The Southeast Piedmont is a geophysical province that stretches from the Mid-Atlantic region to central Alabama. This broad swath of rolling terrain separates the Appalachian Mountains, to the west, from the Coastal Plain, to the south and east. For geologists, the formation of the Piedmont has long been a puzzle.
The Piedmont's bedrock is a mosaic of Precambrian and early Paleozoic rocks (1.2 billion to 500 million years old), represented by igneous and metamorphic components. Some of these formations contain rock that is not otherwise found within the North American craton, suggesting that they arrived as exotic terrains; volcanic components, on the other hand, indicate that they likely formed as offshore, island arcs before merging with the edge of the continent. Finally, areas of volcanic Triassic rock, found near the eastern edge of the Piedmont, are remnants of rifting between Africa and North America as Pangea split apart and the Atlantic began to open (almost 200 million years ago).
The many suture lines between the puzzle pieces of the Piedmont underwent some disruption during the rise of the Appalachians (250-300 million years ago), allowing plutons of magma to form within some of the rock layers. Eventually, as the encasing rock was worn away by erosion, these igneous monoliths were revealed; monadnocks, most common near the edge of the Appalachians, and Stone Mountain, near Atlanta, are excellent examples. Deep soil covers the bedrock in most areas of the Piedmont but, at the fall line (where this province abuts the Coastal Plain), the hard basement of the Piedmont is revealed by waterfalls.
