192 NATURAL HISTORY BULLETIN 



pographic monotony of the plain. The forest areas within the 

 Wisconsin plain are almost restricted to these valleys, the outlying 

 groves being mere thickets confined to the borders of swamps 

 and lakes, or small groves developed on kame-like knobs or ridges. 

 It is a striking fact that where in the upper courses of streams no 

 valleys were cut into the plain no forest-border appears. 



East of the Wisconsin area the lowan drift, with a surface only 

 slightly undulating, presents similar conditions and the river val- 

 leys again offer the only shelter to groves. 



East of the lowan area a belt of Kansan drift extends from 

 Howard and western Winneshiek counties to Dubuque county. 

 This is typical rolling Kansan cut by streams and presents a 

 topography sufficiently varied to give opportunity for the de- 

 velopment of numerous groves. 



East of this Kansan strip, extending from eastern Winne- 

 shiek and Allamakee counties to Dubuque county, lies the so- 

 called driftless area. This is the oldest topography in the state, 

 and presents a very rough surface cut by deep and narrow val- 

 leys. This was one of the most heavily forested regions in the 

 state. 



The southeastern part of the state is occupied by the Illinoian 

 which is limited by a line drawn from Scott through Muscatine 

 and Henry counties to the southeastern corner of the state. The 

 western part of this area is morainic and sufficiently rough to 

 shelter numerous groves. The eastern part is cut by the Missis- 

 sippi and the bluffs on the west side afford a similar protection to 

 forests which developed both on the bluffs and on portions of the 

 adjacent alluvial plain. 



Westward, extending across the state and northward to its 

 northwest corner, lies the great Kansan drift area. This is also 

 an old surface, and is for the most part undulating, becoming 

 very rough in the south-central part of the state where the heavi- 

 est forest-growth also appeared. Locally and over larger areas in 

 the northwestern part of the state, the Kansan has been but little 

 eroded and presents flat surfaces, which were treeless. 



In the western part of the state the Kansan topography blends 

 with that of the Missouri valley bluffs. The latter extend from 

 Sioux City to Hamburg and portions of them are illustrated in 



