472 C. jR. Keyes — Aggraded Terraces of the Rio Grande. 



explanation of which present great difficulties by the other and 

 more familiar agencies. 



There appears to be much misconception regarding the com- 

 position of the deposits which go to make up the aggraded 

 plains of the Rio Grande and other large rivers of the South- 

 west. The substructure is almost invariably spoken of as 

 gravel. This seems hardly correct. To be sure, the entire 

 surface of the plains might easily be mistaken for exposures of 

 gravel-beds. Closer inspection clearly shows that by far the 

 greater part of such deposits is made up of clay, called by the 

 the Mexicans adobe. In this adobe there are commonly a few 

 small pebbles. For the most part the most gravelly surface 

 when turned by the plow gives a loamy soil, such as is pos- 

 sessed by the prairies of the Mississippi valley, except the 

 color. There are some gravel-beds, but on careful examination 

 they are found to be very limited and to represent gravel- 

 trains of former arroyo-courses. They are sharply defined 

 gravel streams traversing the old fans. 



Over most of the arid regions the winds blow away the fine 

 soil at the surface until there is left a layer of closely set 

 pebbles which act as a protection to the further action of this 

 kind. The fact that the plaiss deposits are largely adobe 

 instead of coarser materials, as one would naturally expect 

 from surface appearances and from long experience in humid 

 regions, is at first very surprising. That the deposits in ques- 

 tion are composed chiefly of fine clays is due mainly to the 

 effects of planorosion, or flood-sheet erosion as it is termed by 

 McGee. The deposits of this vigorous geologic process are 

 principally clays rather than sands and gravels. This is probably 

 an explanation for most of the extensive so-called Tertiary and 

 Quaternary "lake-deposits" of many portions of the West. 

 Planorosion is peculiar to arid regions. It is neither fluviatile, 

 nor lacustrine, nor maritime in nature, but sheet-flood erosion 

 in its strictest sense, modified by eolian influences during dry 

 times. 



