Up an down they go, with the tides. Marsh periwinkles, or marsh snails (Littorina irrorata) are found in and around the marshes in North Carolina.
Periwinkles play an important role in the decomposition of marsh grasses (detritus), which helps maintain a healthy salt marsh community. Science considers them an “indicator species.” If periwinkle populations are doing well, chances are the salt marsh is doing well. http://www.thepilot.com/news/2009/may/31/ask-the-aquarium-marsh-periwinkles-indicator/
I am always mesmerized by the numbers and beauty of the ecosystem of our marsh.
Here is a site with macro photography of these marsh snails by Jeffrey Pippen: http://people.duke.edu/~jspippen/mollusca/marshsnails.htm