The wetland-upland complex occupies an old meander on the alluvial plain of the Delaware River, and covers ~500 ha of which ~260 ha are palustrine and riverine tidal wetlands (Tiner 1985). It is bounded by bluffs to the east and by the Delaware River to the west. Crosswicks Creek is the major stream in the wetland but there are numerous smaller channels, such as Watson Creek, that wend through it. The dynamic nature of Watson Creek was described by Abbott (1887: 158-159):

The alluvial flats through which it flows are very uniform in composition; and unless lodged by trees, borne hither and thither by freshlets, have been the cause, there is nothing to show why the creek is not almost a straight line, instead of being as tortuous as a writhing serpent.

That it has changed its course for many a rod, even in historic times, there is evidence in maps attached to old deeds. That it was equally erratic in prehistoric times is also demonstrable, but not with so little labor. In several tracts of the lower lying meadows ancient channels can still be traced, and when ditches have been cut, I have gathered many a curious relic of Indians, left upon what was the bank of a stream, centuries ago.

Sediments are primarily alluvial and aeolian (Wagner, Miller, and Foss 1982; Stewart 1983). Alluvial sediments were deposited from both the Delaware River and Crosswicks Creek. Those from the Delaware River are silts and clays rich in organic matter while sediments from Crosswicks Creek are typically carbonaceous sands with smaller amounts of silt and clay (Owens and Minard 1975). Aeolian sediments were deposited during periods of drought (Wagner, Miller, and Foss 1983; Stewart 1983). Carbon dating of surface sediments show that deposits date from the Pleistocene through Holocene while the deeper sediments are the unconsolidated Cretaceous clays and sands of the Raritan Formation. Below these sediments the basements rocks are mostly amphibolites and gneisses (Owens and Minard 1964). Sediments of higher elevation areas within the wetland-upland complex are generally of different origin. For example, Duck Island is composed of superficial deposits that are classifed as Graywacke I sediments and are interstratified sand and gravelly sand of Pleistocene origin which may be as young as the Wisconsin Glaciation but more likely as old as the Sagamon (Owens and Minard 1975).