flow. At present the channel is a sandy waste with many tortuous subsidiary channels, constantly shifting in position, and there is no low-water flow except in favored places. There seems to be a greater proportionate load of sediment, which under present conditions is silting up the channel."

TERRACES OF ALTAR AND SANTA CRUZ VALLEYS

Santa Cruz Valley and its tributary, Altar Valley, which lies parallel to and west of it, are marked by several common features. Altar Valley lies between the Baboquivari and Coyote mountains on the west and the Tumacacori, Cerro Colorado, Sierrita, and Tucson mountains on the east and drains north to Abra Valley and thence to the Santa Cruz. It is about 40 miles long and ranges in width from 8 to 15 miles. The head of the valley from Buenos Aires south and for 3 to 4 miles out from the mountains on the west side as far north as Las Moras and on the east side as far north as the San Luis ranch is a plain cut on rock-a mountain pediment. (See Pl. XV, B.) About 4 miles south of Buenos Aires is the divide which separates the drainage going south past Sasabe into Mexico. It is smooth and flat. The head of the valley consists of confluent pediments formed by streams from the adjacent mountains, some of which flow north and others south.

Pediments border the east side of the valley and are probably extensive. On the west, however, from Las Moras to the Coyote Mountains, the pediment is narrow and the plains are covered by very coarse alluvium. Along the Coyote Mountains the exposed pediment is from half a mile to 3 miles wide.

The headwater streams in the pediment areas are incised in narrow valleys from 10 to 50 feet deep. These trenches lead from all the mountain canyons to an axial stream which, originating in the union of Arivaca Creek and the stream which follows San Luis Canyon, runs north almost to the Anvil ranch. Each trench is a flat-bottomed valley from a quarter of a mile to 11⁄2 miles wide. The loamy bottom is marked by a well-defined arroyo with banks from 2 to 6 feet in height. The bluffs of the trench have their maximum height of about 75 feet near Buenos Aires, decreasing in height upstream and downstream. Near the Anvil ranch the bluffs are about

Ross, C. P., op. cit. In this paper a complete description of the terraces of Gila River is given.

20 feet high, and thence northward they fade out rapidly, so that at the Robles ranch there is no sharp separation between the plain of alluviation of the main stream and those of its tributaries.

The trench of the main or axial stream shows no definite terrace, but a terrace is common on the tributaries, though owing to the "accidents" of erosion it is absent in many places. West of Pozo Nuevo the tributary valleys are from a quarter to half a mile wide. Each valley has a flood plain in which the stream is intrenched about 10 feet, like the axial stream. Above the flood plain is a terrace about 15 feet high, and then a narrow flat, followed by a gentle slope to the level of the plains, 35 to 40 feet higher.

Near Arivaca the flood plain of Arivaca Creek is bounded by low bluffs about 20 feet high, which constitute the face of a narrow discontinuous terrace. About 40 or 50 feet above the terrace lies a rolling plain cut on rock, which is part of a well-developed mountain pediment. This plain is well shown in Plate XV, A, a view north across Arivaca Creek east of the town of Arivaca. At the town the creek now runs in a trench about 200 yards wide and 15 feet deep. This trench has been formed within recent time, for the flood plain was once a marshy flat. Lieutenant Michler1 visited Arivaca in July, 1855, and his narrative states that "numerous springs lie concealed among the tule," a phraseology which implies that there was a swamp with rushes or "tule" at this locality.

Santa Cruz River rises in Arizona at the southern base of the Canelo Hills and flows southward through a broad depression called the San Rafael Valley, between the Patagonia and Huachuca mountains. Crossing the international boundary, the river describes a loop of 30 miles and reenters the United States 6 miles east of Nogales. At the second crossing of the international boundary the stream has an altitude of about 3,700 feet, and thence northeastward to Calabasas its grade is 40 feet to the mile. Near Calabasas the Santa Cruz is joined by Nogales Wash, sometimes called Potrero Creek, which flows northward from Nogales, and by Sonoita Creek, which heads on the east side of the Santa Rita and Patagonia mountains. This part of the Santa Cruz Valley is a broad lowland about 17 miles wide from the base of the Patagonia Mountains to Atascosa Ridge, in the Tumacacori Mountains. The lowland is composed of long ridges and flat-topped spurs extending from the bordering mountains toward Santa Cruz River and its tributaries. Similar ridges slope from the divide between Nogales Wash and the river. The culminating point of this divide is Mount Benedict, a bold, somewhat conical mountain. Between the ridges are sharp-walled

1 Emory, W. H., Report on the United States and Mexican boundary survey, vol. 1. p. 119, 1857.