Editing Lichenometry (section)

==Methods==
[[image:Xanthoria elegans 97571.jpg|thumb|right|''[[Xanthoria elegans]]'' was one of the first lichens used for lichenometry.]]
Several methods exist for dating surfaces with help of lichenometry; the most simple relies on a single largest lichen though other methods use more. There are also differences in the way the lichen is measured; some scientists suggest that the largest [[diameter]] should be measured, but others prefer the diameter of the largest inscribed circle. A problem in dating lichens is the fact that several [[thalli]] can fuse together, making several minor lichens appears as a larger one of older age.<ref>[http://www.blackwellpublishing.com:443/Microbiology/pdfs/lichens.pdf Lichen, Lichenometry and Global Warming] {{webarchive|url=https://web.archive.org/web/20110708012620/http://www.blackwellpublishing.com:443/Microbiology/pdfs/lichens.pdf |date=2011-07-08 }}</ref> The lichenometrist Tom Bradwell has listed the following five method families as the principal ones into which most other methods can be classified:
*'''Largest lichen (LL):''' When the single largest lichen of a species is used it means that the lichen that is oldest or grows in most favorable conditions is used to date the minimum age of the exposed surface. This was the original lichenometric from which others then developed or used as reference. Despite relying upon a single lichen this technique is praised for its simplicity and allows obtaining an image of the age of rock exposure while still in the field.<ref name=GA>{{cite journal |last=Bradwell |first=Tom |year=2009 |title=Lichenometric Dating: A commentary in the light of some recent statistical studies |journal=[[Geografiska Annaler]] }}</ref>
*'''Largest five lichens (5LL):''' This method is a development of the '''LL''' and was developed in the 1970s to avoid reliance on one single potentially anomalous lichen. It has been proved that neither accuracy nor precision improves significantly by having more than five lichens.<ref name=GA/>
*'''Fixed-area largest lichen (FALL):''' This technique was initially specially designed for dating rockfalls and [[talus cone]]s with no uniform age of deposition. The largest [[thallus]] in a unit area is measured. The sample areas are usually boulders with surfaces of about 1&nbsp;m<sup>2</sup>.<ref name=GA/>
*'''Size-frequency approach (SF):''' The analysis of size and frequency of lichens was initially done in order to study lichen populations and preexisting thalli growing on surfaces, but has since been used as an effective absolute and relative dating method.<ref name=GA/>
*'''Lichen cover approach (LC):''' This method works with the premise of that the area covered by a single species will increase over time, and by measuring the total area percentage covered by a certain lichen species the age of exposure can be inferred.<ref name=GA/>