FORESTRY SURVEY 247 



2. Upland timber soils usually found along the stream 

 courses and covering about 12 per cent of the surface of 

 the county. — These are silt loams varying in color from 

 yellow to grey and containing about 3 per cent of organic 

 matter resulting from the decay of former vegetation. 

 Differing from the prairie soils in color, in low organic 

 content, and in the possession of a more rolling surface, 

 they present a still more striking contrast in the character 

 of their native vegetation which was almost exclusively a 

 forest in which oaks were conspicuous. They usually 130S- 

 sess as subsoil a plastic yellow clayey silt, often mingled 

 with glacial drift forming a gravelly clay frequently lack- 

 ing in permeability. These soils are usually less fertile 

 than those of the prairie type and offer more difficult 

 problems of cultivation and fertilization. 



3. Terrace soils deposited by stream action in the val- 

 leys of the larger rivers. — These are usually to be referred 

 to the action of over-loaded streams, during the melting 

 of the glaciers, partly tilling the valleys. Later the streams 

 cut through this material and developed new flood plains 

 at a lower level. These soils vary from such silts as are 

 seen in the valleys of the Vermilion and Illinois Elvers to 

 the sands and gravels occurring in the valley of the Fox 

 Kiver, but in general they consist of brown or grey silt 

 loams with varying amounts of sand and organic matter. 

 They occur, as their designation indicates, in the form of 

 benches or terraces along tlie larger streams, covering 

 about 3 per cent of the surface of the county, and are 

 usually fertile and well drained. 



4. Swamp and bottom-land soil mostly of a recent 

 development upon the wide level flood plains of the Illi- 

 nois River. — As a rule this type of soil is poorly drained, 

 often submerged during periods of high water, and con- 

 sists of brown silt loam containing about 4 per cent of 

 organic matter. It is however most fertile, producing 

 heavy crops wherever the drainage permits. Very little 

 more than 2 per cent of the county is covered with soil of 

 this type. Here may be included small areas of peat less 

 than one square mile in extent, 



5. Residual soils formed by the accumulation of loose 

 material from the weathering of rocks in place. — While 

 such soils cover less than 3 square miles. La Salle County 



