126 IOWA DEPARTMENT OF AGRICULTURE. 



have prevailed everywhere. The effect of the successive ice sheets which in 

 turn covered nearly the whole of Iowa, was to tone down and conceal the 

 preglacial, rock-carved topography by spreading over it a deep mantle of 

 drift. The drift-covered area occupies much the larger part of the state. 

 In this region the topography is young as compared with that of the drift- 

 less area; it is in no way related to geological structure; its characteristic 

 features are due partly to the manner in which the load of glacial detritus 

 was distributed and deposited by the ice, and partly to the effects of ero- 

 sion and other modifying influences acting on the mantle of loose materials 

 since the glaciers disappeared. There were, however, not less than five dif- 

 ferent episodes of ice invasion for Iowa, each of long continuance, and 

 separated one from the other by still longer interglacial periods, from which 

 it follows that among the different sheets of drift consequent on the suc- 

 cessive stages of glaoiation, there are enormous differences in age. The 

 glaciers of the later stages were not so strong and did not extend so far as 

 those belonging to the earlier part of the glacial epoch. On many accounts 

 it may be regarded as a fortunate circumstance that the geographical posi- 

 tion of Iowa was so exactly related to the magnitude and movements of the 

 later ice sheets that not less than three of them successively entered her 

 borders and terminated by melting before advancing over more than a small 

 fraction of her entire area. The terminal margins of these later glaciers 

 have been mapped with a high degree of accuracy, and it turns out, for- 

 tunately again, that the particular parts of the state which the invading 

 lobes of the later glaciers occupied, were not twice the same. The drift- 

 covered portion of Iowa presents four well-defined areas, each having at the 

 surface a sheet of drift differing in age and, to some extent, in origin, from 

 the drift of either of the others. Iq some places, as, for example, south of 

 a line drawn through Des Moines and Iowa City, the drift is very old; in 

 other places, as in the middle northern counties of the state, the drift is 

 very young. The topographic features of the several glacial areas vary 

 with their age. The older drift, which has been long exposed to the action 

 of weathering and drainage waters, has the upper zone profoundly changed, 

 and the whole surface has been carved into an elaborate system of drainage 

 trenches and deep stream valleys. The withdrawal of the latest ice sheet 

 from Iowa is an event so recent that the surface of the younger drift is yet 

 unaltered; it has not been affected in any way; it remains precisely as the 

 waning glaciers left it. 



The oldest glacial deposit known in the state does not appear at the sur- 

 face anywhere. It is effectually covered by the drift of tbe second ice inva- 

 sion, and is revealed only through the erosion of stream valleys and the 

 making of artificial excavations. The second glacial invasion and the result- 

 ing sheet of till have come to be known in geological literature as the Kan- 

 san. The Kansan ice, flowing in this region from the northwest, covered 

 the whole of Iowa except the small fraction belonging to the driftless area; 

 it extended southward half way across Missouri; it spread westward into 

 Nebraska and Kansas; eastward it joined other glaciers which radiated from 

 centers of accumulation into Labrador, and so formed a continuous sea of 

 ice reaching from central Nebraska to the Atlantic ocean. Outside of the 

 comparatively small areas occupied by the younger sheets of till, the Kansan 

 drift gives character to the surface of Iowa. The topography of the Kansan 



