According to the Rita K Sutton book, "Early Carters in Scott County, Virginia", "Agnes Allen may have been a widow. Dr. Samuel Stallard made this entry in his Family Bible: 'My mother's father was Norris Carter. I think he came from Fauquier Co., VA. * My mother's mother was named Agnes Taylor before marriage to said Norris Carter.'"
Rita Sutton's book also says that Norris married Agnes Davidson.
* First hand info - not from The Books.
PS Norris is proven to Peter "of Fauquier" and Judith Norris Carter, all early settlers of Southwest Virginia.