1870. | The Formation of Cape Cod. 501 
Besides the till, or boulder-drift, it has been stated that strati- 
fied gravel and sand, nearly or quite free from boulders, make up 
a large part of these series of morainic hills, including their 
highest portions on Long Island and Cape Cod. We thus see 
that the ice was aided in the accumulation of its terminal deposits 
by streams laden with vast quantities of modified drift. These 
streams appear to have been formed during the meltings of sum- 
mer upon the surface of the ice-fields, especially at the period 
when they yielded and were driven back by a warmer climate. 
To understand how such rivers could get their freight of gravel, 
sand and silt, we must consider what the ice-sheet was. The 
interior of Greenland is now covered by a similar field of ice, 
which rises steeply at its edge, but after a few miles changes to a 
gently inclined plateau, elevated above the highest peaks of the 
land on which it lies, and apparently of immeasurable extent. 
Dr. Hayes found the angle of ascent on this plain to decrease 
from six to two degrees in thirty miles, at which distance he 
reached an altitude of about five thousand feet. The ice of the 
glacial period had a similar, but probably less steep, ascent from 
its terminal front northward. The temperature of its area was 
changed so that the snows of autumn, winter and spring were 
not entirely removed in summer, but very slowly increased in 
depth, their lower portion being changed to solid ice. This con- 
tinued through so long a period that the surface of this ice-sheet 
was lifted above the highest mountains of New England. At the 
White mountains, two hundred miles north from its border, it 
rose to a height at least 6300 feet above our present sea-level. 
Its greatest thickness was far to the north, from which region the 
vast pressure of its own weight caused it to flow slowly outward. 
Even its lower portion, which rested on the uneven surface of the 
land was thus pushed forward over all our hills and mountains, 
transporting boulders and marking its course on the ledges. Over 
British America, as far north as the surface geology has been ex- 
plored, and to the most southern limit reached by the ice-sheet, 
which coincides nearly with the course of the Columbia, Missouri 
and Ohio rivers and the south coast of New England, the direction 
of its motion, as shown by the strize, was generally southward, being 
nearly due south in British Columbia, south-west in the region of 
the great lakes, and south-east between Hudson river and the _ 
Gulf of ‘St. Lawrence. The loose materials which covered the — 
