MODE OF FORMA TION OF TILL 5 5 



other evidence, negatives the idea that the basal portion of the 

 glacier "plucked " these huge masses of rock from the ridges and 

 transported them in its body. 



Sometimes the pressure of the ice resulted onlv in a slight 

 folding, or a tilting and faulting of the rock strata without mov- 

 ing them to a distance from their original position. This may 

 affect them to a depth of twenty or thirty feet, and ma}- include 

 some of the solid, heavy-bedded, unweathered layers. In these 

 cases, at least, no question can remain that the action was extra- 

 glacial or marginally subglacial. Their importance lies chiefly 

 in the fact that they prove beyond dispute that even the Kansan 

 ice-sheet could, at times, exert powerful forward pressure on the 

 rock over which it moved. 



5. The forward movement of the unbroken rock mass was 

 rarely continued many scores of vards before some obstruction 

 was encountered, often in the form of an upward slope of the 

 land, increasing the friction to such a degree that the transported 

 ledge was unable to withstand the pressure and general fracture 

 resulted.' In a few cases, the impression made on the observer 

 is that the mass remained unbroken while the stress accumulated, 

 until, becoming too great, the entire ledge was suddenly frac- 

 tured into innumerable small angular pieces, under a well-known 

 principle recognized as a condition of the formation of certain 

 limestone breccias in various portions of the earth. Usually, 

 however, there is clear evidence of a kneading or rolling over 

 and crushing of the mass, producing an internal structure which 

 cannot be simulated by the ])roduct of an}^ other known process. 

 When a series of semi-decayed or loosely compacted strata being 

 forced forward in front of the ice, are checked by some obstruc- 

 tion, they tena to corrugate in a manner somewhat similar to the 

 Appalachian type of mountain building ; and the disturbance is 

 greatest in the vicinity of the obstruction. But when the ice 



'The deposits representing this stage are quite numerous in the Pecatonica basin, 

 over an area about three miles in diameter, lying immediately southwest of the village 

 of Dakota, and another west of the village of Everts. They are usually in the form of 

 dome-shaped knolls and short ridges, and forming conspicuous features of the drift 

 topography, it is unnecessary to more minutely describe their localities. 



