86 PROCEEDINGS OF THE CHICAGO MEETING 



formably on the sand. At Weston this boulder bed appears high up on the 

 Missouri River bluff at an altitude of 900 feet, near Tracy (Platte City sta- 

 tion) at 846 feet; near Hoover, 835 feet, and east of Smithville, at the mouth 

 of Crow Creek, at 800 feet — an average gradient of 10 feet per mile. The 

 boulders in this valley are mainly of local limestone, but some northern ma- 

 terial is present. Todd,'' noting the boulder bed at Weston, attributed it to 

 rapids of the Missouri at this point, but its relations show clearly that it is the 

 deposit of another stream that crossed, at right angles, the site of the Missouri 

 before the latter stream was formed. The stream that deposited the boulders 

 and superjacent bed evidently had access to glacial debris, but formed the 

 deposit before the region was glaciated. 



Glacial and postglacial Changes 



Of the changes in the ancient Platte River during glaciation the Missouri 

 work has so far revealed nothing, probably because the course of the "Platte" 

 was pushed far to the west in Kansas, as suggested by Todd.'^ The history of that 

 part of the present Missouri east of Kansas City during this interval is clearer. 

 With the Kansas River this part of the Missouri formed approximately the 

 southern limit of glaciation, though the ice pushed south of the present valley 

 in several places. In Jackson County are two abandoned valleys that seem to 

 ■ have been temporary outlets, and it is a significant fact that, so far as known 

 to the writer, wherever glacial till occurs south of the Missouri River it lies 

 north of one of these abandoned valleys. Another abandoned outlet appears 

 to have been near Grand Pass, Saline County, and there are possibly others to 

 the east, as there are known to be to the west, in Kansas. 



The factors that determined the present course of the Missouri between 

 Omaha and Kansas City are obscure and the writer has no suggestion to offer 

 at this time. That the river has occupied its valley for some time is evident 

 from the depth to which its rock floor has been eroded. From the data on 

 borings previously mentioned, it is known that erosion has cut down to ap- 

 proximately 100 feet below the existing floodplain, and the valley then filled 

 with that amount of clay, sand, and gravel. The bulk of this valley-filling 

 material may have been derived from the rapid melting of some late ice-sheet 

 in the northern States and the subsequent overloading of the Missouri. 



POST-ILLINOISAN DRIFT IN NORTHERN ILLINOIS WEST OF THE MAPPED 

 WISCONSIN MORAINE 



BY MORRIS M. LEIGHTON ^ 



{Abstract) 



As a part of the new program of the Illinois Geological Survey in the re- 

 examination of the Pleistocene of Illinois, recent studies have been made by 

 the author in northern Illinois where the drift has an uncertain status. Sev- 



* J. B. Todd: Formation of tlie Quaternary deposits. Missouri Geological Survey, vol. 

 10, 1896, p. 205. 



^ J. E. Todd : The Pleistocene history of the Missouri River. Science, new series, vol. 

 39, 1914, pp. 263-274. 



1 Introduced hy P. W. De Wolff. 



