CHAPTER XVII. 



SOILS. 



SOURCES OF SOIL MATERIAL. 



The soils of the glaciated portion of the Ohio River Basin and of the 

 Lake Erie Basin are very largely derived from the glacial drift and the 

 loess and lacustrine silts that cover the drift. The underlying rocks are 

 indirectly a source of much material, since their decomposed surface por- 

 tions were incorporated in the drift, but they constitute a minor source so 

 far as direct contribution is concerned. 



The great agencies involved in producing the soils of the glaciated 

 district — the ice sheet, the glacial lakes, and the glacial streams — have long 

 since ceased to operate, but modei'n streams are still at work spreading 

 alluvium over valley bottoms in their flood stages. The small lakes that 

 remain in the depressions of the drift are precipitating marl deposits and 

 carrying on then- borders a vegetal growth which will some day yield a 

 rich soil as the lakes are lowered. Vegetation has also been enriching the 

 soil with humus over much of the plane-surfaced drift from the time it first 

 gained a foothold on the di-ift surface; while organisms of various kinds, 

 both plant and animal, have united with the atmospheric agents to break * 

 up the soil and mix it thoroughly. 



The preceding discussion has shown that there are wide differences in 

 the ages of the di-ift deposits, there being deposits of Kansan or pre-Kansan, 

 of Illinoian, of lowan, of early Wisconsin, and of late Wisconsin age. The 

 exposed portion of the oldest (Kansan or pre-Kansan) drift in northwestern 

 Pennsylvania constitutes but a limited part of the drift surface, amoiinting 

 to but a few hundred square miles. The Illinoian drift of northwestern 

 Ohio and southeastern Indiana extends over several thousand square miles 

 outside the limits of the Wisconsin drift, but it Is covered so deeply bv silt 

 of later (lowan) age that it forms the soil only on the valley slopes or in 



