I was just looking around online and found this image. I think it was taken by Alexander Gardner after Gettysburg.
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As I've come to understand it, the photos of post-battle dead, specifically those from Antietam and Gettysburg, are exclusively Dead Confederates. The Union controlled the field after those battles and the Confederates left their dead unburied and moved on. Interestingly enough, I've seen pictures where burial parties dug graves for Union soldiers but left the Confederates unburied, at least for the moment... like the case of this photo:
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Anyway, back to the subject of the first photograph. Recently, I forget where, I saw this photo captioned as dead union soldeirs after Gettysburg. I think this was determined by their dark jackets. Maybe this is true (and notice the bodies are missing shoes). But I also notice these men are wearing shell jackets instead of the typical infantry 4-button sack coat. As I understand it, shell jackets or frock coats were more common among Confederates. Perhaps these men are wearing dark gray, 7-button, Richmond Depot shell jackets like this one:
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