GRASSES OF IOWA. 367 



heaped up the loose loess which forms the surface material, till sharp, 

 conical peaks, precipitous bluffs and choked river valleys abound. This 

 topography is excellently shown in the vicinity of Sioux Citv and Council 

 Bluffs. 



THE YOUNGER DRIFT. 



Approximately the northern half of the state is covered by the newer 

 drift. It is in this region that the marks of the ice are most abundant. 

 Across the central portion of this northern area are the marks of a great 

 tongue of ice which crossed the state line with a front stretching from 

 western Worth county to Osceola, and extended down to the present site 

 of Des Moines. This was the last ice sheet which invaded Iowa, and the 

 heaped-up, moranic knobs around the border and within the area covered 

 by it are as fresh and sharp as many moraines of living glaciers. The 

 country is marked by many lakes, and much undrained territory. The 

 hills are rough and irrelgularly placed, and have no constant relations to 

 the streams. Much of the country is wholly uninvaded by streams, and 

 consists of great, level flats dotted by shallow bogs and small swamps set 

 with water-loving grasses. Everywhere there is evidence of the recent- 

 ness of the ice invasion. 



East of the moraines which border this area from the north state line 

 to Hardin county, and stretching from them to the narrow border of 

 older drift west of the driftless area, is a region which seems not to have 

 been so recently occupied by the ice; the lakes have disappeared, and un- 

 drained areas are few. There are no moraines proper, though along much 

 of the border there are peculiar ridges and heaps of loess which are in a 

 measure genetically akin to them. The river valleys are very broad and 

 very shallow. The stream courses are erratic, and the drainage basins un- 

 balanced. The streams occasionally turn aside from a broad, even plain to 

 cut through clusters of high, rocky hills. One frequently ascends from 

 the plain when approaching the river valley, though the stream itself is 

 of course below the level of the plain. The peculiar hills, the strange 

 behavior of the rivers, and many minor factors indicate that it has not 

 been long that the ice has left the country to the fashioning of the rivers, 

 but the amount of work which the latter have done indicates that the 

 period has been appreciably longer than in the north-central part of the 

 state. 



In the northwestern portion of the state, west of the moraine and 

 from Sac county to the Minnesota line, is an area which is in many ways 

 to be linked with that just described. There are no corresponding loess 

 ridges, so far as the area has been investigated, and the stream courses are 



