CHAPTER XVIII. 



THE PLIOCENE PERIOD. 



The most distinguishing formational feature of the Pliocene is 

 its aggradation deposits. 1 This is a consequence (1) of the excep- 

 tional deformations which took place during the period, and just before 

 its beginning, and (2) of the recency of the deposition which has saved 

 the formations, to a large extent, from removal. There is little doubt 

 that similar deposits were made in similar amounts during and after 

 other periods of comparable deformation, but they have been largely 

 swept away by subsequent erosion. The Pliocene deposits will suffer 

 the same fate if the continent remains quiescent until another base- 

 leveling, like that of the Cretaceous, is accomplished. 



Simple and obvious as the method of terrestrial aggradation is, 

 and illustrated in a small way in almost every tract of diversified topog- 

 raphy, its results are less clearly recognized than those of most other 

 phases of sedimentation, and their identification, correlation, and pre- 

 cise interpretation are attended with difficulties much beyond those 

 which attend typical marine, lacustrine, and fluviatile deposits. Of 

 the major examples of Pliocene deposits of this class, those formed 

 in the intermontane basins, abundantly exemplified in the Great basin, 

 are the most obvious and unquestioned, though largely misinterpreted 

 as lacustrine deposits. Lacustrine deposits are, however, present 

 and extensive in this region. 



1 In its broadest sense, all sedimentary formations on land or under water are 

 aggradational, but deposits under seas and lakes have their own distinctive terms, 

 marine and lacustrine, and deposits made in the channels or on the flood plains of 

 rivers have their designations, fluvial or fluviatile, and alluvial. The term aggra- 

 dation is coming into use to designate a group of complex deposits that take place 

 on land partly by overburdened rivers, but quite largely by temporary streamlets, 

 slope-wash, " sheet-wash," and miscellaneous agencies that' remove material from 

 uplands and deposit it on flat lands, and it is in this sense that it is employed here. 



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