A large triangle shaped craft, close to the ground, with no sound, at all.

This occurred in August, 1978, about sunset. I was 21 years old and recently completed a 3 year tour of service in the U.S. Army.  Two of my friends and I had completed a day’s work and decided to have a beer, ( one 6-pack, we were by no means intoxicated)  We stopped on a back road outside Carbon Hill Alabama called “ Nubin Ridge “.   Which is the highest point around the small town.                                                        
While we were talking, I was facing the direction this thing came from.  Then I saw a light in the top of the trees.  We were higher than the trees because of the lay of the land.  At first I thought it was a helicopter because the light lit up the tree tops and was coming towards us. I told the other guys to look.  Now this light was getting close (it was probably a quarter mile away when I first saw it ).  It was not going fast at all, Maybe 25 or 30 MPH.                                                                           !
Then the actual light comes in to view.  It’s very bright and low to the ground, 500 ft or less. It gets within 2 or 3 hundred feet of us when I made out the shape of this craft, a triangle, black, and very large, I would say at least as big as a football field or bigger.  It goes right over us; the light actually lights up the road we’re standing on and never makes a sound. Being in the military,  I knew about aircraft, and this was nothing like anything I’ve ever seen, then or now.  I never told anyone for fear of looking stupid, especially back then.  
I was brought up in a christen home so that sort of thing was taboo.