Here is a dictionary definition I found for "slash".

"an open place in a forest, cluttered with branches, chips, or other debris, as from the cutting of timber"

I am not questioning your answer, but is it definitive or could the "slashed timber" referred to on a Civil War battlefield just be residue? This makes a great deal of difference to me in the understanding of the nature of the fighting. It would seem stacked timber would be highly protective for a defender, while "branches, chips, or other debris" would not be protective and would be a hindrance to movement.

Jim