THE SANGAMON SOIL AND WEATHERED ZONE. 129 



seems to indicate that for a time after the Illinoian sheet was exposed to 

 atmospheric action the drainage conditions were good, but that subse- 

 quently they became imperfect and the peat was formed. The possible 

 relationship between the lower silt and the gumbo of the Mississippi Basin is 

 discussed in connection with the latter deposit (p. 32). 



The wood found in association with the Sangamon soil and peat, like 

 that of the Yarmouth, appears to be largely coniferous. Wherever identi- 

 fications have been attempted such woods have been found to occur, and 

 specimens not critically examined have the aspect of the coniferous rather 

 than the deciduous varieties of wood. The aspect of the flora is decidedly 

 boreal. However, as noted in connection with the discussion of the vege- 

 tation found in the Yarmouth soil, it is necessary to guard against the 

 inference that the vegetal remains preserved in the peat and mucky portions 

 of the soil furnish an index of the climatic conditions throughout the entire 

 interglacial stage. They pertain only to the close of that stage when gia- 

 cial conditions were being' inaugurated, and may have an aspect very dif- 

 ferent from that of plants which grew in the midst of an interglacial stage. 



Slight exposures of the Sangamon soil and weathered zone are to be 

 seen on nearly every hillside within the limits of the Illinoian drift where 

 erosion has opened a fresh exposure low enough to r.each the base of the 

 loess. A few exposures have been found within the limits of the Iowan 

 till, but such exposures are far less common than beneath the Iowan loess. 



Several excellent exposures of the Sangamon soil have been made by 

 railway companies at poinfs where the overlying loess has been stripped 

 off to obtain a filling for the railway track. For example, along the Van- 

 dalia Railway, a few miles west of Marshall, in Clark County, Illinois, an 

 area of more than an acre has been stripped of the loess, leaving the black 

 mucky Sangamon soil at the base. A similar exposure, though less exten- 

 sive, is found at West Point, Iowa. The presence of the black muck 

 here seems somewhat remarkable, inasmuch as it occupies the crest of the 

 marginal ridge of the Illinoian drift. The upper 6 or 8 inches is a deep- 

 black color, beneath which there is a deposit of gray gumbo 2 or 3 feet in 

 thickness, capping the Illinoian till. Extensive exjjosures of black muck 

 below the loess have been made by the Chicago, Burlington and Northern 

 Railway at the crossing of Johnson Creek, about 4 miles south of Mount 



mon xxxviii 9 



