“RE-MAPPING” HARLEM’S GLACIAL LAKE IN NORTHERN NEW YORK CITY, NY

Reeds (1927) deduced the location of glacial lakes Hudson and Flushing over the NYC region. Lake Flushing covered much of NYC and extended across Long Island Sound. In papers focused on the engineering properties and design challenges of the glacial soils, Parsons (1973 & 1976) produced a slightly more detailed map over the city, placing the lakes north of the terminal moraine in areas with no rock outcrops.

Detailed mapping from NJ (Stanford 2010, Stone & Others 2002) eliminated Lake Flushing and lake deposits from upper NYC, instead placing Lake Bayonne over lower Manhattan until an outlet to Long Island Sound opened at Hell Gate, allowing the lake to drop to the Lake Hudson level that was largely confined to the Hudson valley.

None of this newer mapping accounts for the extensive (~2 mi²) glacial lake deposits present over most of Harlem and parts of Inwood and the Bronx. Pleistocene glaciers scoured out deep valleys and basins in the bedrock where tight NE trending folds brought the relatively softer Inwood Marble to the surface, and the NW trending faults along 125^thand Dyckman Streets fractured the rock. Glacial lake deposits are often present in these mapped areas (Baskerville 1992 & 1994), filling in the basins where top of bedrock is below elevation 0’.

An island of harder schist along Park Avenue roughly separates Harlem’s lake into 2 segments. To the west, the top of the glacial lake deposits is generally between El. 0’ and -15’. To the east, where the basin is more open to Harlem and East River drainage, top of lake deposits is typically between El. -15’ and -25’. It is not yet clear to what extent, if any, the Harlem lake connects to Lakes Bayonne and Hudson.

While the stratigraphy varies by location, there is a strong trend across the area. A small area around E 105th Street has a thin layer of Gardiners Clay above the rock, which is otherwise covered with till, with boulders especially common in the deepest valleys. Layers of varved brown and sometimes gray silt and clay typically lie above this. The varved deposits often grade upward to a layer of silty fine sand. In some locations layers of outwash sand and/or till are mixed in with the lake deposits, marking readvances of the ice front. Changes in some engineering properties of the soils, vertically and laterally, also indicate loading by ice and/or desiccation as glacial lake levels dropped.