836 



GEOLOGY 



mittently through the Pliocene, though the movements which toolf 

 place during the period are not always distinguishable from those 



of earlier and later times. De- 

 forming movements often extend 

 through long periods, and the 

 Pliocene movements were in many 

 places probably no more than 

 continuations of those begun 

 earlier. 



About the close of the period, 

 movements were extensive and 

 great, resulting in increased height 

 of land. The interval of active 

 erosion which followed is some- 

 times known as the Ozarkian or 

 Sierran epoch. 1 The region cov- 

 ered by the Lafayette formation 

 was somewhat higher than now, 

 and in reaching this position, was 

 perhaps somewhat deformed. The 

 coast line was probably farther 

 east than now, perhaps at the e 

 of the continental shelf, aci 

 which streams may have flowed. 

 To this epoch, the submerged 

 continuations of the St. Lawrence, 

 the Hudson, the Delaware, the 

 Susquehanna, and the Mississippi 

 are commonly referred. Some 

 of these submerged valleys 1 

 great depth, and it has i 



Fig. 556. Map showing supposed 

 distribution of land and water 

 on the Pacific coast of the United 

 States during the Pliocene peri- 

 od. (Ralph Arnold). 



assumed that their depth was a measure of the elevation of the i;md 

 when they were excavated. But if the considerations set forth in 

 Chapter XXXI have force, it is not necessary to postulate such 

 extraordinary changes of level by uplift and depression. < 



Hershey, Science, Vol. Ill, p. 620, 1896, and LeConte, Jour. Geol 

 VII, p. 529. 



