DISCUSSION OF ARGUMENTS 475 



later stages of the flood following the Wisconsin stage of the ice-sheet. 

 At that time the Missouri was at a very high flood stage and sufficient 

 to scour and fill to that depth. We can not conclude more than that the 

 deposits were later than the beginning of the Glacial period. 



GLACIAL GRAVELS IN TERRACES 



3. He appeals to the occurrence of drift gravel deposits as affording 

 evidence of preglacial origin of the valley in which they are found. 

 Much that has been said in the last section may be applicable here. To 

 show that the mere presence of stream deposits, including glacial boulders, 

 does not prove the presence of ice in the close vicinity, much less at the 

 identical locality, drift boulders and gravel may be found scores of miles 

 from the edge of the ice which brought the boulders. 



GLACIAL TILLS 



4. The occurrence of typical till or boulder-clay at any point is con- 

 sidered good evidence that the ice has been at that point at a time begin- 

 ning with the age of the bottom of the till; for it is generally conceded 

 that till is on the immediate edge or presence of an ice-sheet. If Dr. 

 Leonard can establish the existence of typical till on the terrace near 

 Hensler, we are bound to believe that an ice-sheet reached that point and 

 remained there long enough to deposit the depth of the till there found ; 

 but this may be true of the locality near Hensler without proving that 

 the valley is the Missouri Yalley. To one who suspects that the Missouri 

 was formed as was outlined in previous section, the finding here of a 

 valley deeper than usual may be plausibly ascribed to the valley of a 

 river leading northeast, which from its greater age or more active drain- 

 age had excavated deeper than adjacent valleys. There is nothing im- 

 probable in the different valleys crossed by the course of the Missouri 

 varying greatly in age and degree of erosion. Why may we not infer 

 that preglacial Heart Eiver turned north in the vicinity of Bismaick 

 and drained independently, or with Knife Eiver as a tributary, into the 

 Souris Eiver ? Such conditions would favor more rapid erosion. More- 

 over, the material in which the basin of the Knife Eiver is excavated is 

 largely composed of soft beds of the Laramie. 



BOULDER BAR NEAR MOUTH OF TOBACCO GARDEN CREEK 



5. He argues that beds of boulders near the mouth of Tobacco Garden 

 Creek indicate the preglacial formation of the valley in which they occur. 

 But boulder bars, as they may be called, in river beds are formed not very 

 uncommonly. They, of course, are evidence that they have been formed 



