FOURTH ANNUAL YEAR BOOK — PART II. 12 < 



has been developed by erosion of the drift mantle. Valleys have been cut 

 in the loose glacial clays to depths ranging from eighty to two hundred feet. 

 The great age of the Kansan valleys is further indicated by the fact that they 

 are broadly U-shaped, and their sloping sides are trenched by numerous 

 lateral channels which branch and re-branch repeatedly until traced to their 

 origin in a multitude of minute twigs up on the divides. The whole surface 

 has been carved and shaped by flowing water and developed into an intri- 

 cate system of rounded hills and ridges separated by steep-sided ravines. 

 (PI. I, Fig. 1.) Every foot of the surface is thoroughly drained. While 

 the Kansan areas everywhere present the same fundamental type of topog- 

 raphy, the erosional features are probably most strikingly developed in the 

 counties drained bv the forks of the Grand. Nodaway, Nishnabotna and 

 other rivers of southwestern Iowa. 



It is a wholly dififerent type of topography from that noted above, that is 

 seen between Wilton and Walcott, around Morning Sun and Mediapolis, 

 between West Burlington and New London. These points all lie in an area 

 of drift which rests upon and overlaps the weathered and eroded surface of 

 the Kansan. An ice sheet having its origin in the Lauren tian highlands 

 south of Hudson Bay, flowed outward until it crossed Illinois and pushed 

 over for a short distance into Iowa. This was the Illinoian stage of glacia- 

 tion, and the detritus left on the surface when the ice melted is the Illinoian 

 drift. The Illinoian drift is more or less trenched around its edges; near 

 the larger river valleys, as between Walcott and Davenport, it has been 

 carved by erosion so as to develop young, narrow and steeply graded 

 ravines. But over the greater part of its area the surface is unchanged; the 

 topographic features are due, not to the carving effect of drainage waters, 

 but to the leveling and moulding influences of glacier ice. The drift of this 

 small area in southeastern Iowa is young as compared with the Kansan. 



The flow of the Illinoian ice across the Mississippi river into Iowa is 

 responsible for another interesting bit of topography. Nichols is located in 

 the midst of a level undrained area, the bed of an extinct lake. The Illinoian 

 ice choked up the channel of the Mississippi from the mouth of the Wapsi- 

 pinicon to the mouth of the Des Moines, and the waters of the great stream 

 were diverted around the glacier front. . Southward from West Liberty and 

 Atalissa there was a low, wide basin which was hemmed in on one side by 

 the high bluffs seen a mile or two west of Nichols, and on the other side by 

 the thick margin of the Illinoian ice. The waters were ponded in this basin 

 and formed an extensive glacial lake in which sediments composed of mud, 

 sand and gravel accumulated. When the ice melted and the Mississippi 

 returned nearly to its old course, the lake was drained, but the level floor of 

 sedimentary deposits remains to bear testimony to former conditions. Lake 

 Calvin, the name given to this ancient body of water, has been mapped and 

 described by Udden; the level floor of the old Illinoian glacial basin attracts 

 the attention of all observant travelers between West Liberty and Columbus 

 Junction. 



Embracing Buchanan, Black Hawk, Bremer, Chickasaw, Mitchell and a 

 number of the other counties in northeastern Iowa, is an area of what is 

 known to geologists as the lowan drift. The evidences of newness, of 

 youth, are much more strongly marked in the lowan than in the Illinoian 

 drift. There has been no alteration of the till and practically no erosion of 



