It's no wonder he's in the American canon, he's got the proper name.
To mark twain means to note that it is safe between these two areas. So it was safe to guide your ship on the river in this manner. To mark twain was to sound the river. Sound is an interesting word in English:
We sound-off when someone is trying to ascertain how we feel. In taking this measurement and we are healthy, we are said to be sound as a bell. When we want serious punishment we call for that person to have a sound thrashing.
It sounds good, but it is still only an impression.
When we are deeply asleep, we are soundly so; it is the depth that whales travel to and it is the body of water in which they might travel (as in the Puget Sound in Seattle).