THE RENSSELAER PLATEAU is populated by glacial features. In particular, the Poestenkill Community Forest has an esker running through the middle of it. (You are standing on it!) An esker is a long and narrow ridge made of sand and gravel. As a glacier passes over land, tunnels on the bottom form from melting ice. These streams run through the bottom of the glacier, and accumulate the sand and gravel sediments over time. When the glacier retreats, the sediment is left behind in an esker.