CHAPTER IV. 



THE CLIMATIC THEORY OF TERRACES. 



The preceding general description of the climate of Arizona and New Mexico, and of the 

 relation of that climate to topography and vegetation, prepares the way for a consideration 

 of the various hnes of evidence which seem to indicate that distinct climatic changes have 

 taken place in post-glacial times and even in the period covered by the written history of 

 the eastern hemisphere. Among purely physical phenomena, unrelated to man or his 

 work, none is so widespread or so important in its bearing on this problem as the alluvial 

 terraces described briefly in the chapter on topography. Until within the past ten years 

 such terraces, if they were discussed at all, were almost invariably assumed to be the 

 result of movements of the earth's crust. As a rule, however, they were dismissed with 

 a word, on the tacit assumption that they were not of sufficient importance to warrant 

 further discussion. W. D. Johnson, and possibly others, had recognized that terraces 

 may originate from climatic variations, not only in glaciated, but in non-glaciated regions.* 

 Nevertheless, the possibility of such origin in specific cases was rarely or never discussed. 

 The fu-st papers to consider the matter with any fullness were two upon Russian Turkestan 

 by Professor Davis and the writer, and another upon Persia also by the present writer. 

 All three were published by the Carnegie Institution in 1905 in a volume entitled "Explora- 

 tions in Turkestan." Since then the subject has received some attention in the writings 

 of Barrellf and others, but no one has yet definitely attempted to test the climatic theory of 

 terraces by applying it to a definite region in America and working out the agreement or dis- 

 agreement of the facts with this theory and with its chief rival. Accordingly I shall do this, 

 even at the risk of repeating some things which I have said elsewhere in regard to Asia. 



Terraces, although a common feature of the landscape in many arid regions, are not of 

 great importance in themselves. As possible indicators of climatic changes in recent geo- 

 logical times, however, they are of the first importance. Geologists have long been keenly 

 alive to the fact that the interior of our planet is in a state of incessant change which 

 manifests itself in the varied phenomena of crustal movements, the bursting forth of vol- 

 canoes, and the transformation of rocks by the development of new magmas or by the 

 processes of metamorphism. Yet, in regard to climate, they have until recently tacitly 

 assumed that, with the exception of a few unique cases such as the Permian and Pleistocene 

 glacial periods, the conditions of the earth have either remained uniform for ages or have 

 been subject only to the extremely slow variations postulated by the nebular hypothesis. 

 Recently, however, a change has taken place, and geologists are beginning to realize that 

 at certain special epochs the climate of the past has been subject to great changes. The 

 discovery of evidences of a Cambrian glacial period by Willis and others, and of a pre- 

 Cambrian period by Coleman, and still more the development of the planetesimal hy- 

 pothesis by Moulton and Chamberlain, have introduced a wholly new conception. Never- 

 theless, except for a few tentative suggestions, such as those of Gilbert, Davis, and Barrell,t 

 the general opinion still remains that climatic changes are very slow in occurrence, that 

 throughout most of geologic time conditions of practical uniformity have prevailed, that 

 the changes of the glacial period came to an end before the beginning of the historic period, 

 and that since that time the climate of the world has remained uniform. If the conclusions 



* W. D. Johnson : The High Plains and Their Utilization. Twenty-first Ann. Kept. U. S. Gaol. Survey, part iv, 1901, 



pp. 626, 628-630. 

 t Joseph Barrell: The Relations between Climate and Terrestrial Deposits. Journal of Geology, vol. xvi, 1908. 

 i G. K. Gilbert: Rhythms and Geologic Time. Proc. Am. Assn. for Adv. of Science, vol. 49, pp. 1-19. 



W. M. Davis: Explorations in Turkestan. Vol. 1, Cam. Inst. Wash. Pub. 26, 1905. 



Joseph Barrell: Origin and Significance of the Mauch Chunk Shale. Bull. Geol. Soc. Am., vol. 18, 1907, pp. 449-476. 



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