1879.] The Formation of Cape Cod. 495 
colored clays, sand and gravel, varying from black to red, brown, 
gray and white. Gay Head township, reaching three miles to the 
east, has a very uneven surface of glacial drift in small elevations 
and depressions, strown with frequent boulders but apparently 
underlain by Tertiary clay and sand at no great depth. The par- 
allel ranges of hills which extend through Chilmark and the 
north-west part of Tisbury, occupying a width of one to three 
miles, have also a very irregular contour. Their surface is gen- 
erally glacial drift with very abundant boulders, but occasionally, 
as at the top of Prospect hill, it is modified, consisting mainly of 
water-worn gravel and sand. The black, red and white Tertiary 
clays underlie these deposits in the hills, and are exposed in the 
cliffs along the north-western shore to the east side of Lumbard’s 
cove, eleven miles from Gay Head. Upon the south side of 
Prospect and Peaked hills they extend to heights two hundred 
and twenty-five and two hundred and fifty feet above the sea. . 
The south-eastern half of Martha’s Vineyard consists of modi- 
fied drift without boulders, lying in extensive level plains twenty- 
five to fifty or sixty feet above the sea. Along the south shore 
these plains are indented by numerous ponds, which are only 
separated from the ocean by a beach, and the shores of the ponds 
are again indented by long and narrow arms or coves, from the 
head of which dry channels extend across the plains in a north- 
erly course. The road from West Tisbury to Edgartown crosses 
several of these depressions, one of which, known as Quampachy 
hollow, may be taken as an example; this starts from the head 
of Oyster pond, a narrow arm of the sea which stretches two miles 
north from the beach by which it is now shut in. The dry hol- 
low, diminishing from twenty-five to ten feet in depth and from 
three hundred to one hundred feet in width, prolongs this valley 
at least three miles to the north. Near Vineyard Haven and 
Oak Bluffs, north of these plains, and on Chappaquiddick island, 
the modified drift, sometimes sprinkled with boulders, is heaped 
in gently-sloping hills fifty to one hundred feet high, which 
appear to have been formed at the margin of the ice-sheet. 
Thence the line of terminal moraine is continued in Muskeget 
and Gravelly islands, which, however, are only low banks of gravel . 
and sand. On Tuckernuck island it appears again in small hills, 
which in part, as seen at North pond and eastward, are unstrati- 
fied, with plenty of boulders, the remainder being modified drift. 
