Mountain of Sand would be Mynydd y Tywod, I would have thought. I live near a place called Mynydd Bach y Glo "Little Mountain of Coal". But I wouldn't use it as name, not for a person anyway. Sorry.
Welsh surnames tend to be a male name that ends with 's' now. After the English enforced translation of surnames, Welsh popular surnames are names like Jones (from John), Davies (from David), Evans (from Ifan), Hughes (from Huw), Roberts (from Robert), etc.
Is there any other surname you'd consider?
Gwlad, gwlad, pleidiol wyf i´m gwlad
Tra môr yn fur
I'r bur hoff bau
O bydded i´r heniaith barhau