The pioneer cemeteries of Iowa stand two and three feet above the fields around them, with a fence round each one, and the step up from the corn to the graves is the topsoil that has left the state since the men in the graves broke the prairie. Iowa was the finest soil on the continent. Ten thousand years of tallgrass prairie, with roots fifteen feet deep, had built a black loam over a foot thick across the whole state, so rich the settlers of the 1850s said you could not kill it. They ploughed it in a generation. The grass went under, the corn went in, and every spring since then the bare field has met the Iowa rain with nothing holding it, and the soil has gone down the gullies into the creeks and the creeks into the Mississippi, which has carried it to Louisiana, where it is the Gulf's problem. The cemeteries were the one piece of each township never ploughed. The prairie grass survived inside the fence, the roots held, and the ground inside stayed where it was while the ground outside went south. So the graveyards rose, or seem to have, as the fields fell around them, and a man can stand at the fence and measure with his eye what a century and a half of corn has cost: half the topsoil of Iowa, by the state's own estimate, more than a foot in places, from the most fertile soil in the world, to grow a crop that is mostly fed to pigs or turned into fuel. The fields are still losing it at ten tonnes an acre a year in the worst years, five times faster than it forms, and the farmer makes up the difference out of a bag. For ten thousand years the bison grazed that grass and the roots held the state together. The men who ploughed it are the only part of Iowa still standing at the height they knew.