1878.] Distribution of Timber and the Origin of Prairies. QI 
NOTES ON THE DISTRIBUTION OF TIMBER IN 
SOUTH -WESTERN IOWA, WITH INFERENCES 
CONCERNING THE ORIGIN OF PRAIRIES’ 
BY- -PROF -J E TODD. 
PON the bluff deposit of Western Iowa is found an unusually 
favorable field for testing the theories concerning the much- 
vexed question of the origin of prairies, or rather the origin of 
forests, for doubtless the former are necessarily the older condi- 
tions of most regions. 
The soil over wide areas is almost perfectly uniform, and so 
deep that no underlying formation can thrust in its influence to 
complicate the problem. The surface is almost infinitely varied; 
the high plain which is the summit of the loess, the low alluvial 
plains and hill-sides and bluff-sides presenting every conceivable 
angle of inclination, and dipping in every possible direction. To 
produce even greater variety, ledges of rock and knolls of gravel 
occasionally appear. 
L 
In such a region timber occurs in the following circumstances: 
(1.) In the hill-regions where the slopes are inclined from 5° 
to 10°, it is found much the most generally on the northern 
Slopes just south of creeks flowing east or west. This was noted 
some years since, by Mr. J. A. Allen. 
Timber is found in the same region a little less frequently on 
western slopes, east of creeks flowing north or south. On the 
same streams considerable timber may occasionally be found on 
_ the west side. All other portions of the hill region are uniformly 
destitute of trees. 
(2.) In the bluff region, where the slopes are from 10° to 45°, 
e 
E, 
a 
d 
through Mills and Pottawattamie counties, with a narrowing and 
_ western ridge of bluffs, leaving the slopes facing the bottom land 
_ bare, except in two well-marked cases; the first, when a lake, 
-~ 1 Read before the Iowa Academy of Science, September 26, 1877. 
