Cahokia Courthouse State Historic Site is located a few miles south of Cahokia Mounds SHS. Follow HWY 3 south from the I-70 exit on the Illinois side to Cahokia. The courthouse/post office and its court clerk/postmaster, John Hay, played an important role in the Corps's preparations for the journey west. The front of the courthouse, top left. An early 19th century furnishing of the courtroom, left. Both Lewis and Clark had letters and supplies sent to and from the post office, above.