<P> The Taconic orogeny was a mountain building period that ended 440 million years ago and affected most of modern - day New England . A great mountain chain formed from eastern Canada down through what is now the Piedmont of the East coast of the United States . As the mountain chain eroded in the Silurian and Devonian periods, sediments from the mountain chain spread throughout the present - day Appalachians and midcontinental North America . </P> <P> 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 . </P>

What happened to the mountains formed during the taconic orogeny