STRATIGRAPHIC RECORD OF GLACIAL READVANCES ALONG THE HUDSON RIVER AT MANHATTANVILLE, NEW YORK CITY, NY

The 125^th Street Fault is a NW trending fault that cuts through a high ridge of Manhattan Schist present along the eastern shore of the Hudson River, before it turns to the SSE and crosses upper Manhattan. Between Broadway and the river the fault was eroded into a deep, irregular, tiered valley that filled with strata that recorded the advance and retreat of glacial ice across the Manhattanville neighborhood.

The bedrock surface often has only a thin layer of weathered rock and little saprolite, suggesting that the overlying sediments are more recent, rather than from an older glaciation that would have protected the rock below from later glacial scour. The lowest tier was covered, and the deepest crevices of the valley were filled with a very dense bouldery and cobbly till, presumably the Wisconsinan basal till. Ice retreat covered the till with varved silt and clay (with some layers of outwash) up to the level of the valley’s eastern and western outlets at roughly elevation -160’.

A readvance of the ice covered the older varved soil with outwash and deposited till through the western outlet and partway into the valley. This was surrounded and covered with outwash before a more extensive layer of till was deposited above, often interlayered with outwash and varved deposits. The amount of interlayering and drift indicates a fluctuating ice front sat over the site before further retreat covered it with varved soil that was not initially connected to a glacial lake in the Hudson.

Another readvance planed off these deposits roughly level with the top of the bedrock along the north and south sides of the valley, and covered them with another layer of till. The underlying outwash is very dense, indicating the entire valley was ice loaded even where thick till is not present above. Till is thick along portions of the valley’s north and south sides and thin along most of its axis, forming a basin. Varved soil started filling in from the SW along the river, suggesting a connection to an unidentified glacial lake in the Hudson, before the varved soil proceeded to fill in the basin and then cover the site.

The surface of this varved stratum is quite dense, typical of a desiccated profile. It is unclear if the water table dropped below the varved surface due to the opening of Hell Gate, the breaching of the Narrows or if the ground rose due to post-glacial isostatic rebound.