Burkes Landed Gentry 1939, a book, states: William Mountcastle left England in the latter half of the 18th century & settled in South Carolina. He married in 1804 Susan Moore & died 1840 leaving with other issue a son: Andrew Jackson Mountcastle born 1816, marr Cornelia Frances Williams & died 1885 having had issue George Williams Mouncastle born 2 Feb 1871 & Eula born 1875. The book then goes on to give more info about George Williams Mountcastle. Is this yr William? Have you since found out where he came from in the UK?