There is a phenomenon called super refraction. It can and sometimes does cause a mirage. It's a bending of light rays that causes mirages, and sometimes will make things appear to float. However they are usually, but not always attributed to conditions of high moisture content in the air.
My point was that even if Custer could have seen the hill tops from 14 miles away, he would not have been able to see the camp in the lower valleys
I truly doubt that was an event during the mid day from 'Lookout Hill'. My stated calcs were simple numbers based on common formulas. But these mirages will only display what could ordinarily be seen normally, they can't lift an object up and out of a valley for instance. One will see the higher points almost as if they were actually visable, but nothing that is normally blocked from normal vision. Think of it like seeing a boat mirage, but you won't be able to see it's anchor. Good point though.
jj