The outstanding feature of Pinhook Bog is the tree-covered mat of sphagnum moss. Sphagnum moss is a stringy, delicate moss of a light-green color. The mat floats on top of the water and can become thick, yet it can have a pocket only a few inches thick in the middle. As the mat thickens, larger and larger plants take root and grow. Under the mat a peat bed develops. The acidic water slows the decay of the sphagnum moss and other plants. With time, the peat may fill the bog from the mat to the bottom. As the moisture becomes less acidic, typical land plants take root and the bog disappears.