finer
sediments with contrasted lignitic, swampy, black clays, shallow water
shells, and cleaner, deeper water-beds, with a considerable molluscan
fauna. Some of the Brooklyn borings show this bed to have a thickness
of 150 feet in the neighborhood of the deep drainage valley under
Brooklyn itself.
Then
followed a period of folding, in which the glacial, beds along the
north shore of Long Island and the island eastward (Gardiner's Island
and Martha's Vineyard) were strangely and conclusively bent, and
contorted, and compressed. This is attributed to the weight and
dragging of superincumbent ice. The layers were clayey, and not so
resistant as the later gravels would have proven, and the ice sheet
was well extended.
The
Gardiner erosion interval sheared off the caps of the Sankaty folds,
and as this has been referred to wave action, the land was
proportionally lower to permit the transgression inward of the ocean.
The Tisbury or Manhasset gravel accompanied a great depression, and
the formation is one of gravel and sand which surmount the folded clays
of the Gay Head deformation. It is found at Hempstead Harbor in sand
pits, stratified with included layers of boulders, and has now some 200
feet elevation above tide. It attains a maximum thickness of 150 to
250 feet.
It
is supposed by Veatch that the well-marked harbors, heading valleys,
and sulcations of the north coast of Long Island were well advanced, as
topographical features, before the Tisbury deposits were laid down.
These deposits served to deface and blur this older relief. "They
continued the filling of the Sound Valley across western Long Island,
which was begun in the Jameco epoch, and buried the deep valleys which
had been developed in the northern portion of Long Island by streams
flowing into the Sound River."
The
Vineyard Elevation came next in the physical evolution of Long Island,
and it is fancied that the choking up of the passages in the old Sound
water-way in western Long Island caused the Housatonic and Connecticut
Rivers to turn east-
