Mctamorphism of Glacial Deposits g 



Pressure. — In the deeper-seated areas of the fragmental zone 

 of the earth's crust, pressure has long been regarded as playing an 

 active part in the alteration of rock. In the case of the superficial 

 sediments under discussion there appear to be three sources of 

 pressure : 



1. The weight of drift overlying a given horizon in a mass 

 of sediments exercises a compressive force; in the deeper-buried 

 sediments this force is stronger. In consequence of this compres- 

 sion there is greater facility in capillary action, that is, waters move 

 more slowly through these sediments, and precipitation is increased. 



2. During the continuance of an ice-invasion, the weight of 

 the ice itself bore down on the unconsolidated materials, thus act- 

 ing as a factor in their alteration. In discussing this, however, it 

 must be granted that an ice-sheet degrades, first of all, the regolith. 

 It is a fact nevertheless that in certain localities, some of the pre- 

 viously aggraded sediment was not removed by ice.^ These de- 

 posits may be the drift of an earlier ice-invasion; in any case, 

 wherever not removed, it was subject to the great weight of the 

 ice-sheet. This weight can be computed only approximately. Some 

 observations have been made on which are based conclusions in 

 reference to the surface slope of ice-caps ; this data includes a study 

 of both existing ice areas and bands of drift constructed by former 

 ice." A conservative estimate of the depth of Wisconsin ice over 

 the Erie basin is at least 2,000 feet. This figure is based on two 

 considerations : the present difference in level between Lake Erie 

 and altitudes south that were covered by ice is about 800 feet. The 

 ice reached south of the Erie basin approximately 200 miles; if its 

 surface sloped even six feet per mile, this would represent a depth 

 of 1,200 feet which, plus the 800 feet due to the difTerenece in 

 altitude, makes approximately 2,000 feet. The basal pressure per 

 square foot for clear ice of this thickness would be 115.500 pounds. 



In New York state, there is a greater difference in altitude, 

 even when we neglect the overdeepened portions of the major Fin- 

 ger Lake valleys. The range in altitude alone would give 1,500 feet 

 of ice; this, in connection with the surface slope of the ice, would 

 give a depth of approximately 2,500 feet, which represents a basal 



9 R. S. Tarr, American Geologist, vol. XXXIII (1904), p. 287. H. L. Fairchild, Bulletin 

 of the Geological Society of America, vol. XVI (1905), pp. 5.3-55. F. Carney, Journal of 

 Geology, vol. XV (1907), pp. 579, 580. 



10 Chamberlin and Salisbury, Geology, vol. Ill (1906), pp. 356-58. 



