“Wet With Yesterday’s Blood” by Ian Knight
"By late afternoon, the great army had begun to retire back towards the Ngwebeni valley, where they had started the day, carrying their loot and their wounded with them. Friends and relatives of the dead dragged many of the corpses into the dongas which flow below the present site of the St. Vincent’s mission, and covered them over. Other corpses were simply left with their shields covering their faces in token burial."
More to the point:...
It would be months before the British returned to bury their dead, covering the bones with piles of stones, the origins of the cairns which are a conspicuous feature of the site today.
Why did it take so long. It may have been possible that a burial party would have found incriminating documents’ that would undermine Chelmsford created cover-up.