Monday, May 01, 2006

Lichenometry

I was thinking about measuring lichen buildup on the rocks in a rock pile - perhaps by measuring the diameter of lichen patches. But where it would get interesting is in comparing measurements with those of other rocks and other structures at the same location. For example, fixed boulders can be expected to have the largest lichen patches. Rocks in a rock pile might have smaller patches, rocks in a nearby stone wall might have patches the same size as in the rock pile. You would not get a sense of absolute age of rock piles but you might get a sense of the relative ages of different things at the site.

7 comments :

pwax said...

I am not saying that patch diameter is the right thing or even meaningful. Maybe a better metric would be #patches per unit area. I am just saying that there ought to be a relation between what you measure and relative age of different things at one site.

Anonymous said...

Lichen growth is also affected by dampness of the its particular micro-environment, amount of sunshine or lack of, and mineralogy of the stone (i.e. quartz & feldspar are strongly resistent to lichen growth.) These three variables need to be account for when doing comparisons within a site. Once you control for the variables then one can begin a meaningful analysis.

James Gage
www.StoneStructures.org

Anonymous said...

Hey Peter!

Near me is a place called Black Rock because it used to be covered in Black Lichen and looked like a big black rock. Now, due to acid rain killing off the Black Lichen, it looks like a big grey rock.
As far as I know, no one is suggesting a name change...

Anonymous said...

Hello, new to this blog (and blogging in general) I've read that a good rule of thumb is thallus (patch) size increases ~ 1mm in diameter/year. Here's one source on dating petroglyphs...

http://mc2.vicnet.net.au/home/date/web/lich.html

Mark

pwax said...

Thanks for the input everyone.

James: I think dampness and sunlight might be assumed to be similar for different rocks within a site. Clearly the angle of the surface is important. My idea was to measure lichen growth on many different surfaces at a site.

Mark: thanks for adding a comment. But ~1mm per year can't be right. 1000 years would make patches a meter in diameter. At Stonehendge the patches are about 8 inches across.

Geophile said...

There are different kinds of lichen, too. It's a fun idea, but for now probably best as one of those things you watch for as you go to each site, noticing the variety of situations and coverage. Over time you could learn more about it from your observations.

That comparison between boulders in place and rocks in piles might be significant at many sites, though.

When there's no lichen at all it is hard to see things as old. I've run across that, too, when people have taken me to things they thought were old.

pwax said...

Geophile:
Presence of lichen seems not to be a good way to estimate age but I think absence of lichen is a good way to estimate youth.

Also, you have caught my drift exactly: lichen growth is something to try noticing a little more carefully - especially in comparing the rock piles with other rocks at a site. For example, in the pictures of crossed pairs I posted.