GEOTECHNICAL EVIDENCE OF MULTIPLE GLACIAL ADVANCES IN NEW YORK CITY'S SUBSURFACE

Most of NY City’s surficial glacial deposits date to LGM ~ 21,000 BP, with no sign of anything older. Mapping in NJ (Stanford 2010) and CT (Stone & Others 2005) document Illinoisan and possibly older layers below the LGM sediments. In NYC multiple glacial deposits are buried in valleys that cut across the city. Several site investigations (Moss & Merguerian 2005, 2006, 2007, 2008, 2009) indicate they are the product of at least 3 different glacial advances.

Changes in engineering properties especially help to separate the strata by event. Consolidation tests and SPT blow counts provide information about former glacial loads on the soil. Weathering of bedrock and till clasts and seismic properties of strata give an indication of relative age. Lithologic changes at a site mark shifts in glacial flow.

Sites in brief:

A SE Manhattan valley contains 2 thin layers of till with different origins. The lower till has gravel clasts of decomposed schist, while the upper contains serpentinite found only to the NW. Stratified drift below the lower till has cross-hole seismic test results more typical of weak rock rather than soil, possibly from weak cementing in an older, more weathered stratum. SPT blow counts increased below each till layer, reflecting soil densified by repeated glacial loading.

Glaciers generally scoured the NYC bedrock clean, but NW Brooklyn has thick decomposed rock below glacial lake deposits that are overlain by 2 separate till layers with very different lithologies. The deepest stratum is old enough to allow generation of saprolite below. The upper till contains Cretaceous clay found only to the NNE.

Plunge pools scoured into the bedrock at the WTC site were filled with till. A silt layer 2/3 of the way up in the till contained a tree branch ¹⁴C dated at 49,500 BP. The pools were carved prior to deposition of till during a glaciation older than LGM.

In a terminal zone such as the NYC area, climate shifts within glaciations may have produced localized advances. However, the major strata appear to represent an older advance (Illinoisan?) associated with weathering, followed by at least 2 Wisconsinan advances. In NYC the LGM (with flow from the NW) is well established, but the lower WTC till originates from an advance 50 Ka or older. The uppermost tills in NW Brooklyn and Queens are from a minor readvance that flowed from a more NNE direction.