The Piedmont physiographic province of Georgia is composed of hard
igneous and metamorphic rocks derived from the recrystallization of
ancient (300 to 600 million year old) sediments that were once deeply
buried and subjected to high temperatures and pressures. They were
re-exposed during a collision between the African and North American
Continents about 250 to 300 million years ago.Locally, low density fluids, characterized by an abundance of silicon dioxide, collected into hot molten magmas and migrated upward through the surrounding denser rock. These magmas cooled in the crust and formed large homogeneous rock bodies that are highly resistant to erosion. Subsequent removal of the landmass by erosion has formed the extensive exposed granites (for example, Stone Mountain) that characterize the Piedmont of the Southeastern United States. Piedmont rocks have been extensively mined for a variety of different minerals and the local granite and marble are shipped worldwide as building stone.
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