I'll try again.

Whist I know nothing of American migrations, Sharp felt that some of the ballads he was collecting in the remoter corners of the Appalachians had been hiding there for several centuries.

Might I suggest looking for scarcer ballads that have material that has died out in Britain, that is possibly closer to the earliest versions from the 17th century.

I'm sure I've seen examples of this and of course my study of 'Bramble Briar' shows this did happen though of course it's not a Child Ballad. Mind you it has better credentials than some of Child's higher numbered ballads.