The Terrane

The Hudson River drains New York, and parts of Vermont, Massachusetts, Connecticut, and New Jersey. The basin contains three subareas: the upper Hudson from Mt. Marcy to Troy, the Mohawk from Rome to Troy, and the lower Hudson from Troy to New York Bay (we try to stick to this terminology throughout the book, but some authors have failed us). The Hudson and Mohawk basins are fresh water; the lower Hudson is an estuary, with water greater than 1 practical salinity unit (psu) usually below West Point.

The drainage and flow pattern of the upper Hudson is complex and consists of a number of streams coursing through the Precambrian and early Paleozoic rocks of the Adirondacks (Chapter 2). By contrast, the lower Hudson takes a reasonably straight shot to its terminus (Figure 1.1a, b, c). The regional geological lineations seem oblivious to the Hudson's flow, which slices across a series of complex terranes. The mid-Hudson cuts through early and middle Paleozoic sedimentary rocks, with the Catskills to the west and the Taconics to the east. Most notably, the river then runs through the Hudson Highlands, early Paleozoic geological formations that trend directly east-west across the Hudson's flow path in the vicinity of West Point. The river's erosion simply cut downward through this cross-cutting terrane with no notice of its geological contrariness.

Below this region the river passes the Triassic volcanic cliffs of the Palisades and then moves past the New York group, a series of Proterozoic and early Paleozoic metamorphic rocks and then a series of terminal deposits of the last glacial epoch. Along the entire stretch of the lower Hudson, one is impressed by the steep banks and even cliffs along the shoreline. Most other estuaries in the United States meander to the sea along a broad low-relief flood plain. The Hudson clearly has cut down through a great deal of bedrock and yet, owing to its relative youthful erosional history, has not formed a large depositional plain near its mouth.

The Hudson's straight southerly course has cut through these disparate geological terranes and, during the time of the glacial ages, formed a classic U-shaped cross section, much like the glacial fjords of southwestern Norway. During the height of the most recent glacial advance during the Pleistocene Epoch, the glaciers scoured the Hudson to a depth of 150-200 m (488-650 feet). Then, as the glaciers retreated, the Hudson obtained the shape of a fjord, with a deep U-shaped valley. Glacially-derived sediment filled the now quiet-water Hudson so that today it is rarely more than 50 m deep, although

a Kingston L

Figure 1.1. The Hudson River Estuary (continued on next two pages).

a Kingston L

Figure 1.1. The Hudson River Estuary (continued on next two pages).

Figure 1.1.

Figure 1.1.

Figure 1.1 (continued). The Hudson River Estuary.

c it reaches greater depths at World's End in the Highlands.

Thanks to the post glacial rise of sea level, the Hudson River Estuary is now a drowned river valley falling only 1.5 meters along the 240 km between Troy and the Battery. The estuary is maintained as a shipping channel, and dredged to a minimum depth of 9-11 m, although portions of the river are much deeper. Slightly more than half the estuary is fringed by marshes and wooded swamps; the remainder consists of mud flats that are flooded at high tide. Wetlands such as tidal marshes are in greatest abundance in the upper third of the estuary (Chapter 20).

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