SURFACE DEPOSITS. 87 



soil, composed of little besides gravel and scattered boulders ; 

 but such places are very small compared with the space 

 occupied by good soil, and would hardly attract attention 

 anywhere except in a region so fertile as Iowa. 



Boulders being more definite in their character than the 

 other materials of the drift, we are able to make more satis- 

 factory observations concerning their distribution. We find 

 that they are more abundant in some places than in others ; 

 and also, that those having a certain composition prevail in 

 some parts while such are entirely absent in others. Besides 

 this, we find that in one limited part of the State, boulders of 

 all kinds are almost entirely absent. The outline of this 

 boulderless region may be approximately described as 

 follows : 



Commencing upon the north line of Winneshiek county, go 

 southeastward to the center of the west line of Clayton 

 county, thence to the northwest corner of Jackson county, 

 and thence to the Mississippi river, near Clinton, the space 

 between this line and the Mississippi river, is the region referred 

 to. It is not entirely boulderless, but boulders are exceedingly 

 rare there. It is, however, by no means a driftless region; 

 because the other characteristic materials of the drift are 

 present, except upon some of the hills and in some of their 

 valleys near the great river. Occasionally the drift clays are 

 found to be of considerable thickness within this region. 



Going westward from this boulderless region, we find 

 inmediately bordering and parallel with it, extending from 

 Mitchell, to Cedar county, another region in which boulders 

 are unusually prevalent as compared with other parts of the 

 State. Even in this region, where boulders are most abun- 

 dant, they are never so numerous as to constitute a blemish 

 upon the fair surface, nor to cause any impediment worth 

 naming to farming operations. 



After crossing the Des Moines river we find, in western 

 Iowa, a variety of boulders that do not appear at all in 

 eastern Iowa. These are of red quartzite, identical with the 

 Sioux quartzite, and are distributed, in connection with the 



