96 BIG SPRING PRAIRIE. 
depth of muck is decreased to such an extent that roots 
of trees can penetrate to a firmer stratum. 
10 Trees require a firm support for their roots. 
Consequently, successful tree growth can not occur 
where muck is deep and loose. 
11. Order of tree encroachment upon prairie was 
as follows: 
(a) On limestone island. 
(b) On sand dunes or beaches. 
(c) On clay islands or clay banks. 
(d) Along drainage ditches. 
(e) On severely burned areas. 
12. Portions of this prairie were formerly wooded, 
as is evident from the buried forests. Forests probably 
destroyed by flooding on account of construction of 
beaver-dams. 
13. The highest portions of natural prairie possess 
the most distinctive marsh characters. 
14. Parallel ditches at intervals of four rods 
needed to drain this area properly for the successful 
cultivation of the best paying crops. 
