A friend of mine gave me an isochrone map of the Mississippi River before it got fixed in place by the levees, locks and dams of the last century. The map, drawn by a man called Harold Fisk, shows the tremendously long history of the river in a single delicious  page of geological spectacle. It's a thing of wonder — the ancient river, immortalised in riotous swirls of pinks, peaches, greens, yellows and blues, roves across time and space, filling the page with its many meanders. Here is every journey the river has travelled over its very long life all at once and everywhere. Until I gazed on the map I'd never seen a better illustration of the Greek philosopher Heraclitus' observation that you never step into the same river twice. ..