The Gravelly Sea...

And beside Acre runs a little river, called the Belyon, and near there is the Fosse of Mynon, all round, roughly a hundred cubits broad; and it is all full of gravel.  And however much be taken out in a day, on the morrow it is as full as ever it was, and that is a great marvel.  And there is always a great wind in that pit, which stirs up all the gravel and makes it eddy about.  And if any metal be put therein, immediately it turns to glass.  This gravel is shiny, and men make good clear glass of it.  The glass that is made of this gravel, if it be put back in the gravel, turns back into gravel, as it was at first.  And some say it is an outlet of the Gravelly Sea. - The Travels of Sir John Mandeville