300 THE ILLINOIS GLAOIAL LOBE. 



seldom reach a height of 25 feet, and many of them are 10 feet or less. 

 The Kaneville esker described above has its trough mainly within this 

 undulatory belt, but the delta lies entirely west of it in a very level country. 

 There are occasional narrow sloughs or depressed areas 10 or 15 feet below 

 the general level, which in most cases have connection with the present 

 drainage lines. The upper course of Blackberry Creek is through a series 

 of slightly depressed marshy tracts which occur among- the low knolls of 

 this undulatory belt. The undulations are maintained as far south as the 

 line of Kane and Kendall counties, the southern terminus being- at a gravel 

 plane leading down Fox River. There is a narrow till plain between this 

 undulatory belt and Fox River from Geneva southward similar to the plain 

 which borders it on the west. 



STRUCTURE OF THE DRIFT. 



The drift of this portion of the composite belt is much more variable 

 in structure than the Marengo Ridge. On nearly every section except in 

 the gravelly belts above noted wells have shown that both till and assorted 

 material are present. The till apparently preponderates over the assorted 

 material, for the latter usually occurs in thin beds. In the gravel plain 

 known as English Prairie and in the gravelly belt between Crystal Lake 

 and Fox River there appears to be little or no till within 100 feet of the 

 surface. The drift is mainly gravel and cobble, there being but little sand. 

 In places fine laminated clays appear at some depth beneath till and grav- 

 elly deposits. The Illinois Central Railway exposes such beds in the deep 

 cuts east of Plato Center. The till appears to be oxidized at surface to 

 greater depth on this portion of the composite belt than on the Marengo 

 Ridge, a feature which is probably attributable to the greater coarseness or 

 porosity of the till, tor it can not be older than the Marengo Ridge. In 

 some cases the oxidation extends to a depth of 50 feet or more. However, 

 the till is not everywhere coarse textured, for on some of the level tracts 

 among the morainic knolls it is very compact. On low tracts near Terra 

 Cotta, in eastern McHenry County, there is a compact silt used extensively 

 for tile and also for terra-cotta ware. At several other places the silt is 

 used for tile. 



The presence of buried soils at great depth which are underlain as well 

 as overlain bv till is a not uncommon feature. In some cases the soils are 



