Do not assume because your ancestor made his mark that he was illiterate. Deathbed wills frequently contain "marks" made by people who before their final illness were capable of reading and writing. Even for those in good health, they may have made their "mark" instead of signing because that was the custom or what they chose to do.
Even someone who signed their name may not have actually been literate. It is always possible that they knew how to "make" their signature and not much else.
And never assume that your ancestor was a "few bricks shy of a load" just because they were illiterate.