Once in the ‘70s, I went to the Manassas Battelfield and located the railroad cut where Union charges were repeatedly thrown back during the second battle. The portion of the cut I found was in a light woods in which the trees were probably less than fifty years old. The cut was well defined, but unpreserved. I walked out about fifty yards from the cut and began noticing rectagonal areas of slightly sunken soil, typically ten to fifteen feet long and five to six feet wide. These were scattered in the thin woods parallel to the cut and going back toward the cut. The impression I had is that these were burial pits, arraigned for the convience of those gathering the bodies for burial.
Is this known or did you figure this out on your own. If you did, maybe others would be interested in what it looks like you may have found.