What is tufa (and how / why is it formed)?
Textbook definitions include:
- "Tufa: n. a chemical sedimentary rock of calcium carbonate, precipitated by evaporation; it commonly occurs as an incrustation around the mouth of a spring or along a stream."
Source: "Collins Dictionary of Geology" (Harper Collins Publishers, 2004) - "Colour: White, yellow, red, brown.
Texture: Compact to earthy; friable.
Structure: Tufa is a porous or spongy rock . . .
Mineralogy: Principally calcite; impirities of iron oxides are responsible for yellow and red colours.
Field relations: These rocks are produced by the precipitation of calcite through water evaporation around hot springs or in caves, where they form thin deposits of no great extent."
Source: "Philip's Minerals Rocks & Fossils" Philip's (2003)
However, these definitions are only part of the story. It is now known that certain species of plants can play an important role in the formation of tufa - and this appears to be the case in Stainforth Cutting. Ferns, mosses and lichens thrive on the shaded, damp limestone walls of the cutting, These catch droplets of water, which slowly evaporate leaving a fine film of calcium carbonate on the plants. The calcium carbonate builds-up to the extent that it starves the plants of sunlight and oxygen, so the plants die. New mosses & ferns grows on the film of calcium carboate and the cycle continues. Eventually the entombed plants decay, leaving behind a limestone 'sponge' (i.e. a delicate calcium carbonate matrix full of holes).
A detailed (academic) discussion on the subject of 'tufa' can be downloaded from the Field Studies Council website via: http://fsj.field-studies-council.org/media/348120/vol5.3_138.pdf
Acknowledgements
Researched and written by Mark R. Harvey.