"I was fascinated that in the 1800s, instead of signatures some relatives added their 'mark' to documents (a 'X') because they couldn't write."
It went on later than that as well, although typically this was more prevalent in groups that moved around a lot compared to most people.
When my dad's paternal grandparents (so my great grandparents) married in 1914 in Gloucester, both my great grandmother and her father (he was one of the witnesses) made their mark rather than signing. Although her mother did sign her own name.
My great grandmother at the time was a "box filler" in the local match factory. She literally filled boxes with matches for the "England's Glory" matches brand.
Her family came from a line of "watermen". They worked on the canals from Gloucester up to Birmingham. Her father is first shown on the 1871 census living on the barge "Anne", which was moored at Worcester, with his parents and an employee. His father was described as a "captain" at the age of 23, so it appears that he had his own barge by then.
By 1881, Henry was a "boat boy" at the age of eleven working for his father.
Henry married in 1890, at the age of 21, and by 1891 they were living in a house a couple of hundred metres away from Gloucester docks. Henry was described as a "waterman".
My great grandmother (the one who made her mark on the marriage certificate in 1914) had eight siblings. Of these, one other also made her mark on her marriage certificate but all the others signed.
Their father, Henry, was a witness at three of the weddings and he made his mark each time.