Geology of the New York City Region

Quaternary History of the New York Bight

As the following discussion illustrates, the Quaternary history of the New York Bight is complex. Sedimentary deposits and geologic features preserved in the region are different in glaciated and unglaciated areas, and are different in areas areas on land compared with areas now submerged beneath the ocean (now and in the past). In addition, as the glaciers advanced and retreated, sea level rose and fell, and the lands surface subsided downward beneath the weight of the ice, and rebounded upward when the ice melted. These variables all add complexity to the interpretation of the geologic history from one location to the next.

The effects of the most recent stages of the Wisconsin Glaciation (Stages 2 and 4) are fairly easy to recognized. Older, pre-Illinoian glacial stage deposits occur in the region south of the terminal moraine of the Wisconsin glacier in central New Jersey. These older deposits are not resolved due primarily to the high degree of alteration by exposure to weathering. The older New Jersey glacial deposits are probably Early Pleistocene in age, but resolution beyond that has not been established.

On Long Island two terminal morainal ridges stand out on topographic or relief maps. An older more southerly deposit is called the Ronkonkoma Moraine which formed during the an early part of the Wisconsin Stage of the Pleistocene Epoch (Stage 4 - prior to about 55,000 years ago). The Ronkonkoma Moraine forms most of the low rolling hills along an east-to-west transect generally on the south side of the LIE (Long Island Expressway). The higher, younger, more northerly morainal ridge is called the Harbor Hill Moraine. The Harbor Hill Moraine represents the terminal moraine of the most recent advance of Wisconsin Stage glacier (Stage 2) which reached its most southward advance about 18,000 years ago. This glacier probably completely melted in the New York City region in the range of 13,000 to 12,000 years ago. Terminal moraine deposits also occur across northern Staten Island and extend westward across New Jersey into Pennsylvania (Interstate 80 generally follows the terminal moraine across much of Pennsylvania). It is interesting to note that the highest elevations in both Brooklyn and Queens are associated with the Harbor Hill Moraine. The highest location in Brooklyn is Battle Hill (220 feet), a morainal hill in Green-Wood Cemetery in the northeast corner of the cemetery above Seventh Avenue and 20th Street. The highest point in Queens, 258 feet, is the top of a morainal ridge in the neighborhood of Glen Oaks on the southern service road of the Grand Central Parkway.

The Unglaciated Landscape

The prior to the onset of continental glaciation the landscape in the New York Bight region was probably more subdued that at present. Weathering processes probably maintained a deep soil profile in areas that were later scoured down to barren bedrock by glacial ice. The fall in sea level with the onset of glaciation allowed stream to carved downward into their channels. With the increased stream gradients erosion accelerated. Early in the Pleistocene the gently southward dipping escarpments of Cretaceous and Tertiary strata may have formed ridges as high as 600 feet above (ancient) sea level in parts of Long Island and the Atlantic Highlands of New Jersey.

For periods of time when sea level was lower, New York Harbor was probably a canyon for a very turbulent Hudson River (when it wasn't buried by glaciers!).South of the terminal moraines of the great ice sheet meltwater carried gravel, sand, and rock flour (silt) away from the melting ice, and deposited it on a broad outwash plain that extended to the coastal areas. Sea level fell each time the continental ice sheets grew, and rose when the ice melted. The ocean's surface elevation possibly dropped as much as four hundred feet, causing the shoreline to migrate seaward to the outer fringes of the continental shelf. During low-stands in sea level, rivers incised into their flood plains and transported their sediment loads across an expanded coastal plain (exposed continental shelf). Along the shelf margin the rivers carved gorges that extended seaward; processes of submarine erosion carved great submarine canyons beyond. The largest of these is the Hudson River Canyon in the outer New York Bight area. Far offshore from southern New Jersey are the Wilmington and Baltimore Canyons (carved by the Delaware River during different stages of the Pleistocene). During the last glacial period the ancestral Hudson River cut a canyon that extended from the Outer Harbor area (near the current tip of Sandy Hook) southeastward across the exposed continental shelf for about 85 miles.

During times when sea level was low, poorly consolidated sediments (deposited during previous cycles of high-standing) seas were eroded and reworked. Cores derived from offshore sites display evidence of "over-compaction" caused by dewatering processes that took place when sea level was low (some of this material is currently eroding and washing up on area beaches). It was during these "low stand" times that groundwater leaching and soil development occurred in some horizons (which, unfortunately, destroyed most of the fossil shell material).

When sea level was high, marine waters were able to flood the outwash plain, and invade the entrenched stream valleys and low areas carved by glaciers north of the terminal moraine. Where marine water was able to flood low areas, estuary environments prevailed supporting abundant marine life. Lakes formed on higher ground where drainages were blocked. In parts of central Long Island, fossiliferous estuarine deposits lie on top of the older glacier till, and are in turn buried by as much as 40 feet of sand and gravel outwash from the latest Wisconsin Stage advance. On Long Island the fossiliferous estuarine deposits are called the "Gardeners Clay." (This fossiliferous unit also underlies Sandy Hook, as noted earlier.)

Three named Pleistocene Formations are exposed along the Delaware River Valley and parts of the coastal plain extending northward from Cape May to Staten Island. These deposits incorporate material eroded from the youngest Tertiary deposits in the region, the Pliocene Beacon Hill Gravel, which caps the highest hilltops on the Atlantic Highlands and other eroded remnant hills on the coastal plain. Younger Pleistocene units are equivalent to erosional terrace gravels and alluvial facies along ancestral river valleys of the modern Hudson, Raritan, and Delaware Rivers in New Jersey. The oldest of these Pleistocene formations is the Bridgetown Formation (probably associated with an interglacial period during the Early Pleistocene). The Pensauken Formation is probably equivalent in age to the Sangemon Interglacial Stage, but may also represent younger fluvial deposition and estuary development. The Cape May Formation represents riverine and nearshore facies south of the Wisconsin glaciers, as well as Holocene deposits. The stratigraphic relationships of these units exposed on land with equivalent offshore marine deposits is not well understood.

An ancient buried barrier island complex is partially preserved on the modern New Jersey inner shelf and coastal zone (see Welner et al., 1993). This barrier complex exists 0.1 to 1.5 miles offshore and is buried by approximately 60 feet of sediment with an additional 60 feet of water above. This ancient barrier is associated with a still stand in sea level, associated with Stage 3 (a Middle Wisconsin Interstage - approximately 50,000-55,000 years ago). This ancient barrier island deposit rests upon older sediments deposited in previous transgression cycles associated with the melting of older continental ice sheets. It is probable that some of the interglacial periods prior to our current one were both warmer and longer lasting. If this is true, then it is likely that sea level may have actually been higher than in the present. This is quite obvious in many arid regions of the world where elevated shoreline terraces are preserved. The demarcation of as the boundary between the Inner and Outer Lowlands and the Central Uplands of the Coastal Plain in New Jersey may represent the high standing sea level of one of these previous cycles (see Figure 113).

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Last modified: 3/11/2019