This mound - cairn? - doesn't get a spot on the Magic map, so I admit I can't comment on its prehistoricness. But there are so many other prehistoric cairns in the vicinity, perhaps it really is one.
Robin Hood's Grave is an oblong mound, seven yards by three. It is situated at the bottom of a narrow rocky dell at the head of Crosby Gill, where the footpath from Orton to Crosby enters the woods, once the chase of Sir Lancelot Threlkeld. It is noticed by Mr Sullivan in his "Cumberland and Westmorland," but he speaks of two heaps: this is, however, a mistake, there being only one. Of this mound he says "It was once customary for every person who went a-nutting in the wood, at the south end of which this heap is situated, to throw a stone on Robin's grave, repeating the following rhyme:-
Robin Hood, Robin Hood, here lie thy bones;
Load me with nuts as I load thee with stones."
From 'The Vale of Lyvennet' by J S Bland, published 1910.