The fellow local blogger Callan Bentley who writes the very informative geology blog Mountain Beltway organized a hike up the nearby Sugarloaf Mountain this past Saturday. My wife and I considered joining them, but for some insane reason they were adamant about starting at 9 am when we would still be waiting for our coffee to brew. So we missed the trip. Hopefully, there will be more opportunities in the future to join Callan in the field during more reasonable hours of the day.
Meanwhile, Callan e-mailed this morning some interesting pictures he had taken on Sugarloaf during the hike. They show what appear to be gastropod feeding tracks in the layers of algae (cyanobacteria) covering the surfaces of rocks.
I suspect the tracks were made by slugs. Compare them with the examples I have in the posts here, here and here.
The underlying rocks are, according to Callan's post, quartzite.
Now, I draw your attention to the following papers:
Danin, A. 1986. Patterns of corrosion and abrasion induced by Mediterranean land snails on limestone rocks. Malacolological Review 19:91.
Shachak, M., Jones, C.G. & Granot, Y. 1987. Herbivory in rocks and weathering of a desert. Science 236:1098-1099.
Jones, C.G. & Shachak, M. 1990. Fertilization of desert soil by rock-eating snails. Nature 346:839-841.
The snails mentioned in the cited papers apparently eat the limestone rocks to extract the endolithic lichens* that are their actual food. The result of this phenomenon is the slow conversion of rocks into sand. But this is somewhat different than what was observed on Sugarloaf Mountain. On Sugarloaf the slugs are eating the algae that grow externally on the rock surface. Moreover, quartz is much harder than limestone (calcite) on the Mohs Scale of mineral hardness. Therefore, the title of this post notwithstanding, I actually doubt the slugs could inflict any significant damage to the rocks of Sugarloaf Mountain.
*Lichens that grow inside, but near the surface of rocks.