MODE OF FORMA TION OF TILL 5 7 



mingled with a considerable quantity of rounded drift of which 

 a large portion consists of Canadian bowlders. The foreign 

 material may reach as high as 50 per cent, of the mass, but 

 usually falls below 10 per cent. The bowlders belong to the 

 "englacial " drift, and the deposits containing them are marginal, 

 but I introduce them here because by a marked readvance of 

 the glacial front, they may be reworked into till. 



8. In other cases, the ice was transporting the red, imper- 

 fectly formed till, described under stage No. 2, contemporane- 

 ously with the angular limestone debris, and they often came in 

 contact, either becoming thoroughly commingled or perhaps 

 only interstratified. When the intermingling is complete, the 

 deposit has the appearance of a very stony till of a light brown 

 color, and the material of which is evidently mainly of local 

 derivation.^ 



9. Just previous to the incursion of ice into the Pecatonica 

 basin, in Stephenson county, its area was occupied, at the lower 

 levels, by several successive extra-glacial lakes, which deposited 

 glacial silts, some of which resemble typical loess, and others a 

 modified or water deposited till. During the glaciation of the 

 region, the abrasion of these comparatively soft silts was remark- 

 ably small, yet sufficient to leave an impress on the character of 

 the till which now overlies them. For it is observed that, while 

 the ground moraine over the northern half of the county, where 

 these silts were poorly developed and rarely reached by the ice, 

 is very similar in color and texture to the residuary clays, over 

 the southern third of the county, where the land lies lower and 

 the extra-glacial silts were strongly developed, the ground 

 moraine is in general of a quite different character. The marginal 

 deposits are similar in both districts (consisting largely of angular 

 limestone debris and stratified water-worn gravel and sand) , but 

 on the inter-incipient-morainic tracts in the southern district, 

 there is often seen a till identical in all important respects with 

 that which is exposed in long and repeatedly glaciated regions 



'Typically developed along the road between Egan ami Lightsville in Ogle 

 county, about two miles north of the latter village. 



