The Well at the World's End by William Morris

On the morrow the Sage led them straight into the rock-sea whereas it seemed to them at first that he was but bringing them into a blind alley; but at the end of the bight the rock-wall was broken down into a long scree of black stones. There the Sage bade Ralph and Ursula dismount (as for him he had been going afoot ever since that first day) and they led the horses up the said scree, which was a hard business, as they were no mountain beasts. And when they were atop of the scree it was harder yet to get them down, for on that side it was steeper; but at last they brought it about, and came down into a little grassy plain or isle in the rock sea, which narrowed toward the eastern end, and the rocks on either side were smooth and glossy, as if the heat had gone out of them suddenly, when the earth-fires had ceased in the mountains.

Now the Sage showed them on a certain rock a sign cut, whereof they had learned in the book aforesaid, to wit, a sword crossed by a three-leaved bough; and they knew by the book that they should press on through the rock-sea nowhere, either going or returning, save where they should see this token.