Three recent books that explain local and regional geology and glacial history are: Geology of New York: A Simplified Account (Isachsen, et. al, 1991); Roadside Geology of New York (Van Diver, 1985); and Written in Stone (Raymo and Raymo, 1989). Also of local interest to NYC residents is The Geology of New York City and Environs (Schuberth, 1968).

Recent evidence supports the theory that continents lie on crust that is less dense than oceanic crust, and "float" on moving pieces of dense crust called "plates." The continents have drifted apart and collided again many times in the past 1.2 billion years (Isachsen et. al., 1991, pg. 11-20). The upland areas in the northeast corner of the survey are underlain by a backbone of hard, nearly vertically-tilted, fractured, thinly-layered rock (Fig. 6). This Ordovician-aged, Staten Island Serpentinite bedrock is exposed in a badly eroded trail between the marsh and the golf course driving range. It is a fragment of the African-European continent which was welded onto the North American continent after they collided (Rogers et. al, 1990).

Geologic deposits on top of the bedrock ridge include dense and friable (non dense) glacial till, glacial outwash, stratified drift (interlayered tills and outwash), glaciofluvial deposits, loess, and recently deposited alluvium (Fig. 6,7). The glacial tills and outwash were deposited between 14,000 and 27,500 years ago (Isachsen et. al., pg. 167-180) by a continental glacier that advanced from the north. The moving glacier first scoured, then smoothed and polished the bedrock. Very recently deposited mineral alluvium and organic sediments occur on top of the outwash in the Rippowam floodplain soils and in the Wallkill and Ipswich marsh soils (Fig. 7).

Figure 6. Pattern of soils, geologic parent materials and landform on both steep and level glacial till deposits.

Figure 7. Pattern of soils, geologic parent materials and landform in marshes and on nearby steep hills.