What evidence was there around Waknuk of the civilization of the Old People?

A road....

The bank was no puzzle to me then: it was far too big for me to think of as a thing that men could have built, nor had it ever occurred to me to connect it with the wondrous doings of the Old People whom I sometimes heard about. It was simply the bank,coming round in a wide curve, and then running straight as an arrow towards the distant hills; just a part of the world, and no more to be wondered at than the river, the sky, or the hills themselves.