MADISON COUNTY. 423 



depend very largely on the modification of the drift surfaces and these 

 details can, in many cases, be very well explained without any recourse 

 to the underlying beds. All of the questions that concern the county, 

 whether relnting to its topograply, its soils or its water-supply, con- 

 nect themselves with the origin and history of the deep drift-deposits, 

 by which its entire surface is now covered. 



DRIFT AND SOILS. 



The subject of the Drift has been taken up so many times, and from so 

 many points of view in the reports of the survey already published, that 

 it is unnecessary here to treat of it from a general or theoretical point of 

 view. 



The deposits of the Drift in Madison county fall uiider the ordinary 

 heads. The lowest and oldest of these deposits is a heavy bed of bowlder 

 clay, which covers the face of the country universally. It is a tough, 

 waxy, dark-blue clay, in which scratched and striated pebbles and 

 ' bowlders are abundantly distributed, and occasional seams of sand and 

 gravel, varying in thickness from one inch to two feet are found, but 

 without regularity or constancy. 



This member of the drift series exceeds the rest very largely in volume 

 and also in the importance of its offices. As has been before stated, 

 borings of 60 feet are sometimes made without exhausting the bowlder 

 clay. These facts seem to indicate that the average thickness of this 

 member of the series is not less, certainly, than 60 feet. 



In considera:ble areas of the county, the bowlder clay forms the present 

 surface, or rather the bowlder cl«.y as modified by the action of the atmos- 

 phere and of vegetable growth and other organic agencies, upon it. These 

 areas constitute the coldest and most Intractable lands of the county. 

 The soil formed from their weathered surfaces is a black clay one foot or 

 a foot and a half in thickness. The action of the atmosphere is shown to 

 have reached below the surface in the conversion of one or two additional 

 feet of the blue clay into yellow clay.' These weathered deposits pass by 

 insensible gradations into the underlying deposit. The lands of this 

 description are less varied in the natural forest growth than the other 

 lands of the county. They are susceptible, however, of considerable 

 amelioration byunderdraining, and possess all the elements necessary for 

 long continued productiveness. 



By far the larger part of the county is covered with another order of 

 drift-deposit, viz., those that have been modified and re-arranged 

 during a period of submergence to which the original beds have been 

 subjected. If a bed of the yellow clay already spoken of as formed from 



