. Carnegie Institution of Washington publication. 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 the
. Carnegie Institution of Washington publication. 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. I
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