From Perrin's 1883 History of Alexander, Union and Pulaski Counties, Illinois. Page 286:
The county seal when explained, tells how the county came to be named Union. The figures upon the seal represents two men standing up and shaking hands. One of them is dressed in the old-fashioned shad-bellied coat and vest, broad brimmed hat, and long hair. The other is in the conventional ministerial suit. It represents a meeting of a Baptist preacher named Jones, and George Wolf, a Dunkard preacher, as one of two men, first in this county. Jones had been holding a remarkable series of meetings, and Wolf and he met,
shook hands, and agreed to hold or continue the meeting, the two joining in the work, and calling it a Union meeting. This was held in what is now the southeast portion of the county. The seal illustrating this historic incident in the county was designed and adopted by the County Commissioners in 1850, and it was, it is said, the suggestion of Gov. Dougherty. The meeting of these pioneer preachers that thus became historical, probably occurred about 1816 or 1817.