The actual memorial has him as John G. Clark and the nearby sign telling his story has him as John J. Clark, either way, here it is.
John J. Clark was born in Canada in 1844. He enlisted as a private in Company F of the New York Infantry in 1862, serving actively in the Civil War. After being shot in the hand and contracting typhoid fever, he was honorably discharged on March 6, 1863 as a sergeant in Company B of the New York Calvary.
Following his discharge, Sergeant Clark emigrated to Southern California. While traveling from Bakersfield to Salt Lake City on a buckboard (a light, four-wheeled carriage), he stopped in Valley of Fire, tied his horse to the back of the wagon and wandered around, possibly looking for water. Eventually, he crawled under his buckboard and died, presumably from thirst, several days before his body was found on June 30, 1915.
This is located right off the highway in Valley of Fire State Park in Nevada.