8o STUDIES FOR STUDENTS 



and is chiefly deposited on the lower slopes or in the flatter por- 

 tions of the valley. The secondary deposit is therefore chiefly a 

 low slope deposit and a valley-bottom deposit. It therefore con- 

 forms to the natural configurations of the valley ; indeed, the 

 new deposit is the very element that gives to the valley its proper 

 configuration under the new conditions. It therefore blends 

 quite perfectly into the common configuration of the new and old 

 surface formations, and the distinction between the new and the 

 old thus blended in configuration is pre-eminently calculated 

 to escape observation. 



The formation does not arise from simple displacement or 

 reworking of any one deposit, but is a recomposition of material 

 derived from everything exposed to wash in the portion of the 

 valley undergoing erosion at the time. It may be in itself a 

 consistent formation with definite characters and quite free from 

 signs of post-depositional disturbance. As its material was 

 derived from that which prevails on the upper slope of the same 

 valley, it is very liable to resemble it more or less closely, and 

 this, taken with the harmonious topographic configuration, adds 

 to its liability to be confounded with the original deposit. For 

 example, in a valley whose upper slopes 'are mantled with loess, 

 with few exposures of older formations, the slope wash is neces- 

 sarily loess-silt in the main, and the recomposed formation on 

 the lower slope or in the valley must be made up of similar silt 

 merely modified by exposure and transportation. The older 

 formations can only make such contributions to the new deposit 

 as their exposure and erosibility permit, which in the case sup- 

 posed — a common case — is small. The new aggregation must 

 necessarily bear a close general resemblance to the original loess 

 and may easily be mistaken for it. 



The criteria for discrimination are found in the effects of 

 partial weathering, in the element contributed by the older 

 formations, and in the structure of the new aggregation, besides 

 the special features which most individual deposits possess as 

 their personal characteristics, so to speak. In a valley whose 

 upper slopes are till or other form of drift, similar recomposi- 

 tion formations of an illusive nature take place. 



