CHAPTER XXIX 

 THE PLIOCENE PERIOD 



The most distinguishing feature of the Pliocene, so far as the 

 present continents are concerned, is the predominance of terrestrial 

 deposits. This is a consequence (1) of the exceptional deformations 

 which took place during the period, and just before its beginn 

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

 to a large extent, from removal. Similar deposits in similar 

 amounts during and after other periods of comparable deformation. 

 have been largely removed by subsequent erosion. These dep< 

 of the Pliocene are perhaps most obvious in intermontane regions 

 such as the Great Basin. They have sometimes been interpn 

 as lacustrine deposits, and such deposits no doubt exist; but o 

 areas much greater than those occupied by lakes in Pliocene tii 

 and over tracts which never formed parts of well-defined flood plains. 

 broad aprons of detritus accumulated. Such accumulations 

 now most considerable at bases of mountains whose steep si- 

 join plains of low gradient, and where the climate is sub-arid, the 

 rain falling in sudden and copious showers, largely concentrated 

 on the mountain heights. The thirsty plains below, covered with 

 porous debris, quickly drink up the descending floods and strand 

 the detritus they bring down. Most of the western mount air 

 America are flanked by such deposits, some of which are of Plio- 

 age, and some younger. In lake basins, subaerial sediments nu 

 into lacustrine, and the two may be interstratified. The < lop- 

 in question merge so insensibly into flood-plain deposits that 

 cannot be separated from them in all cases; nor should they 

 since they are of the same essential nature. Pliocene deposi- 

 this type are doubtless concealed beneath later accumulate 

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

 all the steep slopes in the western mountain region. 



828 



