BIG SPRING PRAIRIE. 41 
Horizontal lines indicate time. Vertical lines in- 
-dicate rapidity of filling up or change. 
a—First bulrush or tattail making its appearance. 
6 to c—Curve of advance from shallow lake to 
marsh. 
c te d—Curve of advance from marsh to semi-marsh. 
d to e—Curve of advance from semi-marsh to meso- 
phytic prairie. 
Under artificial drainage this curve must be modi- 
fied to meet the new conditions, and hence the follow- 
ing diagram will illustrate it: 
XEROPHYTIC — if 
Cc f 
MESOPHYTIC — Biers 
es 
fb 
Apa FOREN GT MC 9 oe a 
FIGURE7T. DIAGRAM OF ADVANCE AND RETROGRESSION UNDER ARTIFI- 
CIAL DRAINAGE, 
a—Appearance of first bulrush or cattail. 
a to b—Advance under natural drainage. 
6 to c—Effect of artificial drainage. 
c tod—Settling of soil and erosion. 
But the condition of greatest water content of soil 
will not be reached until the outlet ditch becomes com- 
pletely clogged up with plant remains whose accumu- 
lation will be greater here than where there is less 
moisture. Consequently a barrier retarding drainage 
will be built up and the conditions on the originally 
‘drained portion will for a time become more hydro- 
phytic as indicated by the curve ‘“‘d fo e.”” When “‘e” 
greatest degree of marshiness has been attained, the 
natural process of advance will again tend to carry the 
