26 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



than the second; it is less than half as high, and has a much more gentle slope. Below it 

 there is a little terrace only about 5 feet in height, which scarcely deserves to be noticed. 

 Next comes a fifth, which is the most pronounced of all; it corresponds to the one on which 

 the city of Tucson is built. Below it there is what may be termed an incipient sixth terrace 

 corresponding to the edges of the new channel at Tucson. 



TERRACES OF OTHER ARID PORTIONS OF NORTH AMERICA. 



Forty or 50 miles to the southwest of the northward-draining Pantano the main branch 

 of the Magdalena River in Mexico drains to the south. Here again terraces are highly 

 developed, as may well be seen at Cocospera (see Plate 1, c, page 34), near one of the old 

 Spanish missions which are so picturesque a feature of all this region. The total number 

 of terraces here amounts to seven, in addition to the terraced edge of the present flood-plain. 

 One of the seven, however, is only 3 or 4 feet high, and is not continuous, so that it scarcely 

 deserves to be counted. Two of the others he close together and usually merge into one. 

 All alike are composed of gravel and appear to be the same type as those of the other 

 valleys of the region. 



Terraces of the kind here described are not confined to the valleys mentioned above; 

 they occur in that of the San Pedro to the east of the Santa Cruz and in that of the Gila to 

 the north. In northern Arizona the Grand Wash in the western part of the State contains 

 one or two terraces which appear to be of the same type, and so does the Kanab Canyon 

 in southern Utah, which I have elsewhere described in connection with the terraces of 

 Persia and Turkestan.* In the northeastern part of Utah the Provo River, which rises in 

 the Uinta Mountains and flows southwestward through the Wasatch Range to Great Salt 

 Lake near Provo, traverses a valley which shows a fine series of terraces of the same type 

 prevalent farther south. The same is true of Kamas Creek, flowing north to the Weber 

 in the same region. In Montana similar phenomena of the upper Yellowstone and other 

 valleys may perhaps be connected with glaciation, but this can not be true of those already 

 described in Ajizona nor of those of the Rio Grande, Tularosa, and other streams in New 

 Mexico. Still other terraces which can not be of glacial origin occur in various parts of old 

 Mexico and will be described later. All these are only a part of the cases of non-glacial 

 terracing which the writer has liimself seen. 



From the descriptions of others it appears that there are many other valleys in the semi- 

 arid portions of America which are similarly characterized by terraces. For example, 

 since the completion of the original manuscript of this chapter the Journal of Geology for 

 October 1910, vol. xviir, pp. 601-632, has a^jpeared containing an article by J. L. Rich, 

 who describes terraces of apparently the same sort in Wyoming. Mr. Rich has also 

 presented before the Association of American Geographers a paper in which he discusses 

 the process of terrace-making as it has occurred during the last half centmy. In general 

 he adopts the theories of the present writer as set forth in "Explorations in Turkestan," 

 and in a paper on "Some Characteristics of the Glacial Period in Non-glaciated Regions."! 



TERRACES OF FOREIGN COUNTRIES. 



Turning to foreign countries, in the pubhcations already named, I have discussed similar 

 terraces distributed from Turkey on the west to China, 3,000 or 4,000 miles away, on the 

 east. In Greece they occur along rivers such as the Alpheios, and appear to have had a 

 share in the burial of the ruins of Oh'mjjia.J Apparently some terracing process has been 

 very active in the arid or semi-arid mountainous regions of the world in the most recent 



* Explorations in Turkestan, Expedition of 1903. Carn. Inst. Wash. Pub. 20, 190.5, p. 272. 



t Bulletin of the Geological Society of America, vol. 18, pp. 351-3SS. 



i The Burial of Olympia, by Ellsworth Huntington. Geographical Journal, London, vol. 36, 1910, pp. 657-686. 



