Well Winter Hill is the 'source' of this folklore.
Perhaps we could not do better than take as our guide that eminent local antiquarian Dorning Rasbotham.Quoted in 'Horwich: its history, legends and church' by Thomas Hampson (1883).
"Sept. 12th, 1787, I went this day to visit a remarkable stone, and took with me the landlord of the alehouse at Moorgate (Horwich) as my guide. In this excursion, after having the Winter Lads some time on our left, we proceeded over Winter Hill in which situation was about sout-west or north by south. The stone lies upon the declivity of a hill in the township of Turton. It goes by the name of the Hanging, or Giant's Stone.
The tradition of the common people is, that it was thrown by a certain giant upon a certain occasion (the nature of which they do not specify) from Winter Hill on the opposite range to this point, and they whimsically fancy that certain little hollows in the stone are the impressions made by the giant's hands at the time he threw it; but I own I could not find out the resemblance which was noticed to me. It appears, however, to have long excited attention, for that it is a heavy, gray, moor stone; a rude mark of a cross, that about 7 inches by 6 inches, appears at a very distant time to have been cut upon it. It is elevated upon another piece of rock, and its greatest length is 14 feet, its depth in the thickest part 5 feet, and its greatest breadth upon the top, which is nearly flat, is about 9 feet. The height of the highest part of it from the ground is about 5 feet 8 inches. A thorough going antiquary would call this a Druidical remain.
'Certain little hollows' might sound like cup-marks. But rock art with folklore? Let's not get overexcited.