112 The American Geolofjist. i^eb. isoo. 
glacial pebbles. When considerable sections are exposed, as is the 
case at two or three points, the upper part of the deposit for 15 or 20 
feet is seen to be altered to a yellowish or grayish color, while the mass 
below is of the characteristic blue color of such clays. This formation 
does not rise above the level of the sea save in a small portion of the 
surface of the island. Its highest point seen is in an excavation for 
road material in the west part of the village of Nantucket, where it 
rises about 30 feet above the sea. 
Upon the undulating surface of the lower clay lies a mass of irreg- 
ularly stratified sand and gravel, shaped in the hillocks and short, 
tumultuously grouped ridges called kames, with prevailing trends east 
and west and enclosing occasional ponds or swamps in the intervening 
hollows. Where the hills are steepest, often having slopes of 20° to 
30°, they usuailj' are very stony, large angular blocks being scattered 
ever their surfaces, which, with the peculiar contour, show clearly that 
this belt is part of'a frontal moraine. 
The most peculiar feature of the southern plain is found in broad 
channels extending from the moraine to the ocean shore. These chan- 
nels are digitate, two or more often uniting in their southward course. 
They range from 5 to 20 feet in depth, and have flat bottoms from 200 
to 800 feet wide, sloj^ing toward the sea with an irregular descent of 
about five feet in a mile. Their seaward extremities are in all cases 
below the level of ,the ocean and contain lagoons or ponds which are 
barred from the ocean by beaches of sand. 
Evidences of decay are observable in the pebbles of these drift 
deposits, by the crumbling of many of the varieties of crystalline and 
fragmental rocks, by the dissolved look of the surface of the rocks 
which resist interstitial decay, and by the development of joint planes 
in the pebbles, so that, though they may be but little decayed, they 
often split into fragments on being removed from their bed. Professor 
Shaler believes these changes to be twice or three times greater than 
in more northern localities, as abolit Fall Eiver, Mass., or on the coast 
at Boston and northward. But the suggestion that the drift of Kan- 
tucket has been proportionately longer exposed to subacrial weather- 
ing, and that the recession of the ice-sheet from Nantucket was twice 
or thrice as long ago as from northern New England, seems unwar- 
ranted ; for the outer portion of the drift would contain a large inter- 
mixture of preglacial gravel from stream beds and the sea shore, and 
of other superficial deposits that covered the country before the ice 
age, all of which had already been long subjected to weathering, so 
that these characters of the pebbles may have been acquired then. On 
the other hand, the drift farther north, where glacial erosion and 
transportation were more efiicient, would scarcely include any peb- 
bles formed by preglacial weathering, and stream and sea erosion, 
and its rock fragments, derived by glacial planation and plucking 
from the bed-rock, would exhibit only that slight weathering and 
chemical change which they have received during post-glacial time. 
