THIRD INTERVAL 143 



such as gophers, ants, and earthworms; and changes may go on with 

 comparative rapidity. Whatever the time required to produce weather- 

 ing to a depth of 4 feet, the time required to bring about changes to a 

 depth of 8 feet will be very much more than twice as great. The Kansan 

 is altered to an average depth of more than 8 feet ; the weathered zone of 

 the Illinoian rarely exceeds 4 feet ; again we conclude that the Yarmouth 

 interval was more than equal to all post-Illinoian time. 



Third interglacial Interval, the Sangamon 



Another interglacial interval, the Sangamon of Leverett, followed the 

 withdrawal of the Illinoian ice. There are no very satisfactory deposits 

 of Sangamon age in Iowa, but the interval is very clearly represented at 

 a number of points in Illinois. This interglacial horizon, like the 

 Aftonian and the Yarmouth, is indicated by its buried forests, its soil 

 and weathered zones, its pond silts, and its peat beds. One of the most 

 instructive sections showing the relations of the Sangamon stage is seen 

 in a railway cut on the Toledo, Peoria and Western railway, 7 miles east 

 of Peoria. The exposure is described by Leverett in his monograph on 

 the Illinois glacial lobe, and is illustrated in plate XI, figure B, opposite 

 page 128, of the work cited. The section shows in ascending order (1) 

 typical yellow Illinoian till; (2) stratified silt, evidently laid down in a 

 quiet, shallow pond; (3) peat, with great quantities of tamarack roots, 

 recording conditions which followed the silting up of the pond and its 

 conversion into a marsh; (4) loess, probably Iowan or early Peorian; 

 (5) Wisconsin drift, and (6) Wisconsin gravels. Numbers 2 and 3 rep- 

 resent the Sangamon. The weathered zone at the top of the Illinoian, 

 and underneath loess which may be of Peorian age, is a feature seen at 

 numberless points in Iowa as well as in Illinois, and represents changes 

 which were wrought, in part at least, during the Sangamon interval. 

 Muscatine, Montpelier, and Davenport may be named among the Iowa 

 localities where this phase of the subject may be studied. At the brick- 

 yard east of Mud creek, in Muscatine, the Illinoian is overlain by beds of 

 stratified sand which, there is little doubt, belong to the Sangamon; 

 they may represent, however, only the beginning of the Sangamon, the 

 melting phase of the Illinoian ice. In turn the sands are overlain by 

 loess. 



Iowan Drift 



extent of the iowan 



Following the Sangamon interval there was a recurrence of glacial 

 conditions, and the Iowan drift, the fourth of the known till sheets, was 

 distributed over a portion of the previously glaciated area. The Iowan 



