In the summer of 1815 an extraordinary hand-painted map was published in London. Some eight feet tall and six feet wide, brightly coloured - in sea-blue, green, bright yellow, orange, umber - it presented England and Wales in a beguiling and unfamiliar mixture of lines and patches and stippled shapes. It was the product of one man's obsession with rocks, a passion that sustained him whilst the rest of his life slid into ruin.

For nearly twenty years, an Oxfordshire blacksmith's son named William Smith journeyed across Britain investigating and naming the layers of rock beneath his feet. Self-taught and determined, Smith had great expertise in practical geology, and this evolving science demanded a new sort of delineation. The beautifully executed map he produced was the first of its kind and transformed the way in which the world was understood. It was a document that laid the groundwork for the making of great fortunes in oil, iron and tin, and, elsewhere, in diamonds, platinum and silver, and was key to the development of one of the great fields of modern science.

Smith's was a remarkable achievement, and all the more astonishing for having been completed single-handedly and without financial or professional support.