Taconic orogeny Taconic orogeny 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. 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
Grenville orogeny here. Ages are approximated from the magmatic activity associated with the individual cycles of the orogeny. The gaps in the ages of the compression cycles and isotope analysis of hornblende, biotite, and potassium feldspar suggest that extension was occurring when compression had momentarily ceased. Rivers' 2008 paper has now examined the timing of the different periods of the orogeny and reconstructed the timeline based on the spatial and temporal metamorphism of the rocks present. According to this newer version of the timeline which is a composite of Rivers 1997 and Gower and Krogh 2002, the Elzevirian orogeny occurs from 1240