Bogs are most common in the northern states where glaciers formed depressions. Bogs have poor drainage. They are fed by precipitation washing things in from the land around them, so dead organic matter like moss and leave wash in, building up as peat. They are acid with a pH of under 4.2. Plant diversity tends to be low. Sedges, cranberries, northern white cedar, red maple, and black spruce grow in some northern bogs. Sphagnum moss may form hummocks (semi dry islands) where ferns and shrubs can grow. In the south, red maple and insectivorous plants may grow in nitrogen poor bogs. Bogs may range from less than an acre to 100 acres, some holding water only a few weeks while others are almost permanent.