HUDSON RIVER TERRACES DELINEATED FROM ARCHAEOLOGICAL INVESTIGATIONS ON VAN SCHAICK ISLAND, COHOES, NY

Van Schaick Island is located at the confluence of the Hudson and Mohawk Rivers and has a rich history of human occupation. Archaeologists uncovered a PreContact Native American and early Colonial site during a survey for a proposed riverfront development. Archaeological excavations were studied a short distance from the 18th century Van Schaick house across Delaware Avenue from the Van Schaick family cemetery.

Modern fill buried multiple A horizons and it was necessary to distinguish these horizons as from single or multiple terraces. I conducted a reconnaissance geomorphology survey to assess the geomorphological context of the artifact-bearing buried A horizons. Shock’s 1963 surficial geology map proved helpful as he correctly identified the elongate central ridge of the island as exposed bedrock. The bedrock is shale-rich melange of the Cohoes Melange formation (Plesch, 1994; Kidd et al, 1995).

Below this island’s summit ridge, there are two distinct terraces designated T1 and T2 for the lower and higher terraces, respectively. The underlying sediment is fine grained alluvium of unknown thickness. The T2 terrace is at an elevation of 40-42 feet above sea level and may represent the geomorphic 500 year flood plain. The T1 terrace is at an elevation of 29-30 feet above sea level and likely represents the geomorphic 100 year flood plain.

Surface elevations were derived from GPS data recorded at the site and from USGS digital elevation model data. The buried A horizons were identified on 2 distinct terraces - T0 and T0a. The buried T0 surface is at an elevation of 16-17 feet above sea level and likely represents a former low bank of the Hudson River. Fill layers were from 19-20th century dredging, building debris and coal ash. The buried T0a surface is at an elevation of 19-20 feet above sea level and underlies most of the project. Both buried A horizons were developed in fine grained alluvium typical of the Hudson River flood plain. Stratigraphic cross sections portray the terrace surfaces. The geomorphological study proved very useful in the interpretation of the archaeological deposits.

Van Schaick Island is an erosional remnant of Late Pleistocene high discharge along the IroMohawk River that carried Great Lakes discharge eastward prior to opening of the Covey Hill Gap around the Adirondacks.