Beginning in Cambrian time, about 550 million years ago, the Iapetus Ocean began to close. The weight of accumulating sediments, in addition to compressional forces in the crust, forced the eastern edge of the North American continent to fold gradually downward. In this manner, shallow-water carbonate deposition that had persisted on the continental shelf margin through Late Cambrian into Early Ordovician time, gave way to fine-grained clastic deposition and deeper water conditions during the Middle Ordovician. In this period a convergent plate boundary developed along the eastern edge of a small island chain. Crustal material beneath the Iapetus Ocean sank into the mantle along a subduction zone with an eastward-dipping orientation. Dewatering of the down-going plate led to hydration of the peridotites in the overlying mantle wedge, lowering their melting point. This led to partial melting of the peridotites within the mantle wedge producing magma that returned to the surface to form the offshore Taconic (or Bronson Hill) island arc.