of the North American Ice-Sheet. 199 
serpula, mixed with sand, about two feet; gravel and sand 
again, thickly filled with shells, two feet; fine white sand, about 
ten feet; the common yellow sand and fine gravel of the modi- 
fied drift, about forty-five feet, its top being at ninety feet; 
coarse gravel, three feet; ferruginous sand, one foot ; changing 
above into a former surface soil, one foot thick; overlain by 
three feet of dune sand, which forms the present surface, 
ninety-eight feet above sea. The highest part of the bank is 
midway between this and the light-house. From a compari- 
son of the species contained in these two shell-beds, Professor 
Verrill estimates that the temperature of the sea at this place 
- was lowered 15° between the times in which they lived. The 
layer of coarse gravel which occurs here at the height of ninety 
feet, is continuous for a half-mile from this point both to the 
north and south, varying from three to eight feet in thickness. 
About half of its rock-fragments are rounded, these being of 
all sizes up to one foot through; the rest, which are rough and 
angular, range up to two feet, and rarely to four feet, in diame- 
ter. This bed has its greatest thickness and is coarsest at the 
highest portion of the bluff, where it closely resembles till. 
The old surface of black soil and the present surface of dune 
sand are also continuous along the same distance. An eighth 
of a mile south from the shell-beds, the bluff falls to a hollow 
height falls from 105 feet at the middle to about 35 feet at each - 
end. Below the rocky layer it consists of fine modified drift 
much colder; sand and fine gravel are accumulated to a depth 
of more than fifty feet, probably brought by rivers from the 
ests sprang up, as the climate became mild again ; and, lastly, 
the sea has eaten away the east portion of these deposits, while 
= sand of its shore has been swept by the wind over their 
p- 
The whole south side of Nantucket Island consists of nearly 
level plains of gravel and sand, twenty to sixty feet above the 
ea. This expanse, reaching more than es frot 
east, with a width varying from one to three miles, is broken 
by frequent hollows which extend approximately from north to 
south, like those already noticed on the similar plains of Long 
