From Minnie Kendall Lowther's "History of Ritchie County:"

Margaret Morrison Lowther was a native of North Carolina. Her [grand]father was Archibald Morrison, and of Scotch-Irish birth, who emigrated from England to America some time before the Revolution, and settled on the Yadkin river, in North Carolina. Here he married a Miss Fooks. Margaret immigrated to Harrison County in her early womanhood. Near the year 1789 [08/04/1789], she was married to William Lowther. After their marriage, they settled near one mile below West Milford, on the farm that is now owned by the Highlands. Here they reared their family, and here they remained until near the year 1837, when they came to this county [Ritchie], where they spent the evening hours of their lives with their son, Archibald Lowther, at Holbrook. Mrs. Lowther was a woman of a devout religious character, a Presbyterian in faith, and her old Bible, which was her daily companion, is now in the hands of the writer. It bears the date of "1790," and is still held together by the old leather string that she ever kept about it. She passed away on May 13, 1850, and lies at rest in the Lowther burying ground near Holbrook, surrounded by the dust of five generations of their descendants. They were the parents of twelve children, six of whom reached the years of maturity.

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Marriage year listed as both 1787 and 1789:

Margaret Morrison in the Virginia, U.S., Compiled Marriages, 1660-1800

Name: Margaret Morrison
Spouse: William Lowther
Marriage Date: 4 Aug 1787
Marriage Location: Harrison County, Virginia

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Margret Morrison in the West Virginia, U.S., Marriages Index, 1785-1971

Name: Margret Morrison
Gender: Female
Spouse's Name: Wm Lowther
Spouse Gender: Male
Marriage Date: 4 Aug 1789
Marriage Place: Harrison County, West Virginia

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Margaret Morrison in the U.S. and International Marriage Records, 1560-1900

Name: Margaret Morrison
Gender: Female
Birth Place: NC
Birth Year: 1768
Spouse Name: William Lowther
Spouse Birth Place: VA
Spouse Birth Year: 1769
Marriage Year: 1789
Marriage State: VA
Number Pages: 1

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I don't think this is the correct Margaret but am listing it as someone's Ancestry tree has it connected to Margaret. This person was the same age but was born in VA and died in Randolph County:

Margaret Lowther in the U.S., Federal Census Mortality Schedules Index, 1850-1880

Surname: Margaret Lowther
Year: 1850
County: Randolph CO.
State: WV
Age: 82
Gender: F (Female)
Month of Death: May
State of Birth: VA
ID#: 197_198616
Occupation: NONE LISTED
Cause of Death: PALSY