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210             GEOLOGY OF NEW YORK CITY
the bluff has an altitude of about seventy-five feet (Fig. 50). It declines landward into an undulating plain, which largely represents drift reassorted by water.
There is a morainal bank setting round the serpentine emi­nences and riding up and over their slopes from Tompkins-ville to West New Brighton on the north shore. Its lower portions, where cut through, show a stiff till formation. It has been broached and escarped by waves and water, and now forms a prominent terrace on the south shore of the Kill van Kull channel, declining to tide-water where an interior north­ward drainage at Snug Harbor has cut through and reduced it. Formerly its discharged contents crowded the shore with boulders.
The modified aspect of the drift is quite contrasted with the exhibitions of its unmodified character- seen on Long Island and alluded to above. A very admirable demonstration of this latter is afforded by the cut, for instance, through the Coney Island Railroad passes, on an ascending grade, from Bay Ridge. The hill also at Prospect Park was, not many years ago, far more extensive, covering the tract through Park Place. It is gradually being lowered, and the numerous cobble-stones recovered from it are broken and crushed and used in asphalt pavements. The iron and lime in the mo­rainal mixture frequently form a cement, and bind into rigid conglomerate the pebbles and cobble-stones. The relative positions of the included stones often show the absence of rearrangement of the moraine by water, since the heavier stones remain in zones above the smaller and lighter ones below.
It will repay the teacher to take a handful of the smaller fragmental material of the moraine and, washing the earth or clay from it, note, under a hand-glass, the stone particles remaining, and attempt an enumeration of their kinds.
In the bluff at Princess Bay Light there is an underlying detrital mass of stones and sand which Professor Salisbury assigns to the "Pensauken formation," of which he says: