MARSTERS, PHYSIOGRAPHY OF THE PERUVIAN ANDES 243 



tion was predominant near the edge of the coastal chain and also in the 

 lower part of the Atico Valley ; hut it should be added, in the latter case, 

 that much of the upper part of the sediments may be post-Tertiary and 

 associated with glacial waste. At least, in the Atico-Caraveli section, we 

 see nothing of the oldland upon which these late sediments rest, until we 

 approach the vicinity of Caraveli. From these sections, it would appear 

 that the West Slope in this locality was never reduced, or worn down to a 

 grade, whereby even an approach to an even sky-line was attained, as 

 noted in our northern section, but was occupied by valleys and hills of 

 large dimensions when Tertiary sedimentation and volcanic eruption be- 

 came prominent phenomena. The pre-Tertiary foothills were only partly 

 buried in sediments, but later they became almost obliterated by the ac- 

 cumulation of mud flows of great thickness and areal extent. 



It is thus evident that the physiographic history of the West Slope is 

 by no means simple. To get a clear insight into the possible variations 

 and succession of events, sections should be studied in every department 

 and province facing the Pacific coast. 



In the main, however, I believe that we have noted the chief succession 

 of events and the principal factors involved, namely : 



(1) The development of an oldland surface upon Cretaceous and 

 probably older formations and corresponding to the western slope of the 

 West Cordillera. 



(2) Its elevation and deep incision of valleys. 



(3) The depression of the shoreward portion, which, during Tertiary 

 time, was in part at least covered with Tertiary deposits, and portions 

 of which now form the present coastal plains. 



(4) The differential elevation of the coastal plain deposits and the 

 extension of the drainage of the oldland to the present coast line. In 

 all probability, no small part of the original area of Tertiary deposition 

 may now be beneath sea level. 



HIGHLAND PLATEAU OR INTEKCOEDILLERAN BELT 



To the belt of country lying between the eastern and, western ranges of 

 the Andes, I have applied the terms Highland Plateau or Intercordilleran 

 Belt. These terms are used in the broader sense and for the need of 

 better ones. Although presenting a complex geologic history, it can be 

 traced from central and northern Bolivia throughout the entire length of 

 the Peruvian Eepublic and into Equador; its width, in general, varying 

 from 100 to 200 kilometers. 



