which means they have been shoved westward over autochthonous basement rocks on the order of many tens or even hundreds of kilometers. These rocks were originally deposited as sediments in a deep water basin. Cameron's Line represents the trace of a subduction zone that ceased when the Taconic island arc collided with, and became accreted onto, the eastern margin of North America. Many of the rocks east of Cameron's Line were once part of the floor of the Iapetus Ocean. When the Taconic Orogeny subsided during Late Ordovician time (about 440 million years ago), subduction ended, culminating in the accretion of the Iapetus Terrane onto the eastern margin of the continent. This resulted in the formation of a great mountain range throughout New England and eastern Canada, and perhaps to a lesser degree, southward along the region that is now the Piedmont of eastern North America. The newly expanded continental margin gradually stabilized. Erosion continued to strip away sediments from upland areas. Inland seas covering the Midcontinent gradually expanded eastward into the New York Bight region and became the site of shallow clastic and carbonate deposition. This tectonically quiet period persisted until the Late Devonian time (about 360 million years ago) when the next period of mountain-building began, the Acadian orogeny. Southern Appalachians In the southern Appalachians of Alabama, Georgia, and North Carolina, the Taconic orogeny was not associated with collision of an island arc with ancient North America (Laurentia). Geologists working in these areas have long puzzled over the "missing" arc terrane typical of Taconic-aged rocks in New England and Canada. Instead, the Iapetus margin of this part of Laurentia appears to have faced a back-arc basin during the Ordovician, suggesting that Iapetus oceanic crust was subducted beneath Laurentia - unlike the New England and Canadian segments of the margin, where Laurentia was on the subducting plate. In contrast to the Ordovician geologic history of New England, rocks in Alabama, Georgia, and North Carolina, including those of the Dahlonega gold belt (GA and NC), Talladega belt (Alabama and Georgia), and eastern Blue Ridge (AL, GA, and NC), are not typical of a volcanic arc in its strictest sense. Instead, these rocks have geochemical and other characteristics typical of back-arc basins - which form behind the volcanic arc on the overriding plate. The presence of these Early-Middle Ordovician (480 - 460 million year old) back-arc basin rocks in direct or