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158 GEOLOGY OF NEW YORK CITY
markable
invasion—their popular expression—are described in the accompanying
section, but underneath the morainal heaps a succession of gravels,
clays, and sands interpret climatic and physical mutations connected
with the advance, the retreat, and wasting away of the land glaciers.
The so-called Pleistocene formations on Long Island have been
separated into the following divisions from top to bottom:
Wisconsin Stage.—Morainal lines, outwash and kettle plains.
Vineyard Interval.—Elevation of land, erosion.
Tisbury Stage.—Depression, great deposits of sand and gravel.
Gardiner Interval.—Ice withdraws, land lower than to-day, erosion.
Gay Head Folding.—Ice, and crumpling of the surfaces of older deposits.
Sankaty Stage.—Ice withdrawn, clay and sand beds.
Jameco Stage.—Ice; filling of Sound Valley, gravel, boulders.
Past Mannetto Erosion.—Ice withdrawn, long erosion, land about 300 feet above present sea-level.
Mannetto Stage.—Ice; 300 feet depression, gravel of West and Wheatley Hills.
The above scheme should be read from the bottom to the top in order to grasp the sequence.
The
Mannetto quartz gravels have been largely rearranged or removed through
the processes of change in the succeeding epochs, and they are
preserved in the Mannetto (West) and Wheatley Hills. These eminences
are in the middle of the island, south of Huntington. Following the
Mannetto gravels, the land was raised and erosion reinaugurated with
the glacier outline somewhat shrunk.
The
Jameco gravels succeed, and they are found underlying Brooklyn and in
western Long Island. The Long Island Sound valley was in a measure
filled up with these dark sands and gravels, the water from the ice
edge, which stood near the. north shore of the island, pouring westward
in channels which may have been deeply gullied beneath Brooklyn. On the
south the Jameco beds form important water-storage reservoirs.
The Sankaty Stage saw a renewal of the ice advance and
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