The Col. George Foster mentioned is George P. Foster, a militia colonel in Yell County, bc 1823 SC, d. 10-1-66 Yell County.
The Gustavus Elgin mentioned is probably Gustavus Samuel Elgin (1829-66), a KY-born merchant in Clay County, MO who was later qm on Clark's staff.
I haven't been able to find "W. B. Cory"