THE TOPOGRAPHIC INFLUENCE OF ARIDITY. 19 



The bahadas consist primarily of innumerable detrital fans deposited by the streams 

 at the point where they issue from the mountains. In moist countries such fans can not 

 attain large dimensions, for they are soon washed away by the steady flow of the streams. 

 In dry regions, on the contrary, they tend constantly to increase in size. None but the 

 largest streams are permanent; for the great majority come to an end soon after leaving 

 the constricted valleys of the mountains. Emerging from the uplands, their speed is 

 checked so that they deposit their load of waste and are divided into many distributaries. 

 Thus fans are formed in whose thirsty gravel most of the water is lost, while the remainder 

 runs on a few miles farther with constantly diminishing volume until it finally spreads out 

 into thin sheets, forming playas which soon evaporate. Except in the case of occasional 

 floods which reach the main streams and run through to the sea, every bit of material that 

 most of the streams bring down from the mountains is deposited in the lowlands. Thus 

 year by year and century by century the fans grow in size, and finally coalesce into what 

 appears to be a single great slope, a vast apron or glacis surrounding all the mountains, 

 and ever rising higher as the mountains themselves are worn lower. In time the waste 

 from the higher mountains may bury the lower ones, cutting them off at fii'st and forming 

 the gravelly passes which make it so easy to cross the minor ranges at frequent intervals. 

 As time goes on, many small mountains are so buried that they merely stick up as little 

 pointed buttes in the midst of a rising sea of gravel and silt. Doubtless in past ages many 

 hills have disappeared entirely, for the deposits washed down from the mountains to the 

 lowlands have a depth of over 1,000 feet not far from Tucson, as shown by the records of 

 wells dug by the Southern Pacific Raikoad.* 



Close to the mountains the bahadas consist of coarse material in the form of subangular 

 boulders with a matrix of cobbles and sand. Farther out, as the slope decreases, the 

 boulders disappear, although in some cases they are washed to a distance of 5 miles or 

 more. Then the cobbles diminish in size and finally vanish, leaving only gravel, and that 

 in turn gradually gives place to the fine sand and silt which alone are found in the playas 

 where the slope is reduced almost to zero and the waters come to rest. The bahadas, 

 playas, and half-buried mountains of the southwestern part of the United States reproduce 

 exactly the topographic forms of other deserts in distant regions, such as Syria, Persia, and 

 western China. In all parts of the world these great piedmont deposits preserve full 

 records of the cUmatic vicissitudes to which they have been subject. Manifestly the 

 nature of the materials laid down under various conditions of climate is bound to vary, 

 even though a certain degree of aridity may have prevailed at all times. If the mountains 

 were at some time denuded of trees by excessive di'ought, a great amount of soil must 

 have been washed down in ensuing years. If the amount of vegetation became greater 

 than now, and the streams became more constant by reason of greater rainfall, deposition 

 at the immediate base of the mountains must have diminished, while farther away it must 

 have increased. Thus the depths of the bahadas must preserve a record of all manner 

 of changes. In the present volume this subject will not be taken up, because it does 

 not bear upon our immediate problem of recent climatic changes, but evidently any com- 

 prehensive study of the climatic conditions of the geologic past demands a careful examina- 

 tion of complete sections from the bahada slopes not only of America, but of all parts of 

 the world. 



(.3) The Terraces.— The bahadas by no means always merge into playas, nor do they 

 universally coalesce with one another. In fact, they usually fail to do so. Once all the 

 bahadas coalesced smoothly and merged into playas or flat valley bottoms, but now their 

 smooth slopes come to an end in terraces and are constantly interrupted by small valleys 

 and gulUes of recent origin. These valleys may be just wide enough for a small torrential 

 stream, or several miles wide. Their depth may be a few feet or hundreds. Their sides 



* Cam. Inst. Wash. Pub. 99, 1908. 



