I believe the marker is referring to the Great Swamp Wildlife Observation Center trails within the refuge. It's a series of boardwalks that lead to several wildlife observation blinds and totals around 2 miles. The trails aren't listed in any books that I know of, though you can find a map here: http://www.friendsofgreatswamp.org/Docs/RefugeMap.pdf . It's a nice, easy hike through the refuge, and wildlife is abundant. When we went, the swamp was all but dried up due to a periodic drought, however, I hear the trails are worthwhile when the swamp is more "swampy".