Read it through once

The mud deposit gradually extends the land--but only gradually; it has extended it not quite a third of a mile in the two hundred years which have elapsed since the river took its place in history. The belief of the scientific people is, that the mouth used to be at Baton Rouge, where the hills cease, and that the two hundred miles of land between there and the Gulf was built by the river. This gives us the age of that piece of country, without any trouble at all--one hundred and twenty thousand years. Yet it is much the youthfullest batch of country that lies around there anywhere.