Bella Coola Lichen

A gravestone with lichen growth. Source: designscience.com

I was reading the charming Bella Coola Blog the other day and came across an interesting post discussing how lichen had been used to date glacial features in the area.  So-called lichenometry is a well-established dating technique which relies on the principle that lichen grows in roughly circular patterns, and that there is a linear relationship between the diameter of the circle and the age of the lichen patch.  Now, such patches can be many hundreds of years old.  So for an event where rock is newly exposed and then lichen grows on it, the lichen will give a minimum date on when the bare rock was first available as a lichen environment.  Anytime you have a bare rock and a dating technique for when the rock became bare, the archaeology antennae rise.

Churchyard at Hagensborg, Bella Coola Valley. Source: bellacoola.ca

Since lichen species and growth conditions will affect how quickly the lichen grows, then a local baseline of the lichen growth rate is needed.  A classic way of doing this is to measure lichen on gravestones, which normally come with a date carved right on them — very convenient!  So, if one gravestone has a date of death of 1870, and has lichen with a diameter 5 cm, and another has a date of death of 1920, and has lichen with a diameter of 3 cm, then you can start to create a graph relating size to age.

In the case of the Bella Coola valley studies, the lichen on the gravestones at Hagensborg was used, and then these growth curves were applied to glacial moraine features from the Little Ice ages, ca. 600 years ago.  The research was done at the University of Victoria by Dan Smith and colleagues.  You can download the paper here, and view other related papers for download here.

Lichen and moss on petroglyph panel, Gabriola Island. Photo: Dan Leen.

Obviously, this technique has potential applications in archaeology – rock art (petroglyphs, and possibly pictographs) could be dated this way, as could alpine features like stone wall hunting blinds.  Potential lichenometry is one reason why it is unfortunate that so many petroglyphs have had their grooves rubbed clean, chalked, or scoured – apart from the damage to the art itself this can eliminate the crucial parts of the lichen for dating purposes.  I have chatted more than once with Cairn Guy about doing lichenometry on burial cairns as well – at least establishing minimum dates for these archaeological features, and in a completely non-destructive, non-intrusive way.  I am sure it is not quite as easy as the researchers make it sounds, on the other hand, looking at their confidence interval in the figure below makes it seem like a worthwhile approach  It would be interesting to collaborate with the lichen guys and see what the possibilities were — a good interdisciplinary Master’s project perhaps.

Dating curve established from lichen growth. Source: Larocque and Smith 2004. Click to enlarge.

References:

Smith, D.J.; and Desloges, J.R. 2000. Little Ice Age history of Tzeetsaytsul Glacier, Tweedsmuir Provincial Park, British Columbia. Géographie Physique et Quaternaire, 54(2):135-141. [Download paper PDF]

Larocque, S.J.; and Smith, D.J. 2004. Calibrated Rhizocarpon geographicum growth curve for the Mount Waddington area, British Columbia Coast Mountains, Canada. Arctic, Antarctic and Alpine Research 36(4):407-418. [Download paper PDF]