DR. Leonard's arguments 473 



the valley floor. Some of the railroad cuts show the boulder-clay to he 80 to 

 40 feet thick. 



"Before the time of the earlier ice-invasion, when the ice-sheet advanced 

 40 to 50 miles beyond the Missouri River, that stream must have flowed in 

 its present broad, terraced valley, and on the floor of this valley the glacier 

 deposited the boulders, gravel, and till so well exposed at many points. These 

 deposits, shown in the railroad cuts of the terrace, lie about 40 feet above 

 the ordinary stage of the river- and vary considerably in thickness." 



5. "Additional evidence that the present valley is preglacial, and that the 

 trench of the river was excavated to its present depth at the time of the 

 earlier ice-invasion, is shown by the boulder bed less than half a mile below 

 the mouth of Tobacco Garden Creek. This bed of boulders, which lies just 

 above the river level, is at least 12 to 14 feet thick and extends along the 

 water's edge for a distance of 100 yards, while scattered boulders and fer- 

 ruginous gravel occur at intervals for another 200 yards. Overlying the 

 boulders are 15 feet of gravel. 



"While some of the boulders of this deposit may have been brought here by 

 floating ice, it is probable that most of the deposit was left here by the pre- 

 Wisconsin ice-sheet when it advanced south of the river. The finer materials 

 of the drift, if they were ever present, have been carried away, leaving the 

 gravel and boulders." 



Dr. Leonard has recently noted very many other exposures showing 

 the relation of the drift to the earlier rocks.^ 



Discussion of Arguments 



dr. leonard's argumexts inadequate 



The arguments ottered seem insufficient to establish the position taken 

 by Dr. Leonard, for most of them are negative, not having a positive 

 effect upon the problem. 



NO TRACE OF OLD CHANNELS 



1. The first point is that no trace has been found of any important chan- 

 nel leading north or east^ and from the present knowledge of facts, viewed 

 from his standpoint, he concludes the evidence is strongly in favor of his 

 position. It should be remembered that there are few deep wells or drill- 

 holes on the Coteau du Missouri. Judging from similar areas else- 

 where, we may be very doubtful of this being a positive argument. Till 

 or boulder-clay is frequently found 200 to 500 feet in thickness, as, for 

 example, in northeastern South Dakota, on the Coteau des Prairies, or in 

 central Iowa, on the divide northwest of Des Moines. For aught we 

 know, there may be a buried valley 200 feet deep across the plateau east 

 of the Missouri River into the Souris Valley or James River Valley. 



3 Journal of Geology, vol. 24, no. 6. 



