IOWA ACADEMY OP SCIENCES. 
71 
south and southwest, and also to the west and northwest across 
the Big Sioux river into South Dakota, he may locate almost 
every one of the little ravines with which the slopes bordering 
the deeper ravines are scarred, by the dark lines of bur oaks. 
The number of these smaller ravines which are tributary to 
some larger one is often so great that a pinnate arrangement of 
these dark lines results. The shrubs in that vicinity are found 
generally on the northern and eastern slopes, where they are 
best protected from the prevailing strong southwesterly winds, 
and the surface soil on these slopes is much deeper and finer, 
and is also covered with a richer fiora. On the other hand, 
many of the western and southern slopes are strewn with 
granite boulders, and a scant vegetation barely covers the sur- 
face, which almost lacks a finer soil— stratum number 3 form- 
ing the surface. These groves would probably have formed 
nuclei of greater forests had not man interfered, for, in the 
northern part of the state at least, the bur oak seems to be 
the pioneer among trees, being followed by the red oak, which 
now forms the greater part of our northern and northwestern 
upland groves. 
The conclusion seems warranted that while soil largely 
determines the character of a fiora, the converse is equally 
true that the fiora will in time affect the character of the soil, 
and that the influence of vegetation upon superficial geological 
changes should not at least be disregarded. 
The conclusions drawn from the observations here briefly 
recorded are the following: 
1. The boulder-bearing stratum marked 3 formed the sur- 
face at one time throughout the region studied. Before vege- 
tation had taken possession of it the finer material was sifted 
from the upper part of the stratum, concentrating the boulders. 
2. Subsequestly a comparatively scant vegetation took pos- 
session, making possible the retention of a somewhat coarse 
soil, — stratum 2. 
3. A richer vegetation then followed, enabling the retention 
of a finer soil, — stratum 1. 
4. Forests, where occurring, followed next in order, being 
ushered in in the manner suggested by the present bur oak 
scrub-tracts. 
5. The agency concerned chiefly in accumulating the finer 
surface soils was wind, the material being retained in place by 
vegetation. 
