usually a very porous gravel, and it is thought by some that a hard-pan 

 layer must have been formed near the surface to prevent the water 

 from sinking in. The flats are now drained by dredged ditches, and 

 the area is devoted to agriculture. The map shows the relation of the 

 level areas and the islands or sand areas, and the general appearance 

 suggests the manner in which the floods from the melting glacier 

 spread the alluvial material over the broadened valley. The Macki- 

 naw River has had a share in the work of deposition ; its drainage 

 channel may at one time have proceeded by way of Crane Creek into 

 the Sangamon. 



The island between Crane Creek on one side and Quiver Creek 

 and the Black-jack Ditch on the other, is not so clearly defined as the 

 one on the other side of Quiver Creek, nor has its northern end been 

 studied. The writer has been through most of its southern half, and 

 the boundaries given on the map for it are fairly accurate. The other 

 large island, lying between Black-jack Ditch and the river, is very 

 sandy, especially near Saidora. The other parts of the same region 

 have not been studied. 



PHYSICAL FACTORS OF THE SAND PRAIRIE ENVIRONMENT 



The physical factors of a land environment have been classified by 

 Schimper ('03 : 1 59-161) into two groups — climatic (geographic) 

 factors, which operate over very broad areas, and edaphic (local) fac- 

 tors, which effect local modifications of the plant life (and the animal 

 life as well) within any such broad region. The ecological type of 

 the vegetation (whether forest, grassland, or desert) is primarily de- 

 pendent upon the climatic factors — temperature, rainfall, and humidity 

 of the air. The ecological type of the animal life is also dependent 

 upon these factors directly, and indirectly through adaptation to the 

 ecological type of vegetation ; we may, therefore, speak of forest or 

 grassland animals just as we speak of forest or grassland plants. 

 The climate of Illinois favors the growth of the deciduous forest 

 (Gleason, '10:117; Schimper, '03:162-175; Transeau, '05). The 

 sand prairie has been able to persist because of the presence of local 

 conditions of aridity. 



The local factors to be considered in any regional study are heat, 

 light, water and humidity, wind, soil, and topography. These factors 

 make up what may be called a local environmental complex ; they are 

 so closely interrelated that they can not readily be analyzed and classi- 

 fied. It might be well to include them all under the term "physi- 

 ography", for local environmental conditions are primarily matters of 



