100 ILLINOIS ACADEMY OF SCIENCE 



In the course of our forest survey certain relations in 

 the distribution of soil types and native vegetation have 

 impressed themselves upon the writer as affording some 

 elucidation of one phase, at least, of the problem of the 

 relative limits of the distribution of forests and grasslands 

 in a portion of Illinois which lies in the tension zone of the 

 great forest and grassland formations of this continent. 



In the portion of the county covered in this survey the 

 surface soil is dark brown in color and in a soil survey* 

 is referred to as an upland prairie soil known as the 

 "brown silt loam." It is described as composed largely of 

 wind-blown loessial material to a depth of three to five feet 

 the upper 6 or 8 inches having a humus content of about 

 6%. In depressions where drainage is poor a black silt 

 loam is found. These together cover 80% of the entire 

 area of the county. 



In these soils streams are cutting their channels and 

 developing their valleys. In the northern part of the 

 county the cutting is slight and little or no valley has been 

 developed, but as the Fox and Illinois Rivers are 

 approached the tributary streams are found to be from 50 

 to 80 feet below the upland. As these streams have 

 developed shallow valleys have been formed, portions of 

 the surface silt loam being removed to depths varying 

 from a few inches to several feet. As the usual depth of 

 the prairie silt loam is about three feet it is clear that 

 along the streams that have eroded definite valleys there 

 will be exposed a strip of the soil immediately below that 

 covering the upland. This soil naturally varies somewhat 

 from that which overlaid it and this variation is manifest 

 in differences of color and texture, and what is probable 

 of much greater ecological significance, in slope and 

 drainage. This difference in topography doubtlessly 

 affects in a very material way the water content of the soil 

 at various seasons of the year and these differences of soil 

 moisture will naturally react upon the vegetation. 



The Department of Soil Survey of the State Agri- 

 cultural Experiment Station in making a soil survey of 

 the county has recognized the differences between the 

 upland prairie silt loams and the soil of slightly lower 



* Hopkins, Cyril G., et al. La Salle County soils. Soil Report No. 5, Univ. 

 111. Agr. Exper. Station, pp. 45, maps. 1913. 



