Hydrology of the Wolf and Reese Preserve

The story of Long Island’s current land surface began about two million years ago, at the beginning of the Pleistocene Epoch in the Quaternary Period. Just prior to this time, the land surface was comprised of continental shelf sediments deposited during the Cretaceous Period (65 to 144 million years ago) and, as sea level was much lower, the shoreline of the Atlantic Ocean was located many miles to the south. A series of glaciations began in the Pleistocene, resulting in multiple advances of glaciers over the land surface, each of which was followed by a warming period characterized by melting ice, glacial retreat, and sea level rise. Each glacial advance and retreat (or recession) modified the land surface and left behind soils and landforms that tell the glacier’s story.

Long Island’s surface records evidence of two major glaciations. The first glaciation occurred between 140,000 and 40,000 years ago and resulted in deposition of glacial drift materials (sand, clay, and rocks deposited directly from the glacier) in the Preserve area. These older drift deposits are not visible on the surface but affect groundwater conditions beneath the Preserve. Following this glaciation there was a warm period with higher sea levels and considerable erosion of the recently-deposited drift. The erosion cut channels into the drift that became filled with fine-grained marine sediments (silts and clays) as sea level rose.

The most recent glacier, the Laurentide Ice Sheet, reached its greatest extent 22,000 years ago, late in the Pleistocene Epoch. This glacier advanced from southern New England to the South Fork of Long Island, moving from northwest to southeast, shaping the land contours of the Preserve and leaving behind glacial drift of its recessional moraine on the land surface. When the Laurentide Ice Sheet advanced over the older glacial drift it scoured the surface somewhat but did not remove the fine-grained sediments deposited in the underlying channels. When the glacier began its final retreat, it left behind a recessional moraine of drift deposits you see at the surface of the Preserve. Similar deposits from the same glacial retreat can be seen on Little Hog Neck to the west and Shelter Island and Gardiners Island to the east. Radiocarbon dating confirmed that the Laurentide Ice Sheet had retreated to the latitude of Westchester County by 19,000 years ago, by which time Long Island was ice-free.

As the ice receded, the land-sea boundary of Long Island also changed. While the glaciers were at their maximum advance, with a considerable amount of the world’s water locked up in the ice, the ocean shoreline was many miles to the south near the edge of the continental shelf and much of what is now Peconic Bay was dry land exposed to erosional processes. A major channel on the east and north sides of Great Hog Neck was cut by the ancestor of today’s Peconic River and is a relic from this time. As the glaciers melted, their meltwaters ended up in the ocean, raising sea level and causing landward migration of the shoreline – much as we are seeing across the globe today. The ocean gradually covered the continental shelf and submerged the land over a period of about 6,000 years, nearly reaching the modern ocean level in that time.

The advancing shoreline did not reach the Wolf/Reese Preserve, leaving the drift deposits exposed to surface weathering and erosional processes that formed the soils present at and just below the current land surface. Much of the Preserve is underlain by loamy soils of the Riverhead and Haven Series – deep, well-drained medium to coarse-textured soils that formed over a mantle of coarse sand and gravel on moraines and outwash plains. However, an area of Scio silt loam is present in the Reese portion of the Preserve and an area of Muck (poorly drained organic-rich soil) is present in the Wolf portion of the Preserve. These soils are found in aligned low-lying areas that are likely underlain by fine-grained sediment that accumulated in channels in the older drift between the two glacial intervals. These fine-grained sediments reduce groundwater infiltration, resulting in accumulations of “perched” groundwater above the regional water table which results in the property’s surface waters. A highly compacted till (unsorted sand, clay, and rocks) at the base of the older drift enhances this effect.

In summary, the landforms and surface water we currently see at the Wolf/Reese Preserve result from natural cycles of global warming and cooling and erosional and depositional environments associated with glacial periods with low sea levels and inter-glacial periods with high sea levels.