EVIDENCE OF TWO WISCONSIN AGE GLACIAL ADVANCES IN A VALLEY BENEATH THE NEW YANKEE STADIUM, BRONX, NEW YORK

In NYC, valleys and depressions cut into the bedrock are usually filled with glacial sediments, often reflecting multiple glacial events (Moss 2011; Moss & Merguerian, 2005, 2006, 2007, 2009). Multiple layers of till and varved lake deposits, and changes in SPT blowcounts that signify glacial loading are commonly found as evidence of more than 1 glacial advance over a site. Occasionally organic material is present that can provide a time frame for the deposits.

Borings made for the foundation design of the new Yankee Stadium indicate that a valley in the Bronx also has sediments from 2 glacial advances. The strata and degree of weathering present suggest the following sequence of events:

Pre-Sangamon age erosion scoured a deep narrow valley along with a depression (similar to whirlpools and ledges seen below the World Trade Center site) into the bedrock along the Inwood Marble/Fordham Gneiss contact. The valley was filled with likely Sangamon age sediments (analogous to the Gardiners Clay found scattered across the region to the south). Lignite, ¹⁴C dated at >48,000 BP, and organic soil was preserved under a ledge at the downstream end of the basin. The glaciation associated with the Wisconsin age Ronkonkoma advance scoured out the sediment not trapped under the ledge, and deposited basal till above the bedrock. Its retreat left pockets of glacial lake varves above the till. This retreat +/or the subsequent Harbor Hill advance filled the valley with outwash sand that was overrun by the Harbor Hill glacier. Thus, the strata below this upper till/varve level have high SPT values due to glacial loading. Retreat of the glacier left behind layers of unloaded varved lake sediments and outwash. Post-glacial streams and sea-level rise filled the center of the valley with organic river clay.

There is no sign of later glaciation and additional loading, so this section of the Bronx apparently was not affected by a minor readvance that moved over NW Queens and Brooklyn, flowing along the East River to the south. Unless the Ronkonkoma advance took place prior to 48 Ka without leaving any sediments behind, thus allowing the site’s organics to date to an interstadial, then either the Queens readvance took place while the Bronx was still under ice, or the glacial ice flow was confined to the vicinity of the East River and did not extend further to the northwest.