I actually have July 9, 1787 for Ephriam and Martha's marriage, but it's close enough. I have Ephriam's parents as William Elder and Ann, no other information. But William Elder's parents were Peter Elder, Jr. and Elizabeth. Peter Elder Jr's father was Peter Elder, Sr. I have information passed to me from my grandmother that states Sr was deeded a track of land from captain david Munnsell in Essex County, VA on March 22, 1665 (then known as Old Rappahanock County)
The Elders belonged to the MacIntosh Clan which was founded by McDuff, earl of Fife.
I hope this helps-- please share if you find any information to help fill in the gaps.