132 Bulletin of Laboratories of Denison University [Vol. XIII 



and more leisurely field-study gives greater prominence and ex- 

 actness to this curve of local lobation first examined by Leverett. 



/;; Licking County. — Save in the valleys, the Illinoian drift 

 near its front is so attenuated that mapping it is a problem of 

 elimination, or the careful study of the rather maturely dissected 

 divide areas. The lesser details of topography in the marginal 

 zone appear to have had slight influence on the outline of the ice- 

 front, while obviously exercising a considerable control over the 

 duration of the ice in its position of maximum reach. This latter 

 fact necessitates patient observation, particularly where the strati- 

 graphy did not encourage differential-weathering effects previous 

 to glaciation ; it is evident that on slopes of heterogeneous rock 

 structure facing the direction of ice-movement, benches of the 

 more resistant formations, weathered into semi-detached spires 

 and blocks,' would have suffered some from ice- work, even though 

 the products of residual decay did not receive a noticeable admix- 

 ture of glacial drift. But among the hills, where the rock struc- 

 ture is more uniform, and the weathered slopes correspondingly 

 even, the absence of foreign material must be established before 

 drawing the drift-line ; and in these higher areas an unexpected 

 localization of erratics surrounded completely by territory in which 

 the most diligent search has not revealed any evidence of glaciation 

 is somewhat puzzling, but very convincing of the fact that the final 

 demarkation of the glacial boundary is a problem of time. 



In establishing the relationship of these valley dependencies of 

 the Illinoian ice-sheet to the Scioto lobe, and in determining 

 whether they are tongue-like extensions of the ice-mass at its 

 period of greatest development, or at a later retreatal stage, three 

 townships. Perry, Hanover, and Mary Ann, of Licking County, 

 have been carefully studied, while like attention has been given 

 to portions of adjacent townships. In valleys trending in general 

 with the direction of ice-movement, the problem is one of distin- 

 guishing the unmodified drift from the deposits of entirely extra- 

 glacial waters, and of determining the drift-covered portion of the 

 valley walls. 



1 F. Carney, Bulletins of Denison University, Vol. XIII (1906), p. 124. 



