Where the rocks and soil are rich in calcium carbonate, there are likely to be lots of snails, because snails build their shells out of calcium carbonate. Unfortunately for me, however, calcium carbonate containing rocks are rare where I live in Maryland. So, when I recently read in Mike High's The C&O Canal Companion (1997) that outcrops of a limestone conglomerate called Potomac marble could be seen at a location along the C&O Canal, I headed for the spot late Friday afternoon.
I parked my car at the Monocacy Aqueduct and got on my trike to travel on the canal towpath. My destination was a place called Camp Kanawha about 5 miles upstream from the aqueduct and reached from the towpath by a short trail that goes over the train tracks. Here is the entrance to the camp surrounded by boulders of Potomac marble. It was just like the description in High's book.
And here is a close-up of the rocks. I didn't have my acid bottle to test the rocks for calcium carbonate, but they are definitely a type of conglomerate.
I did a brief search along the bases of the rocks for snails. Here are the 4 species I found, clockwise from the lefthand corner: Gastrocopta armifera (live), Pupoides albilabris, Ventridens ligera (live), Stenotrema sp. The photos are not to scale; Gastrocopta and Pupoides were about 4 mm long, while Ventridens and Stenotrema were larger, about 7-8 mm wide.
The soil along the rocks was soft and damp and there was plenty of plant cover, providing an ideal habitat not just for snails but also for earthworms and all sorts of arthropods. Can you spot the tiny mite below and to the right of Pupoides albilabris?
It's not possible to evaluate the snail species richness of a location from a 10-min cursory "survey". But I suspect there are many more species to be found around rocks of Potomac marble. One confounding factor in this case is that crushed limestone is used as ballast under railroad tracks and if snails were indeed abundant at this particular location, it would be difficult to determine if the contributing factor were the presence of Potomac marble or railroad limestone, which was abundant.
Here is the MARC train taking commuters to Point of Rocks, the next station only a mile away.