Mark Twain, one of my favourite writers. Much has been said about his wit and wisdom. His nom de plume has also inspired many stories.
Some conjecture that he got his pen name from working on the Mississippi river boats. The boatmen used to 'mark' - 'two fathoms' of water that was considered the safe depth for the boat to pass.
They used to shout 'Mark Twain' were 'twain' was an archaic form of 'two' across to the boats and this probably stuck with Mr Clemens.
So 'Mark Twain' in fact may be the cry that referred to the state of the river. The very river that inspired many of his stories and travelogues. It may all be bunkum, of course, but it is believable bunkum, nevertheless.