40 BIG SPRING PRAIRIE. 
ing pass off as gases, and thus a correspondingly less 
amount of humus is accumulated. 
The rapidity of the above mentioned process was 
strikingly exhibited by a pond on the Lawrence Straw 
farm in the Sandusky River Valley. This pond came 
under the writers observation in 1895. At that time 
there was still a small area of clear water in the center 
for the greater portion of the year. The remainder of 
pond was covered with quite shallow water, in which 
there flourished a rank vegetation of cattails, calamus, 
and various sedges. At present there is no longer any 
clear area of water, at any time of the year; and the great- 
er portion of the original pond is now above the perma- 
nent water level for by far the greater portion of the 
year. This advance has been made inten years. As 
this dead pond is enclosed on two sides by high hills, 
and as several deep gullies discharge their waters into 
this area, the erosive material carried into it wasan im- 
portant factor in its rapid advance. But the like factor 
existed formerly in regard to Big Spring Prairie, as West 
Ridge fills in the inner curve of the bow, and North 
Ridge extends along its eastern border. The present 
broad, shallow drainage valleys were formerly narrow 
and deep gullies carrying into the old lake a large 
amount of silt consisting of sand and clay, which were 
deposited in different places in varying proportions. 
The following figure will illustrate the foregoing 
graphically under the condition of natural drainage: 
XEROPHYTIC 
MESOPHYTIC 
HYDROPHYTIC 
FIGURE 6. DIAGRAM OF CURVE OF ADVANCE UNDER NATURAL DRAINAGE, 
