Kindly Professor Tolkien answers your question:
Quote:
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Away south upon the Hornburg, in the middle night men heard a great noise, as a wind in the valley, and the ground trembled; and all were afraid and no one ventured to go forth. But in the morning they went out and were amazed; for the slain Orcs were gone, and the trees also. Far down into the valley of the Deep the grass was crushed and trampled brown, as if giant herdsmen had pastured great droves of cattle there; but a mile below the Dike a huge pit had been delved in the earth, and over it stones were piled into a hill. Men believed that the Orcs whom they had slain were buried there; but whether those who had fled into the wood were with them, none could say, for no man ever set foot upon that hill. The Death Down it was afterwards called, and no grass would grow there. But the strange trees were never seen in Deeping-coomb again; they had returned at night, and had gone far away to the dark dales of Fangorn. Thus they were revenged upon the Orcs.
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(TT, The Road To Isengard)