ABSTRACTS OF PAPERS 101 



recession of the ice-sheet, and the ensuing second very high stage of the lakes 

 occurred probably during the culmination and gradual disappearance of the 

 ice-fields. This second and less prolonged part of the Glacial period included 

 the maximum Kansan and Illinoisan stages, with the ensuing Iowa and Wis- 

 consin glaciation, being terminated when subsidence of the formerly high ice- 

 covered land brought the final melting of the continental glacier, its departure 

 being attended by the formation of very remarkable terminal and recessional 

 moraines. Time ratios, based on studies of the Falls of Saint Anthony and 

 Niagara by Winchell, Gilbert, and Wright, with De Geer's measurement of the 

 late Glacial stages in Sweden, demonstrate the geologically recent end of the 

 Ice Age and suggest for its entire duration an estimate of about 250,000 years. 



Presented by title in the absence of the author. 



GLACIAL FORMATIONS ON TEE COTEAU DES PRAIRIES 

 BY FRANK LEVERETT 



(Abstract) 



The Coteau des Prairies leads from the head of Big Sioux River, in South 

 Dakota, southeastward across southwestern Minnesota into Iowa, and finds its 

 uatural continuation across Iowa in the high divide between the Missouri and 

 Mississippi drainage. The crest is about 2,000 feet above sealevel in South 

 Dakota, but declines southeastward to about 1,600 feet at the Minnesota-Iowa 

 line and to about 1,300 feet in southern Iowa. The highest known altitude of 

 rock formations along it is about 1,700 feet; but this altitude is attained in 

 only a few places, usually where the Sioux quartzite is prominent. Along 

 much of its course the glacial deposits are 500 to TOO feet thick. In western 

 1 owa they range from 300 to 700 feet. 



The Coteau is so situated as to embrace the younger as well as older drifts. 

 The Wisconsin drift reaches a little beyond the crest of the Coteau in South 

 Dakota and Minnesota, but falls a little short of reaching the divide in Iowa. 

 This drift forms but a small part of the glacial material, probably an average 

 of less than 50 feet. 



A drift of debatable age, but apparently somewhat older than the Wisconsin 

 drift, and referred provisionally to the Iowan stage of glaciation, extends a 

 few miles beyond the undoubted Wisconsin drift in South Dakota, Minnesota, 

 and Iowa and covers the Missouri-Mississippi divide nearly half way across 

 Iowa. Its border passes under the Wisconsin near Wall Lake, Iowa. At the 

 north it passes under the outer moraine of the Dakota, or James River, lobe 

 near Florence, South Dakota. This drift is thinner than the Wisconsin and 

 has an average thickness in its exposed portion of not more than 20 feet. 



The next older drift is the Kansan, and this is found to have a thickness of 

 50 feet or more in places where least eroded, but its original thickness appears 

 to have been less than 100 feet. 



The Kansan and later drifts appear to embrace only 100 to 150 feet of the 

 300 to 700 feet of drift on the Coteau. The pre-Kansan drift thus has a much 

 greater thickness than all the later drifts combined, the average thickness 

 being not less than 250 feet and the greatest thickness over 500 feet. 



Presented extemporaneously. 



