CHAPTER XXVIII 



THE PLIOCENE PERIOD 



FORMATIONS AND PHYSICAL HISTORY 



Subaerial Formations 



The most distinguishing feature of the Pliocene formations, so 

 far as the present continents are concerned, is the predominance of 

 terrestrial deposits. This is a consequence of (i) the exceptional 

 deformations which took place during the period, and before its 

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

 deposits, to a large extent, from removal. Similar deposits after 

 earlier periods of comparable deformation have been largely removed 

 by later erosion. These deposits of the Pliocene are perhaps 

 most obvious in intermontane regions such as the Great Basin. 

 They have by some been interpreted as lacustrine deposits, and 

 such no doubt exist; but over areas much greater than those oc- 

 cupied by Pliocene lakes, and over tracts which were never parts 

 of well-defined flood plains, broad aprons of detritus accumulated. 

 Most of the western mountains of America are flanked by such 

 deposits of Pliocene age, or younger. Pliocene deposits of this type 

 are doubtless concealed beneath later accumulations of a similar 

 sort in nearly all the large basins, and at the bases of nearly all the 

 steep slopes in the western mountain region. 



In the Mississippi basin, far from all mountains, there are 

 patches of gravel on various hills and ridges which are interpreted 

 as the remnants of a once more or less continuous mantle of river 

 detritus. Definite correlation of these gravels is not now possible, 

 and they may not all be of the same age. They are not older than 

 Cretaceous, and are older than the glacial drift. Their similarity 

 to the Pliocene gravels farther south suggests their correlation with 

 that formation. The material of these gravels, almost wholly 

 quartz, quartzite, and chert, is partly local, and partly from the 

 north. The leading topographic features of the Mississippi basin 



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