THE RENSSELAER PLATEAU is made of greywacke, a type of dark grey sedimentary rock composed of angular quartz and feldspar crystals. Greywacke often forms at the edge of continental shelves since it forms from sediments deposited in underwater avalanches. The greywacke on the Rensselaer Plateau formed in the Cambrian Period, when New York was underwater. The Taconic Mountains were moving to collide with North America, and this active margin caused a lot of rapid erosion and deposition. That is partly why the crystals in greywacke are angular: they have not traveled far from the source and were not able to erode into more rounded crystals.