642 PROCEEDINGS OP THE BALTIMORE MEETING 



unpolished limestone might be due to the effects of weathering in post-Glacial 

 times. In localities where the drift is very thin and freezing in consequence 

 had penetrated the rock, the irregular condition of the limestone may repre- 

 sent normal disintegration ; some of the rough surfaces observed might be thus 

 accounted for. In the area recently stripped, however, the depth of the drift 

 precludes this explanation. So far as it is possible to tell, the area over almost 

 its entire length was protected by a uniform mantle of debris. This sudden 

 transition from polished to irregular and rough surfaces can not be accounted 

 for by the effects of post-Glacial weathering. 



Fickleness in the abrasion work of glacier ice is an old observation. Sur- 

 faces which lithologically should register the striations of rock-shod ice are 

 frequently without this evidence ; several miles of such outcrops are known. 

 But when ice passes over limestone horizons we expect to find it registering 

 the slightest work of tools carried in its basal area ; where such horizons have 

 been protected from weathering during post-Glacial time by drift, even the 

 weak striae should now be seen. Irregularity in the intensity of glacial scoring, 

 or even its presence and absence over long distances, is not remarkable. But 

 when this variation is found within a mile it arouses our interest ; further- 

 more, when the variation is so striking, as is the case on Kelleys island, we 

 would seek an explanation. 



For several square miles about Sandusky limestone outcrops. The numerous 

 islands in this region are evidence of pre-Glacial stream erosion topography, 

 modified to some extent by ice work. 4 This irregular surface lies apparently 

 not far from the axis of the Erie basin, and therefore has the proper exposure 

 for inviting active ice erosion. On Marblehead, at least, it was observed that 

 the major joint planes are but slightly discordant with the direction of the 

 moving ice (figure 2, plate 109). Under these conditions it is apparent that 

 locally the basal parts of an ice-sheet might be overloaded even to the point of 

 becoming stagnant. 5 Such a mass of stationary ice would interfere with the 

 movement of the ice immediately in its rear. At first this coming Ice would be 

 checked, but later it would move upward, shearing across the stagnant area. 

 As the overloaded mass gradually lost velocity, the zone of ice directly above 

 it moved onward or sheared over it; the ice in the rear on moving upward 

 proceeded with this ice. The weight of the superincumbent mass would 

 account for a downward movement on the leeward side of the stagnant area. 

 If this downward moving line of ice were properly shod with debris, the rock 

 surface beneath it would suffer unusual erosion ; thus the surface on the 

 leeward side of a stagnant area under these conditions would be strongly 

 glaciated. 6 



An overloaded mass of ice loses its velocity gradually, for it acquires its 

 load gradually ; in consequence of this the rock surface covered as the mass 

 reaches the stage of stagnation does not suffer further abrasion, at least not 

 until after* this stagnant mass has been removed, probably through being grad- 



* J. S. Newberry says : "They are all wrought by glacial action." Geological Survey of 

 Ohio, vol. 1, 1873, p. 111. 



5 Chamberlin and Salisbury : Geology, vol. i, 1904, p. 271. 



6 Chamberlin, in discussing the "Removal of scoring debris from action," mentions that 

 In the opinion of some glacialists there may be an "upward movement through the body 

 of the glaciers." Seventh Annual Report, U. S. Geological Survey, 1885, p. 239. 



Chamberlin and Salisbury : Loc. eft., p. 282. 



