On the top of a hill to the west of Llangadfan, there are the remains of a very large carn not less than sixty yards in circumference. It now consists chiefly of small round stones, the larger evidently having been carried away by some of the farmers. The name of the place is Pen Cad Cymry, the head battle of the Welsh; and the tradition in the neighbourhood respecting these remains is that there was a church there at one time. This tradition may have originated from the circumstance that it was at one time a place of interment.
He also mentions Garneddwen (white) and Garnedd las (blue) cairns that are not far away, and a great number of smaller barrows. Rather than head, I think 'Pen' refers to the bare mountain top?
From 'History of the Parish of Llangadfan' by the Rev. Griffith Edwards, in 'Collections historical & archaeological relating to Montgomeryshire, Volume 2' http://books.google.co.uk/books?id=tS4LAAAAYAAJ&pg=PA441