GRASSES OF IOWA. 363 



No large bodies of water touch the borders of the state and none 

 but small lakes are found within it. The region is one largely fashioned 

 by the action of running water, and the river valleys form, throughout a 

 large portion of it, the natural physiographic features. One other agent 

 has been important, almost as important, in moulding the present sur- 

 face contours; this agent was ice in the form of vast glaciers or ice sheets. 

 In that portion of geologic time known to the geologist as the Pleistocene 

 or most recent a large portion of North America, including nearly all of 

 Iowa, was buried below the slow moving ice sheets which crept down and 

 over it from the north. Any study of the physiography of the state must 

 be largely concerned with the results of this invasion. From this point of 

 view the state is divided into three separate districts each with certain 

 characteristics common to its whole extent, and each again divisible into 

 smaller areas, if minor differences be taken into account. These areas 

 are (a) the driftless, (b) the region of the older drift, and (c) the region 

 of the younger drifts. The first occupies the extreme northeasten por- 

 tion of the state, the second lies mainly in the southern portion, and the 

 third includes the remaining area. The approximate limits of these 

 divisions is shown on the accompanying map. 



THE DRIFTLESS AREA. 



The driftless area includes portions of Allamakee, Winneshiek, 

 Clayton, Dubuque and Jackson counties, with a very considerable por- 

 tion of southwestern Wisconsin, and smaller parts of Minnesota and Illi- 

 nois. It is a region of strong characteristics and of marked individuality 

 when compared with the country surrounding it on all sides. It stands 

 in sharp contrast with the drift-covered country. The land forms seen 

 in the driftless area are almost exclusively due to river erosion. The 

 exceptions are certain ridges and sand dunes which have been heaped up 

 by the winds. The Iowa portion of the area lies on the west side of the 

 Mississippi river, and is thoroughly cut up by the numerous tributaries of 

 that stream. The streams have taken full possession of the area. The 

 larger ones have cut to grade and the work of reducing the inter-stream 

 divides is being actively pushed. From the crest of the encircling range 

 of dolomitic cliffs to the sandy flood plain of the Mississippi is an interval 

 of 6oo feet. A? the distance is rarely more than thirty miles, the streams 

 have high gradients and are busy in the preliminary cutting characteristic 

 of young, torrential streams. Sharp mural walls one hundred to two 

 hundred feet deep are common in the southern portion of the area. To 

 the north, owing to the rise of the strata, the valleys are still deeper. The 

 Oneota or Upper Iowa has cut 350 feet below the immediately adjacent 



