Glennwood contains a circumneutral bog, one of only 15 in Indiana. The bog once was a lake, but it is now nearly filled with a springy mat of mosses, ferns and sedges resting on a bed of peat. Poison sumac, pin oak, willow and red maple surround the bog area. In the forest around the bog grow Canadian mayflower, shiny club moss and sarsaparilla, as well as cinnamon, crested shield and royal ferns. The preserve is noted for spectacular fall color.