THE CLIMATIC THEORY OF TERRACES. 



27 



geological times, and the activity seems to have persisted down almost to the present day, 

 or even to be still in operation. Clearly the truth can not be ascertained without a realiza- 

 tion of the fact that the phenomena are widespread. It is scarcely going too far to say that 

 in the dry, non-glaciated portions of North America and Eurasia regions containing high 

 mountains of unsubdued topography are usually characterized by a peculiar type of terraces 

 which bear so close a resemblance to one another that they all appear to be due to a single 

 cause. Hence the terraces are of great importance because they represent one of the latest 

 and most widespread of geological processes. In attempting to explain them it must con- 

 stantly be borne in mind that we are deaUng with a phenomenon which is as widespread 

 as glaciation, but which has taken place in non-glaciated, arid regions during the same 

 period of time which has seen the intermittent advance and retreat of the ice-sheet from 

 the moist lands of the north. 



THE STRUCTURE OF THE TERRACES. 



Before proceeding to discuss the two theories of tectonic and climatic origin of terraces, 

 let us first consider the general structure of the terraces themselves. In doing this the facts 

 vnW be di-awn only from a Umited area in southern Arizona and northern Mexico, but the 

 general statements apply with equal truth to other parts of North America and to Asia. 

 The upper portions of the terraces universally consist of alluvial material varying in texture 

 from coarse cobbles and boulders to fine silt, according to the distance from the mountains. 

 In the majority of cases the entire terrace is alluvial from top to bottom, although the 



Fig. 5.— Cross-section illustrating the Formation of Climatic Terraces. 



different layers may vary greatly in texture, a part, for instance, being composed of coarse 

 gravel, while an underlying layer consists of fine clay. In many cases, however, solid 

 rock crops out below the alluvial material. In such cases it is clear that the gravel which 

 lies on the rock and forms an upper terrace, and that which lies against it at its base and 

 forms a lower terrace, are not of the same age. This can be plainly seen on the right of figure 

 5, where the deposition of the gravel of terrace A was clearly separated from that of C by a 

 period of erosion, or in fact by two periods of erosion in this particular case. In other cases, 

 such as that already described at Tucson, a lower terrace may consist of fine silt, while 

 the one just above it is composed of gravel. Here again a period of erosion must have 

 intervened, as is illustrated on the left of figure 5, for otherwise it would not be possible 

 to have an unconformity such as that which exists between the coarser deposits of B and the 

 finer ones of C. In still other cases the deposits of two contiguous terraces, such as A and 

 B, are so similar that it is not possible to ascertain whether they were separated by a period 

 of erosion or whether they are parts of a single deposit which was first laid down in suffi- 

 cient depth to reach the level A, and was then in part eroded to the level B. Terraces of 

 all kinds, or combinations of the various types, may be found together, and a single 

 terrace may pass from one type to another, as it is followed up or down stream. 



