MODE OF FORMATION OF TILL 59 



which occupies the southern third of the county. The existence 

 of this basin I recognize as the cause of the much larger propor- 

 tion of foreign drift pebbles in its vicinit}', for it favored sub- 

 glacial transportation of them, but I do not think it alone can 

 explain the phenomena connected with the above described belt 

 of typical till. For (i) when we go east on the line of glacial 

 movement, the characteristics of this till disappear, and in the 

 extreme eastern portion of the county, what little till we do find 

 in the Pecatonic basin, is mostly of the very imperfectly formed 

 "semi-residuary" variety; and (2) if we study the limits of this 

 characteristic till belt and the direction of ice movement, in con- 

 nection with the limits of the main body of the extra-glacial 

 lake clays and silts, we find such a relation between them as to 

 clearly indicate the derivation of a large portion of the former 

 from the latter. 



In short, the composition of the till in the belt now under 

 discussion, points to its being only a stage in advance of that 

 described under stage No. 8, this advance being due chiefly to 

 the introduction of a large amount of clay and silt, the result of 

 an accidental passage of the ice over a semi-plastic lake deposit, 

 which resisted erosion comparatively well, but yet suffered some 

 abrasion at its surface. Therefore, as the yellow till occurs over 

 and in the immediate vicinity of the stratified clays, there is no 

 room for an englacial transportation, which is, moreover, quite 

 unnecessary, and in opposition to the comparative abundance of 

 striated pebbles. 



In summarizing the evidence presented in the preceding 

 pages, I would subdivide the product of direct glacial action, in 

 the central Pecatonica basin, into classes as follows : 



Class I. Semi-residuary drift. Till composed largely of 

 rearranged red residuary clay and chert, with comparatively few 

 foreign pebbles. 



Class II. Angular local limestone debris. Varies between 

 contorted, tilted, and transported unfractured rock ledges, and 

 fine limestone breccia. 



Class III. Consisting largely of a combination of the two 



