From "The Daniel Dennistons of Beverley Manor and Descendants", by John B. Robb:
2. ANN DENNISTON (Daniel and her husbands PATRICK GILLASPY, and ANDREW PAUL 2 1)
Patrick and Ann (Denniston) Gillaspy settled in Frederick County, which from its creation in 1743 until 1772, encompassed all of the lower (northernmost) part of the Valley of Virginia, from just north of present-day Harrisonburg to the Maryland border. Presumably, Patrick, who made his will only a few years after marriage, was already established in the southernmost part of the county.
http://www.johnbrobb.com/Content/DennisonReport.pdf
