landed UFO off I29 between Sioux Falls, SD & Sioux City, IA in late summertime 1978

I was traveling with one other person from Sioux Falls, SD back to Sioux City, IA, using I-29 going south.  We came on a UFO landed in a harvested corn field just to our right.  Off the side of the road, down the ditch & just past the fence line.  It was daylight,about 2 pm. Several other cars had pulled off the road to observe this craft.  Craft was round & large hovering very low to the ground/landed.To my memory it was black with black reflective (windows?).
We pulled over to the side of the road & observed the craft for about 10 minutes. Then the driver in our car left & we went on back to Sioux City.  Other cars were still there when we left.  (I would love to go under hypnosis to remember it better.)  
I was about 28 then, I am 63 now.  I would love to know if anyone else has written in on this particular sighting.  It has bugged me all my life.  I was too young then to know/persue it.  Would love to talk to someone if they have written in to you on this one.  
Date is approximate, but I know for a fact it was late summertime 1978.
Witness indicates that the date of the sighting is approximate.  PD