40 



BIG SPRING PRAIRIE. 



ing pass oflf 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 1893, 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 in ten 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 was an 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 



d. ^= — ^ 



c. 



PIGUEE 6. DIA&RAM OF CUBVE OF ADVANCE UNDER NATURAL DRAINAGE. 



