WISCONSIN DRIFT 151 



pressing itself through the younger drift mantle. The surface records 

 simply the eccentricities of ice deposition, and nothing else. There are 

 many square miles of level plain without signs of knobs or ridges, but 

 with thousands of acres of shallow, reedy marshes and ten of thousands 

 of acres of flat meadow land, above which rise, with scarcely perceptible 

 slope, to a height of very few feet, the low swells which may be cultivated 

 under natural drainage conditions. Efforts at reclamation by means of 

 artificial ditches have been made in places with greater or less success; 

 but so broad are the level plains, so slight the grade, and so far away the 

 possible outlet for the ditch waters that the problem of drainage is a 

 serious and difficult one. Lakes ponded in depressions among the mo- 

 rainic knobs (plate 5, figure 2) and saucer-shaped kettle-holes, varying 

 from a rod or two to half a mile in diameter, distributed over the flatter 

 parts of the lobe, are other characteristic surface features of the Wiscon- 

 sin drift. 



There seem to be two phases of the Wisconsin represented in Iowa, 

 and these may possibly correspond to the Shelbyville and Bloomington 

 sheets differentiated in Illinois by Leverett. The well defined moraine 

 passing southward through Dickinson, Clay, Buena Vista, Sac, and Car- 

 roll counties marks the western edge of the ice during the latest phase of 

 the Wisconsin. But outside the moraine west of Euthven, in Osceola, 

 O'Brien, Clay, and some of the adjacent counties, there is an area occu- 

 pied by drift having all the characteristics of the Wisconsin, except that 

 there are few morainal features : and marshes and kettles are much less 

 numerous than in the inter-morainal portion of the lobe. This area be- 

 longs to the Wisconsin ; the drift may represent the earlier Wisconsin ; 

 but it is quite possible that the position of the moraine may indicate sim- 

 ply a recession and halt of the ice, and that the Wisconsin drift in Iowa is, 

 after all, but a single sheet. But whether there be one or two phases of 

 the Wisconsin, this last of our known drift sheets presents characteristics 

 of extreme youth. 



THE WISCONSIN COMPARED WITH THE KANSAN 



As you know, the old sub-Aftonian or pre-Kansan drift is exposed 

 only in stream valleys or in artificial excavations. It gives character to 

 the surface of no large area as do the several till sheets which followed it. 

 It was everywhere covered by the widespread mantle of Kansan drift, and 

 erosion and weathering, so far as it was concerned, came to an end at the 

 time of the second ice invasion. In the case of each of the other drifts, 

 there are extensive areas over which they have been exposed to the action 

 of the atmosphere and drainage waters ever since they were deposited; 



