In the year 880 a man named Eldad Ha-Dani turned up in the North African city of Kairouan claiming to have come from a land somewhere near Ethiopia called Havilah. According to Eldad, Havilah was where the Ten Lost Tribes lived. Close by was the land where the children of Moses lived. Nobody has ever met them, because their country is cut off from the rest of the world, by the impossibly mystical river Sambatyon, a river made not of water, but of sand and stones. Eldad described the river Sambatyon as a kind of fortress, a terrifying torrent, impossible to cross because of the violence with which it hurled its rocks many feet into the air. Miraculously though, for all its fury, the river ground to a halt on Shabbat. At sunset on Friday, as Shabbat drew in, a cloud would descend over the river; it would cease its raging and calm would descend. But now there were other impediments to crossing the river. Not only did the cloud envelop and obscure the river, hiding it from view, but even if it could be found the river still could not be crossed, because it was Shabbat. And everyone knows that rivers are not to be crossed on Shabbat.