Tufa deposits are rather common in limestone karst areas, where limestone is dissolved by water and redeposited again. It is the subaerial form of the subterranean dropstones, but much more common. Tufa is deposited as soon as the karstification begins, even if there are no well-developed caves.
The process of solution depends on the carbon dioxide amount in the water, and processes which influence the carbon dioxide amount also influence the limestone solution. The solution starts when carbon dioxide rich water reaches the limestone, the carbon dioxide originates from the atmosphere, enters the drops of rain water while they fall to the ground. In covered karst the soil covering the limestone is full of life, and like all life based on oxygene the microorganisms feed on carbonates, oxydize them, and exhale carbon dioxide. So the amount of the gas is much higher in the soil than in the atmosphere.
In combination with the carbon dioxide the water becomes a very weak acid, carbonated mineral water has the typical smell of an acid. In the limestone the acid dissolves limestone until it is neutralized. Chemists say the water is saturated with limestone. But when the water reaches the surface, the environment changes, carbon dioxide leaves the water and so the water is over-saturated. This means, the water contains more limestone than it can hold, and the limestone is precipitated and deposited as soon as possible.
There are various reasons why carbon dioxide is removed from the water. The most important and common is the exchange with the surrounding air, which has a very low carbon dioxide content. In the soil the amount of carbon dioxide is ten times higher than in the atmosphere. As a result the water contains ten times as much carbon dioxide. As soon as it reaches the surface, the carbon dioxide moves between water and atmosphere, the water surface works like a membrane. The water continually looses carbon dioxide until it has the normal amount which is equivalent to the amount in the atmosphere around. The process of loosing carbon dioxide is accelerated by fast moving water, as any form of spraying increases the size of surface and thus accelerates the exchange. The increase of temperature adds to this process, as the amount of carbon dioxide which is dissolved in water decreases with rising temperatures.
And finally there is a biological component. As soon as the water reaches the surface, plants grow in the water. It starts with algae, then moss and finally higher plants like ferns and bushes. Especially the algae and moss, which grow in the flowing water, consume the carbon dioxide. They reduce it with photosynthesis and keep the carbon for their growth, the oxygen is released into the water.
As a result, the loss of carbon dioxide immediately after the spring is immense. The supersaturated water precipitates and deplosits an huge amount of limestone while it flows. Calcite crystals grow on the floor of the brook, on the plants, at the shores and in the flowing water. Most of the calcite covers the algae and moss, just to be overgrown by the next generation of moss.
The structure of the limestone is typical. Covering plants which subsequently are decomposed, the limestone has numerous holes of all sizes. It is rather soft and lightweight, the reason why it is dubbed tufa or tuff. When it is still wet, it is possible to cut it with saws or chains. But when it dries it becomes harder and lighter and is a very good rock for building houses. Because of its porosity is a very good isolator. It was often used to build barnstables, as the limestone neutralizes the emissions of the animals. Today tufa is not used anymore, because the quarrying is a lot of work and thus expensive, and the deposits are small and most of them protected.
Other places of high limestone deposition are steps in the floor and along the shores. The growing limestone forms dams across the brook, across the valley, and along the river. Sometimes the whole slope is covered by tufa, sometimes channels and dams are built on which the water flows.