A couple of weeks ago we visited Cunningham Falls State Park near Frederick, Maryland. When during a hike I stopped to look at a trail sign, I noticed some peculiar marks on the back of the sign on the other side of the post.
It didn't take too long to recognize them as the feeding tracks of slugs. Here is a close-up.
The brownish layer covering the back of the sign is apparently some sort of algae (cyanobacteria), which slugs like to eat. You can see the marks left by the individual teeth of the radulae of at least 2 slugs, one large and one small.
This post was also about slug feeding tracks.