MAPPED EXTENT OF GLACIAL LAKE SEDIMENTS OVER INWOOD, NEW YORK CITY, NY

Modern geologic maps show only glacial Lakes Bayonne and Hudson over New York City (Stanford 2010). Bayonne formed as the ice retreated north of the terminal moraine and dropped to the Lake Hudson level when an outlet opened to Long Island Sound through Hell Gate. These maps do not show the lakes associated with the deep deposits of varved silt, clay and fine sand present over the Harlem (Moss 2019) and Inwood neighborhoods of Manhattan.

In NYC the bedrock consists of tight, steep NNE trending folds of hard schist/gneiss and softer Inwood Marble, that were then cut by NW trending faults. Erosion scoured deep valleys and basins out of the marble and fault zones, producing a highly irregular bedrock topography (Baskerville 1992) that controlled the location of the Inwood lake. In general, lake sediments are found along the upper Harlem River and westward into the Dyckman Street Fault in areas where the bedrock and overlying till are below El. 0’. Like the Harlem lake to the south, continuous lake deposits aren’t present above El. 0’.

In the Inwood basin the bedrock is generally covered with glacial till. Varved silt and clay is present above the till and grades upward into varved fine sand and silt. This is often covered with a layer of fine to silty fine sand. The modern Harlem River cuts through the basin, so in places the glacial lake sediments were eroded out and overlain by river sand and estuarine organic silty clay and peat.

The varves are often gray and clay rich immediately above the till, especially in areas where the basin is below elevation -50’. The gray soil is a thin coating in shallower zones near the edges, a thicker layer in the deep basin. Moving upward the soil color changes to brown, with some gray and red brown varves, which is the color mix more commonly seen across the city. The transition from gray to brown is fairly sharp, marking a rapid shift from a local bedrock sediment source to a more regional one. This suggests the basin was isolated until the lake level rose high enough to connect with the otherwise similar Harlem lake to the south via the Harlem River +/or Lake Hudson just to the west (most likely through Spuyten Duyvil). Stanford (2010) places the Lake Hudson shoreline at Spuyten Duyvil at ~ El. -10’, though it may have been up to roughly 20’ higher before the till at the Hell Gate outlet eroded down to the bedrock, so a connection is possible.