Hi, I too am interested in learning more about Mary McMahan and Zachariah Carwile. Mary is believed to be a sister of Anna W. McMahan (my gg grandmother) daughter of William McMahan of Greenville Dist,SC.. In our Genealogy Library in a book of records from Abbeville,SC I found:
Mary McMahan Carwile b/19 Sep 1755 in VA d/12 Dec 1833 in Abbeville,SC in 1771 m/Zachariah Carwile b/1750 in Goochland,VA (it became Albemarle,VA) d/1841 in Abbeville,SC they were listed as having 9 children: William David Carwile James Rebecca Mary Anna John Schuyler Josiah Samuel Zachariah,Jr.
Do you have more on Mary and Zichariah? I would like to share more if you are interested.