THE PEAIEIES 193 



plate IV, figure 2. and plate VII, figure 1. This bluff region is 

 very rough, but as already noted, the exposed western faces are 

 treeless, while the groves appear in sheltered places only. 



The flat alluvial plain of the Missouri, varj-ing from two to 

 eighteen miles in width, also shows fringes of trees along the 

 tributary streams, and groves of the more or less xerophytie Cot- 

 tonwood on the sand-dunes and sandy flats. 



The absence of trees from the upper courses of many of the 

 streams and from the valleys of the western streams, and. indeed, 

 the distribution of forest and prairie in Iowa, is entirely consist- 

 ent with the argument that prairies are due to exposure to the 

 chief evaporating agents. The more extended flat areas are all 

 treeless. Along the upper courses of the streams in the VTiscon- 

 sin and lowan areas, and othei-s similar to them, no deep valleys 

 have been cut. and no shelter is offered to groves. The drainage 

 of the state suggests an explanation of the comparative abund- 

 ance of trees in the eastern and southern parts of the state, and 

 their absence from the river valleys in the western part. The 

 streams in the eastern and southern areas flow toward the south- 

 east, or their valleys are so short and tortuous that they are not 

 exposed to the prevailing summer southwestern winds. Their 

 valleys are therefore more or less sheltered, and trees thrive in 

 their less-disturbed and hence here more moist atmosphere, for 

 the wind quarters the valleys. 



But in the western part of the state the principal valleys extend 

 toward the southwest and as they are comparatively straight the 

 wind sweeps their entire length without hindrance, and as a re- 

 sult of this, combined with exposure to the sun. no forests appear 

 on the flat bottom lands, but such groves as do occur are in the 

 sheltered ravines and pockets, often at a considerable altitude 

 above the plain. 



In many places there are northern and eastern slopes which are 

 almost devoid of groves. Sometimes this is due to the fact that 

 local topography causes a lateral deflection of the prevailing 

 winds in such a manner that these slopes are more frequently 

 swept by them. The deflection is sometimes downward, as along 

 surfaces which slope downward gradually toward the north. In 

 such cases the currents of air follow the sloping surface in accord- 



