546 J. E. TODD — PLEISTOCENE PROBLEMS IN MISSOURI. 



Moines and Mississippi, after their lower courses had been dammed by 

 the Illinois glacier either directly or by the deposition of its sediments. 

 Perhaps even before the plain was raised to its present level an overflow 

 may have begun down some previous channel corresponding to the 

 Mississippi between Quincy and the mouth of the Illinois. This outlet 

 was rapidly excavated by the latter streams, probably accelerated by the 

 northward elevation of the region. This same elevation threw the 

 western streams to the southern side of the plain and over the divide 

 into the valley of the Gasconade. These changes probabl}^ took place 

 subsequent to the deposition of the high-level loess and gray loamy clay, 

 and before the glaciers north had vacated the principal moraine. 



While this was going on the ravines were cutting back from the lower 

 Gasconade and Mississippi valleys through the dividing ridge on the 

 east. This process w^as naturally assisted by the seepage of water from 

 the higher level, the easy erosion of the lower Silurian shales, the porosity 

 of the Silurian sandstone, and the easy solution and cave-forming habit 

 of the Saint Louis limestone. Today this cutting back of ravines has 

 but recently reached the edge of the plain near Bowling Green and 

 Edgewood in Pike county, but we may suppose that in the axes of the 

 Cuivre and Salt rivers, drainage and erosion may have begun very early 

 though not before the loess had been mainly deposited. 



Objections to the Hypothesis. — Several difficulties also present themselves 

 in the consideration of this hypothesis. Why should there not be more 

 traces of stratification ? We are accustomed to conceive of all deposi- 

 tions formed by moving water as necessarily stratified. If there are no 

 exceptions to this rule we should expect, according to this hypothesis, 

 more or less stratification both in the bowldery and the loamy clay. 

 We have learned in the case of loess and silt that stratification is not 

 very manifest where all particles are very minute or where flocculation 

 comes in to modify. May not further observation and study reveal the 

 principle that if coarse and very fine material are both distributed at the 

 same time, at least with the aid of ice they may be deposited together 

 without marked signs of stratification ? Cases have been found in the 

 terraces of streams where unstratified pebbly till occurs in circumstances 

 where it cannot be clearly traced either to subglacial or lacustrine action. 

 Even without the aid of floating ice deposits of gravel are made upon 

 soft clay by rapidly flowing streams. Treacherous bars of this sort are 

 not very infrequent in clayey regions. In short, a velocity sufficient to 

 transport pebbles is not always able to remove finfe clayey material before 

 dropping the same. 



Why should there be no traces of high-level channels? According to' 

 this hypothesis we should expect traces of vacated channels in the struc- 

 ture of both the bowldery and the loamy clay. There should be evi- 



