Old Rappahannock County Virginia was formed out of Lancaster County in 1656. [Hening Statues, i. p. 427 and Hening Statutes iii.p. 104] It contained the present Virginia counties of Essex (formed 1692) and Richmond (formed 1692), and parts of Westmoreland (formed 1653), King George (formed 1721), Stafford (formed 1664), Caroline (formed 1728), and Spotsylvania (formed 1721) Counties. It had lands on both sides of the Rappahannock River, and was considered the Upper Parish of Lancaster County in 1656. It became Sittingbourne Parish in 1661, and it is by this name that it is referred to in many of the primary documents of this period. Old Rappahannock ceased to exist in 1692. The present Rappahannock County Virginia was created in 1833, and must be distinguished from the "Old".
From the above comments, the great difficulty for the genealogist doing early Virginia history is presented. The settlement pattern varied, often from year to year, with counties and their parishes changing names frequently. It is important to make these distinctions when trying to short out the family tree. Two important and helpful references are:
Hening, William Waller. Comp. The Statutes At Large: Being a Collection of All the Laws of Virginia From the First Session of the Legislature in the Year 1619. Richmond, 1810-1823, 13 Vols.
Robinson, Morgan P., Virginia Counties: Those Resulting from Virginia Legislation, originally published as Bulletin of the Virginia State Library, Volume 9 (January, April and July 1916), Richmond, Virginia. Reprinted by Genealogical Publishing Co., Inc., 1992.
For a history of Old Rappahannock County see:
Warner, Thomas Hoskins : History of Old Rappahannock County Virgina 1656-1692, Tappahannock, VA, 1965.