Let us remember that the bank is first introduced in the first chapter of the novel and is used as a kind of play area for David and it is where he first meets Sophie. David mentions that he uses the bank as a kind of boundary marking out the area where he plays and from the rest of the territory where he does not play, as he regards the land beyond the bank as "foreign" for some reason. However, note what he tells us about the bank:

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.

We are thus given but tantalising glimpses of the bank and what it could be. The older David, looking back, seems to indicate that it was made by man for a specific purpose, which would point towards it being some kind of defensive fortification or man-made structure rather than being the result of the holocaust.