A geochemical cascade
The Petrifying Well of Knaresborough is a geological feature showing rapid mineral deposition. The process is not petrification, where organic material is replaced by minerals, but rather a rapid encrustation. Water sourced from an underground lake flows through an aquifer, dissolving minerals from gypsum and limestone deposits of the Permian period. Specifically, the spring water emerges from the Cadeby Formation dolomite but has high sulfate levels from the overlying Edlington Formation gypsum.
This water has a high concentration of dissolved calcium carbonate. When the spring water emerges and is exposed to the air, it releases carbon dioxide. This change in gas pressure reduces the water's ability to hold dissolved calcite, causing the minerals to precipitate out of the solution. The water deposits a thin layer of this mineral, a form of calcite called tufa, on any surface it touches. The process is fast enough to visibly coat objects in weeks. A small teddy bear can be covered in a hard, stone-like shell in just three to five months. Larger, non-porous items might take up to 18 months to be fully encased. The constant flow rate of 3,200 liters per hour ensures the process is continuous.
England's oldest attraction
The well has been a documented curiosity since at least 1538, when the antiquary John Leyland noted its existence. At the time, its properties were believed to be magical or to have healing powers. In 1630, the land was sold by King Charles I to a local man, Sir Charles Slingsby, who began charging visitors for tours. This makes the site England's oldest continuously operating tourist attraction.
For centuries, local residents and visitors have placed objects in the well's cascading waters. Items left to be encrusted range from everyday trinkets to more unusual objects. A Victorian top hat and a lady's bonnet left in 1853 are now permanent, lumpy features on the rock face. In 1923, Queen Mary left a shoe to be coated. More recently, items have included a bicycle and even a handbag belonging to the novelist Agatha Christie. The mineral deposits build up so quickly that the well face must be scraped down regularly to prevent the overhanging rock from becoming too heavy and collapsing, as it did in 1816 and 1821.