Some of the springs that issue in Hackfall are very alkali and are saturated with calcite. Tufa has been, and still is being, deposited from these springs and along the streams that take the water away from them.
Tufa is a soft porous rock that is deposited by a chemical process from this calcite rich water. There are a number of theories as to what causes the chemical reaction necessary for the calcite to come out of solution. One hypothesis is that certain mosses and algae growing close to the springs extract carbon dioxide from the water as they photosynthesise. This removal of carbon dioxide causes the super saturated calcite to precipitate. While there are areas where there would appear to be sufficient moss and algae to have this effect such as at the “alum spring near to Kent’s Seat there are other places where this is an implausible explanation. There must therefore be other processes which can also cause or contribute to precipitation.
You can download a reference paper on Tufa in .pdf format here
THE TUFA DEPOSITS OF THE MALHAM DISTRICT, NORTH YORKSHIRE
A Pentecost (1981)
A number of geologists (and other scientists) have suggested alternative theories such as temperature change (Ek, and the appropriately named, Pissart, 1965), PH changes (Barnes, 1965), Evaporation (Kindle, 1927). On balance it would seem that where the spring water is super saturated with calcite it does not require much change for that calcite to be precipitated out of the water and form layers of Tufa.
The next question is where does the calcite come from. Although there is no limestone in the area, the nearest limestone is thought to be 400 meters below the site, it is known that the underlying geology of Hackfall includes two significant marine shell beds namely the Ure Shell Bed and the Cayton Gill Shell bed.
Both the Ure Shell bed and Cayton Gill Shell bed are bands of marine sand stone containing very many shell fossils including bivalve Brachiopods. It is possible to occasionally find one of these Brachiopods in the tufa on site and therefore we are fairly safe to assume that the calcium in the springs derives from either or both the Ure Shell and Cayton Gill Shell beds.
The following are a series of pictures of Tufa deposition in Hackfall.