The rocks that form the knife-edged 'backbone' of the Chimney Tops are visible beneath lush vegetation. Most of the rocks in Great Smoky Mountains National Park are sedimentary and were formed by accumulations of clay, silt, sand, gravel, and minor amounts of calcium carbonate in flat-lying layers. The oldest sedimentary rocks were formed during the Proterozoic Era some 800-545 million years ago. Vast amounts of unconsolidated clay, silt, sand, and pebbles were washed down into lowland basins from adjacent highlands. Rocks of the old highlands were over one billion years old, and were similar to the ancient granite and gneiss found in the southeastern parts of the park. These early sites of ocean bottom deposition were formed along the ancient margin of the North American continent as an older and larger supercontinent broke apart.