AMHERST CO, VA (made in 1761 from Albemarle County and "certain islands" in the Fluvanna [James] River.
The Act dividing the Counties of Albemarle and Louisa and the Parish of St. Anne was dated March 1761.

That from and after the first day of May next the said parish (St. Anne) shall be divided into two distinct parishes, in the following manner, that is to say, by Rockfish river to the mouth of Green creek, thence a straight line by the house of Thomas Bell to the Blue mountain, and that all that part of said parish that lies above the said river and line shall be one distinct parish, and shall be called and "known by the name of Amherst": and that all other part of said parish that lies below the said river and line shall be one distinct parish and "retain the nane of St. Anne".

{In 1761 the border between Albemarle and Amherst Counties would seem to be the Rockfish River, which I think is now the border between Nelson and Albemarle}
In October 1778 the parish of Amherst was divided by a line running from Fluvanna [James] River up Piney River to the mountains; the part on the lower side to be known as Amherst Parish and on the upper side, Lexington Parish.

lays next to

Nelson County was created in 1807, when it was separated from Amherst County.