30 BIG SPRING PRAIRIE, 
This tree is situated in a low woods, adjoining the prai- 
rie and at the junction of a shallow drainage valley 
from West Ridge. It is about two rods distant from 
the ditch draining southern part of prairie. The tree 
is 59 inches in circumference just above the exposed 
roots. The amount of settling indicated is about 2% 
feet. Figure 3 shows another tree 36 inches in cir- 
cumference. It is situated in same woods, but a short 
distance up the slope of this natural drainage valley. 
The amount of settling at this point is about 24 inches. 
Considerable of this settling was due to the last deepen- 
ing of the outlet of this prairie ditch in 1897. 
Several wooded areas of this prairie, whose princi- 
pal genera are Ulmus, Fraxinus, and Acer exhibit a like 
tendency to prominent or exposed roots. Whenever 
the roots of trees are imbedded in a solid clay subsoil, 
and the soil settles, it will leave the upperroots ex- 
posed as illustrated in figures 2and 38. Thus, in part, 
we can read the history of the past of such areas by 
the present conditions. 
Since the settling of the soil tends to cause a rever- 
sion of the plant societies, it is of considerable interest 
to further investigate the subject of drainage and its ef- 
fects; especially as prairie fires which burn the soil as 
well as the vegetation upon it are, upon this prairie, a 
direct result of artificial drainage. Before artificial 
drainage was introduced, the burning of the soil did 
not occur, as the muck was surcharged with water. 
The enclosing ridges and the solid rock bed underlying 
the prairie renders this a peculiar kind of prairie. The 
edge of the prairie is lined by a number of springs, the 
natural outlets of the subterranean drainage of the 
ridges; the rock strata of these ridges, as before ex- 
plained, dipping toward the low ground and hence in 
this region toward the prairie, the subterranean water 
must rise in prairie muck until it rises sufficiently high 
to be drained off by the prairie ditches. For this reason, 
