THE METAMORPHISM OF GLACIAL DEPOSITS 481 
York state and Michigan at the western end of Lake Erie, limestone 
formations come to the surface. These outcrops suffered degrada- 
tion by the ice-sheet. Other limestone horizons farther north, in 
part of this distance, also contributed to the glacial load of débris. 
This content of limestone in glacial sediments was partly dissolved 
even by the cold water; no rock-forming constituent is more easily 
affected by water. The resulting carbonated water actively attacked 
the silicate minerals at least. Solution and later precipitation is 
always an accompaniment of ground-water circulating through glacial 
sediments, and further reactions will give different solutions. 
The decomposition of rock-constituents is usually accompanied 
by hydration. This is almost invariably the case in oxidation and 
carbonation. In unconsolidated materials beneath an ice-cap hydra- 
tion would be.an active agent in alteration. 
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 sedi-. 
ments exercises a compressive force; in the deeper-buried sediments ~ 
this force is stronger. In consequence of this compression 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 acting 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 previously 
aggraded sediment was not removed by ice.t These deposits 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 
tR.S. Tarr, American Geologist, Vol. XXXIII (1904), p. 287. H. L. Fairchild, 
Bulletin of the Geological Society of America, Vol. XVI (1905), pp. 53-55. F. Carney, 
Journal of Geology, Vol. XV (1907), pp- 579, 580. 
