Why Is There a Rock Balanced on a Spire? The Cap Protected What Is Under It

The requirement is a hard layer sitting on softer rock, which is an ordinary arrangement in sedimentary sequences. Rain and frost attack the soft rock, and wherever the hard layer has been breached the soft material below is removed quickly. Where a block of the hard layer survives, it shelters a column of soft rock directly beneath it from rainfall, and that column erodes far more slowly than the exposed ground around it. Over a long period the surrounding surface is lowered and the sheltered column is left standing, capped by the block that saved it.

The combination is specific, which is why they occur in limited places:

• •Alternating hard and soft layers, horizontally bedded
• •Rock soft enough to erode quickly, usually mudstone or soft sandstone
• •A dry climate, so chemical weathering and vegetation are limited
• •Repeated freezing and thawing, which prises the rock apart
• •Occasional heavy rain, which removes what the frost loosened
• •Slopes steep enough for the debris to be carried away

The structures are temporary by nature and their end is built into their formation. The column continues narrowing as it weathers, and eventually the neck beneath the cap becomes too slender to carry it, at which point the cap falls off. Once the cap is gone the whole column is exposed and disappears rapidly. Fields of them therefore contain examples at every stage, from a ridge just beginning to be dissected through mature columns to stumps whose caps have gone. The turnover is fast in geological terms, with estimates for the best known field suggesting individual columns last on the order of a few centuries to a few thousand years.

The formations attract explanation wherever they occur and the stories are worth separating from the geology. The English word arrived in the nineteenth century from an unrelated usage referring to folk magic in the American south, applied to the rocks by settlers and now standard in geology, which is an unusual borrowing. Local traditions in Utah describe the columns as people turned to stone, and that account is attributed to the Paiute. Turkish formations acquired their own legends. The tendency to see figures in them is doing what people always do with irregular shapes, which is finding faces, and the formations happen to be unusually good at prompting it.

Bryce Canyon in Utah holds the largest concentration anywhere, where a plateau edge of alternating limestone and softer rock is being dissected into a natural amphitheatre containing thousands of columns, and the freezing and thawing that drives it happens on roughly two hundred days a year at that altitude. Comparable formations occur in Alberta, in Turkey where the material is volcanic ash capped by harder blocks and the columns were hollowed into dwellings, in Serbia, France, New Mexico and Taiwan. Local names differ enormously and several are far older than the English term, which arrived from a quite unrelated usage in the nineteenth century.

A surviving block of hard rock shelters the soft rock directly beneath it from rain, so that column erodes far more slowly than the ground around it and is left standing as the surface is lowered. Alternating layers, soft rock, a dry climate and repeated freezing and thawing are all required. The neck eventually becomes too thin to hold the cap, and once it falls the column goes quickly.