Before 1980, all
ATL flights used a terminal located on the north side of the airfield, where the hotels and Delta headquarters are located now. This terminal was used by all airlines, not just Delta.
The building opened in 1961 with five concourses radiating out in all directions, a bit like Miami or Chicago O'Hare. A sixth concourse was added in 1962.
The main terminal had a vaulted roof - one of the first in the world to do so - and was topped by a high-rise office building and control tower. Most of the gates at the airport were ground level, and passengers boarded aircraft by walking outside and ascending a stairway to the plane's door. Delta's two concourses were equipped with twelve jetways, which back then was a big deal and gave Delta considerable bragging rights. Eastern (Delta's biggest competitor in Atlanta) installed jetways on the concourse it opened in 1962, but the rest of
EA's gates remained at ground level.
When the terminal shut down in 1980 it was left abandoned for about five years. Two of the concourses were demolished almost immediately to make room for the northernmost parallel runway. Three more came down in the mid-1980s. The last concourse (the one built later, in 1962) stuck around into the mid-1990s and served various general aviation and charter companies. It's gone now too.