Res Gestae

But the part of that same Pontic gulf which is scourged by the north wind and by frosts is so completely bound in ice, that neither are the courses of the rivers believed to flow beneath the ice, nor can men or animals keep their footing on the treacherous and slippery surface, a defect which an unmixed sea never has, but only one which is mingled with water from rivers. But since I have been carried somewhat farther than I expected, let us hasten on to the rest of our story.