16 BULLETIN 142, U. S. DEPARTMENT OP AGRICULTURE. 



they are by no means numerous in northwestern Ohio and northern 

 Indiana. In eastern Wisconsin there are some clifflike outcrops of 

 limestone where the deposits of drift are relatively thin and do not 

 entirely cover the irregularities of the preglacial topography. They 

 are most numerous to the east of the basin of Green Bay, from the 

 center of the Door Peninsula southwestward beyond Fond du Lac. 

 Ledges of limestone and sandstone are exposed also along the west- 

 ern margin of the glaciated area of southern Wisconsin, from the 

 vicinity of Portage to Beloit, Wis. These mark the thin outer edge 

 of the till sheet. 



Within the main glaciated regions there are numerous small areas 

 where the till sheet is only a few feet in thickness, and in some of 

 these localities erosion has so far removed the surface covering that 

 only the surface soils are derived directly from the till, while the 

 deeper subsoils are formed from materials resulting from the partial 

 weathering of the underlying rock. The total extent of these areas 

 which are occupied by rock outcrop or by a thin veneer of glacial 

 material over the local rock is so small that they are relatively unim- 

 portant. 



It is probable that, taking the region as a whole, the depths of the 

 different till sheets average as much as 150 feet and that the area in 

 which the till covering is 100 feet or more in depth greatly exceeds 

 that in which the average thickness is less than 100 feet. The depth 

 of the later Wisconsin till alone varies from a few feet to more than 

 150 feet. 



There is one other common characteristic of the majority of the 

 areas in which the soils of the Miami series are extensively devel- 

 oped. Owing to the irregularities of surface configuration the re- 

 gion is one within which numerous large and small areas of ponded 

 and obstructed drainage exist. In the regions of greatest variation 

 in relief, such as in southern Michigan and eastern Wisconsin, there 

 are numerous large and small lakes and many depressed areas which 

 are either in a swampy condition or have remained poorly drained 

 until within the time of human occupation. Even within the gently 

 rolling to undulating region of western Ohio and central Indiana 

 the hollows and extremely level areas were poorly drained in their 

 natural condition. In all such localities there has been an accumu- 

 lation of partially decayed organic matter which gives a distinctly 

 black or very dark brown color to the surface soils. For this reason 

 the surface of the broad region chiefly occupied by the lighter col- 

 ored soils of the Miami series is frequently and repeatedly inter- 

 rupted by large and small areas of these darker soils which have 

 been correlated mainly with the soils of the Clyde series. Where 

 such areas are of too small extent to be separated on the scale used 



