Mountain system that extends from Georgia to Newfoundland. This mountain system is the result of tectonic activity that took place during the Paleozoic era, between 543 and 245 million years ago. Since that time, the mountains have been continuously eroding, and their deep roots slowly rising in compensation as the overlying rocks are removed. It is surprising to find that although the Delaware Piedmont has passed through the whole series of tectonic events that formed the Appalachians, the mineralogy and structures preserved in Delaware were formed by the early event that occurred between 470 and 440 million years ago, called the Taconic orogeny.
This event was triggered by the formation of a subduction zone off the coast of the ancient North American continent that slid oceanic crust on the ancient North American plate beneath
oceanic crust on the overriding plate, produced magma, and fueled an arc-shaped chain of volcanoes. This volcanic arc existed in the late Precambrian-early Paleozoic along most of the eastern margin of the ancient North American continent. In Delaware, there is some evidence in the Wilmington Complex to suggest that the overriding oceanic plate included a small island cored by continental crust.
As convergence continued, most of the sediments deposited on the subducting plate were scraped off to form a thick pile of deformed and metamorphosed rocks. In Delaware this accreted pile of sediments became the Wissahickon Formation. The many amphibolite layers in the Wissahickon suggest that these sediments may have been mixed with ash falls, basalt flows from the volcanoes, or slivers of underlying oceanic crust that were broken off
Eventually, continued convergence dragged the ancient North American continent into the subduction zone where it collided with the volcanic arc and pushed up a gigantic mountain range. The creation of this range signified the end of the Taconic orogeny along the Appalachians. Today the once lofty mountains have eroded away leaving their roots exposed in the rolling hills of Delaware’s Piedmont. The intense metamorphism that occurred when the root