The story goes that the Great Spirits Spring lay at Windfall Run and that vapors rising from it were known as the Great Spirit's Breath. King Arthur and his men came walking through the forest so he might drink of the Spring's water, as he was sorely wounded. He did so and the water proved efficacious. He remained there, drinking the water for ten years, then he died and was buried. The grave was in the middle of Windfall Run, ten feet deep and Arthur was buried in a stone coffin.
In 1951 some boys managed to open the grave, but it filled with water. In 1959 a politician fishing in the area fell into the grave and had to be hauled out of the water. The Forestry Department, at the politician's behest, filled the grave up.
Regarding how such a legend might have grown up, in Arthurian legend, after the Battle of Camlann, Arthur was taken to the Otherworld by Morgan Le Fay, to be healed of his wounds. In some Celtic traditions, the Otherworld was beyond the Atlantic. If you venture across the Atlantic, you will bump into America. That may have given rise to an idea that America is where Morgan took him.
For those wishing to read the original story, it is to be found in K.S. Goldstein and R.H. Byington, ed., Two Penny Ballads and Four Dollar Whiskey Hatboro: published for the Pennsylvania Folklore Society by Folklore Associates, 1966.