Driving north through Georgia, one crosses three major geophysical regions. The Coastal Plain of southeastern North America cuts across the southern half of the State; this flat topography is underlain with Cretaceous and Tertiary sediments and, along rivers and near the Atlantic shore, with Quaternary sands and gravels.
The Fall Line, which runs from Columbus to Macon to Augusta represents the boundary between the Coastal Plain and the Piedmont; along this line, streams, flowing southward or southeastward, fall from the hard bedrock of the Piedmont to the softer sediments of the Coastal Plain. The Piedmont, comprised of numerous fused terranes, is underlaid with igneous and metamorphic rocks; erosion of these ancient rocks has produced the famous red clay of central Georgia, rich in aluminum silicate and colored by iron oxide.
The Appalachian Highlands cross the northwest corner of Georgia and extend along its northern border. In northwest Georgia, Interstate 75 crosses the Ridge and Valley Province of the Appalachians, the central corridor of that chain; east of that province is the Blue Ridge of the Appalachians, composed of ancient Precambrian rock and harboring the loftiest summits in the State (including Brasstown Bald, 4784 feet, the highest point in Georgia).
