8b PLEISTOCENE OF INDIANA AND MICHIGAN. 



not as far as Newcastle. This older drift is easily recognized below Shelbyville, for its out- 

 crops are numerous and extensive, but above Shelbyville its exposures are rare and of small 

 extent and some uncertainty is felt as to its upper limit. In addition to its occurrence in the 

 Mount Auburn ridge, older drift lies on the border of the East White Valley about 25 feet above 

 the gravel filling in a plain that slopes southward at about the same rate as the valley filling. 

 The Wisconsin drift forms only a thin coating 10 to 25 feet thick on these bluffs and on the 

 inclosed tracts between the channels, and there seem to be no grounds for assuming that its 

 thickness is much greater inside the valleys. 



On this interpretation of pre- Wisconsin excavation some difficulty may be found hi account- 

 ing for the network of valleys. There would seem, however, to have been fully as favorable 

 conditions for the development of such a network on the withdrawal of the Illinoian as on that 

 of a succeeding ice sheet, for the slopes or attitude of the surface appear not to have been 

 markedly different in the two stages. The branching, as in the Flat Rock drainage, seems to 

 have arisen through stream wanderings over a flat tract on which there were no guiding channels. 

 As the earlier drift in this region is much thicker than the Wisconsin it may have more com- 

 pletely effaced the channels which preceded its deposition. 



