McDonald Connection

I have traced my roots back to David Lewis McDonald I and his wife, Martha, who lived in Chester Co, S. Carolina and in Georgia. I have found her headstone and her name is given as Martha Benge Duncan McDonald. Her mother may have been Dorcas Lightfoot Benge Duncan, but I can find no record of Dorcas and a daughter named Martha. Any help would be appreciated. Also, does anyone know from whom Dorcas acquired the Benge name? Was her mother Go-ha-Ga? and who was her father?