Recession of the Late Wisconsinan Ice Sheet

Shortly after deposition of the terminal moraine, the ice front began to recede northward. In less than 2,000 years, Long Island and Staten Island were icefree. The ice front first receded from the terminal moraine to a new ice margin a few kilometers to the north. Here, a recessional moraine was deposited with a lineation of ice contact features, such as proglacial lakes and deltas, lateral meltwater channels, and kames. The ice front of the Hudson Lobe retreated northward from the Harbor Hill Moraine and formed the ground moraine terrain of the Oyster Bay Moraine. The ice stood long enough at the next northerly position, the Sands Point Moraine (Fig. 2.3), to develop an ice margin that cuts across the necks of western Long Island. This positionisdocumentedby the Sands Point and College deltas, the Kings Point bog, now dry land over thick peat deposits, and a lateral, west-to-east meltwater channel, that now separates Lloyd Neck and Eatons Neck from older glacial topography to the south.

As the ice front crossed the East River lowland, it deposited a minor recessional moraine at the City Island-South Bronx position, traceable at least to Central Park. By 19,000 years, the ice front reached the White Plains-Dobbs Ferry margin, where a delta of ice-contact sand, gravel, and till nearly 25 m thick was deposited into the eastern shore of Glacial Lake Hudson. Subsequently, the ice receded to a still stand along the present Croton River-Croton Reservoir Valley. Here, meltwater flowing into Lake Hudson deposited the prominent Croton Delta, a remnant of which still protrudes into the Hudson.

At the next ice margin, a delta, now concealed by downtown Peekskill, completes the northward recession of the ice to the southern edge of the Hudson Highlands and the opening of the fjord (Sirkin et al., 1989;Sirkin, 1999). Here, the ice simultaneously downwasted over the mountains and through the gap to establish an ice margin and a moraine, the Shenandoah Recessional Moraine, along the northern edge of the Highlands (Fig. 2.3; Connally and Sirkin, 1986). As the ice withdrew, further Glacial Lake Hudson expanded northward through the fjord to become Glacial Lake Albany.

The pattern of formation of ice margins, recessional moraines, and deltas continued into the mid-Hudson Valley where deltas were deposited into both sides of the lake (at Cold Spring, Moodna Creek, Marlboro, Milton, Hyde Park, Rhinebeck and Red Hook). About 17,200 years ago, the receding glacier stood long enough to build the Walkill-Poughkeepsie Moraine (Fig. 2.3) and then the Hyde Park, Pine Plains, and RedHookmoraines (Fig. 2.3), and the Rhinebeck and Red Hook deltas at an elevation of 60 m. Identification of ice margin position on the west side of the valley corresponding to the Red Hook stand is complicated by the first reversal in the trend of recession, the Rosendale read-vance. The ice readvanced several kilometers, deforming lakebeds and depositing till around 16,100 years ago.

Glacial Lake Albany continued to expand and deepenbehind the ice, leaving lake clays and shoreline deposits at elevations around 100 m (Dineen,

1986), although the stagnating glaciers in Catskill valleys dammed meltwater as high as 400 m. Ice margins formed at Woodstock, Cairo, and Middleburg (Fig. 2.3) before a second readvance of the glacier, around 15,500 years ago, overrode minor recessional moraines to form drumlins. The higher lakes drained into Lake Albany at an elevation of 100 m through a succession of tunnels in the stagnant ice; the meltwater depositing esker-like ridges (LaFleur, 1979). Later, ice margins developed at Ravena and Altamont before a final readvance in the Albany basin that overrode and deformed lakebeds near Delmar. At the next stand at the Schenectady ice margin, with the lake level at about 95 m, a delta nearly 20 m high was deposited from the west by the Mohawk River drainage. In all, up to 100 m of laminated lake silts and clays now fill the basin of Lake Albany (Cadwell and Dineen,

As the ice front receded and Lake Albany expanded northward toward present day Lake George, melt water was impounded in tributary valleys of the Adirondack foothills by both ice and moraines (Connally and Sirkin, 1971). Proglacial lake sediments and morainal segments cut across the valley and occur along several tributaries of the Hudson north and west of Warrensburg.

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