You can take MARTA from the airport to downtown and on to Fernbank.  While on your journeys, you should get off the MARTA train for a few minutes at the Peachtree Center MARTA station.  Here you will see gneiss that is part of the Clairmont Formation.  This formation underlies part of the eastern Atlanta area and consists mostly of dark, contorted gneiss with inclusions of other rock types.  The Clairmont formation is believed to be a metamorphosed Paleozoic tectonic mélange.   While at the MARTA station, you call also look at architectural facings of fossiliferous black marble.