CLIMATE AND TERRESTRIAL DEPOSITS 375 



also recognize the influence which changes in precipitation and 

 temperature may have had upon the rate of erosion.^ Such shif tings 

 of the gravels as were produced by the bowing would result in a 

 progressively younger age of deposits found in going down the streams 

 from the axis of bowing and therefore the Lafayette deposits of sepa- 

 rated districts may not be of strictly contemporaneous origin. 



As facts which have a bearing upon the relative influences of tec- 

 tonic and climatic causes, it may be pointed out that according to 

 Hershey erosion following a Pliocene uplift opened out basin valleys 

 in the Ozark Highland from 75 to 100 and even 300 feet below the 

 main Tertiary peneplain. These valleys are floored with Lafayette 

 gravels which Hershey states represent the end (italics introduced) 

 of deposition of the Lafayette formation, but the evidence for the 

 latter statement is not given. ^ Following the development of these 

 valleys another uplift, reaching in southern Missouri to at least 

 several hundred feet, enabled the streams to excavate the inner canyon 

 valleys of Ozarkian age. In this locality, then, the Lafa3'ette appears 

 to have been deposited at a time of crustal stability between two 

 stages of uphft. While its presence could be explained by either a 

 tectonic or climatic hypothesis, the latter is perhaps the easier, 

 since at a time of crustal stability a climatic instability, by vary- 

 ing the ratio of erosion to transportation, could make itself most 

 readily felt; now by an excess of erosion depositing material near 

 the headwaters and steepening the grade, again by an excess of 

 transporting power shifting the same to a lower portion of its course. 

 Such a climatic instability marked the close of the Tertiary and 

 increased in intensity until it resulted in the tremendous climatic 

 reversals from glacial to interglacial intervals which marked the Pleis- 

 tocene. 



The chief criticisms which may be brought against this view lie 

 in the entire lack of glacial material in the composition of the Lafayette 

 formation and the good evidence of nearly related crustal movements 

 of greater or lesser magnitude which affected the continent at about 

 this time. But in continental interiors it is only land warpings 



1 Op. cit., pp. 305-8. 



2 "Peneplains of the Ozark Highland," American Geologist, Vol XXVII, 1901, 

 PP- 33. 41- 



