24 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



reached in the following pages are correct, however, changes of considerable importance 

 are still in progress, and there is a strong possibility that throughout the course of the 

 world's whole history cUmate may frequently have been subjected to conditions of high 

 variability. It may have remained constant for hundreds of thousands of years, but on 

 the other hand it may have changed suddenly at any time. Only by an exhaustive study 

 of all the possible evidences of change can we reach certainty on this point, and only then 

 can we test such theories as those of the relation of the carbonic acid of the atmosphere to 

 glacial periods, the effect of volcanic dust in cutting off the sun's heat, or the effect of solar 

 variations upon climate and thus upon the evolution of life. 



TERRACES OF THE TUCSON REGION. 



Let us turn now to our inmiediate problem, the origin of terraces. We will begin 

 with a specific example, taking up first the most recent terraces. Near the city of Tucson 

 the Santa Cruz River flows in a newly cut channel from 100 to 300 feet wide and 12 or 15 

 feet deep. The channel has been excavated by the stream within the last 25 years. Its 

 edges rise steeply from the sandy river-bed to a flat alluvial plain from half a mile to a mile 

 or more in width. The plain is in part covered with a thick growth of mesquite and in part 

 with irrigated fields. Before the cutting of the new channel and the consequent partial 

 draining of the plain, much of it was covered with sacaton, a species of bunch grass, 

 5 or 6 feet high, which flourishes only in places where flood water occasionally keeps the 

 ground thoroughly soaked for a time. The alluvial plain, in its present form, can not be 

 of great age geologically speaking, or even as measured by human standards; for Professor 

 R. H. Forbes, of the Arizona Experiment Station, has found pottery in the banks on the 

 borders of the channel at a depth of 10 feet below the plain, and similar finds have been 

 made elsewhere. The pre-Columbian inhabitants of America can scarcely have dug to a 

 depth of 10 feet, or, at least, would scarcely have done so, without iron tools of any sort. 

 Therefore we infer that since the time when pottery was in use the alluvial plain at Tucson 

 has been built up considerably. It scarcely needs the pottery to prove this, for when 

 Americans first settled here, half a century ago, the plain was flooded with water every 

 year, so that sometimes the mail had to be ferried across a mile of water. The floods were 

 not so muddy at that time as they now are, but a certain amount of sediment was carried 

 and was deposited where the water spread out. 



The alluvial plain is bordered by a gentle terrace of gravel, 10 to 20 feet high, according 

 to its distance from the stream. The top of the terrace forms part of the main bahada 

 slope on w^hich is located the city of Tucson. At first sight the terrace seems to be far 

 more gravelly than the alluvial plain, but this is not so marked a feature as superficial 

 conditions would indicate. The surface of the plain is everywhere composed of fine silt, 

 but in its interior, as disclosed by the cutting of the river, layers of gravel are by no means 

 uncommon. The surface of the bahada, on the contrarj', is almost everj-where gravelly, but 

 its interior contains a large proportion of fine material, partly in the form of ordinary silt 

 and partly in the form of a calcareous silty formation known as cahche. The preponderance 

 of pebbles on the surface is largely the result of the process by wliich the so-called "desert 

 pavements" are formed.* In an arid country eolian erosion gradually removes the finer 

 materials from the surface, but leaves the pebbles. In course of time the process goes so 

 far that the entire surface becomes covered with pebbles which assume the form of a 

 regular pavement and check fm-ther erosion of the underlying fine materials. In the 

 bahada on which Tucson is located the process has not gone so far as to produce a genuine 

 pavement, but it has progressed sufficiently to make the surface much more gravelly 



* See Tollman, op. cit. Journal of Geology, vol. xvii, 1909, p. 149. 



