2S6 STUDIES FOR STUDENTS 



under the most unlike conditions of temperature and rainfall, such, 

 for instance, as the deposits upon the deltas of the Indus, the Amazon, 

 and the Yukon, it is perceived that the climatic conditions existing 

 over the surface of deposition form a factor of primary importance in 

 governing the nature of fluvial and pluvial deposits. A proper inter- 

 pretation of ancient continental sediments cannot be made, therefore, 

 if the climatic factor be neglected, and on the other hand the deposits 

 of ancient delta surfaces should contain a more or less accurate record 

 of the climatic conditions of origin. But a closer inspection of modern 

 deltas shows that different parts of the same delta surface exhibit 

 markedly different conditions of deposition ov^ing to the somewhat 

 varying local geographic features, such as lakes, swamps, and natural 

 levees, which may exist. In desert regions, as exemplified by the 

 delta of the Helmund, the great internal river of Persia, portions not 

 now used by the river are barren sandy deserts subjected to deep wind 

 scour, occasionally in this instance exposing ancient ruins, and the 

 corresponding heaping-up of aeolian deposits, while at the same time 

 such shifting, shallow, and variable lakes as that of Seistan, the reservoir 

 into which the Helmund drains, are giving rise to interstratified, truly 

 lacustrine deposits.^ 



Therefore in the climatic interpretation of ancient continental 

 deposits proper allowance must first be made for the geographic 

 variations which occur within the region of deposition. The field 

 studies must be sufficiently broad to lead to some recognition of the 

 ancient geography before the ancient climate may be determined. 

 To that end the modifications in deposit due to the influence of the 

 local geographic conditions will first be discussed. 



Influence of Nature of Surface of Deposition 



PIEDMONT slopes — AERIAL DELTAS 



These are more usually formed in arid or sub-arid climates, 

 since in such the river waters progressively escape, after leaving the 

 mountains, into the thirsty soil and air. The overloaded stream con- 

 sequently throws down its burden of waste upon a slope which may 

 vary from loo feet per mile in the case of the coarser and steeper fans 



I Col. Sir Henry McMahon, "Recent Survey and Exploration in Seistan," The 

 Geographical Journal of London, Vol. XXVIII, 1906, pp. 209-228. 



