MODE OF FORMA TION OF TILL 5 I 



ach'ance, glacial abrasion was at a minimum, and had the ice 

 then melted from this district, without further movement, its 

 drift phenomena would be insignificant and uninteresting. But 

 it so happened that, during the general recession of the glacial 

 front, there were repeated slight readvances, forming a peculiar 

 variety of incipient moraine, consisting of knolls and short 

 ridges of angular limestone debris and of stratified water-worn 

 gravel and sand. Between the marginal accumulations of a 

 distinctly morainic type, there are other deposits of somewhat 

 similar nature, but which belong to the so-called ground moraine. 

 In other words, the period of most pronounced glacial action in 

 the Kansan epoch in northern Illinois, occupied a position 

 considerably later than the culmination of the epoch. This I 

 attribute to a milder climate, causing a recession of the ice-front, 

 but yet giving more free movement to the glacier and softer 

 material to work upon. I have gone into this short explanation 

 of the glacial history of Stephenson county, so as to exactly 

 locate the age of the deposits the significance of which I propose 

 to discuss. 



We may gain some idea of the condition of the surface pre- 

 vious to the arrival of the ice by a study of a north-south 

 belt along the eastern boundary of Stephenson county, and about 

 thirty miles back from the glacial boundary. Here we will find 

 frequent exposures of the semi-decayed upper portion of the 

 Galena limestone with its overlying undisturbed residuary clay.^ 

 The Galena formation is usually a heavy-bedded, sub-crystalline 

 dolomite, shaly in certain thin layers and extensively jointed 

 and fissured. Within ten to fifteen feet of the surface, weather- 

 ing has opened certain deposition-planes, separating the rock 

 into layers from two to four inches in thickness. As the rock 

 is composed of small rhomboidal crystals, the next stage of 

 decay consists of a solution of the bond between the crystals 

 resulting in a loose mass of angular grains, macroscopically 



' This is finely displayed in a road cutting one mile south of Egan in Ogle county, 

 and in an excavation of the Chicago and Great Western railway, two miles east of 

 German Valley in Stephenson county. 



