Originally, Augusta County was a vast territory with an indefinite western boundary. Most of what is now West Virginia as well as all of Kentucky were formed from it, and it also claimed the territory north and west of those areas, theoretically all the way to the Pacific Ocean. (source: Wikipedia.com)
In addition to much of what is now Ohio, Illinois, Kentucky and West Virginia, present Virginia Counties formed up to 1800 from Augusta County include: Botetourt (1770), Fincastle (formed from Botetourt in 1772), Monongalia (1776), Ohio (1776), Yohogania (1776, abolished in 1786), Montgomery (1776), Washington (1777), Illinois (1778, abolished in 1784), Rockingham (1778), Rockbridge (1778), Hardy (1786), Harrison (1784), Randolph (1786), Russell (1786), Pendleton (1788), Greenbrier (1778), Bath (1790), Wythe (1790), Lee (1792), Grayson (1792), Brooke (1797) Wood (1798) and Tazewell (1800). Source: http://www.myvirginiagenealogy.com/va_maps.htm
This map was based on an 1885 Rand McNally Atlas, with the approximate 1761 boundaries of Old Augusta added.