down by erosion on hard crystalline rocks, then uplifted to form a plateau. The remainder of the state is occupied by the Coastal Plain. This is crossed by foothills and rolling prairies in the central part of the state, where it has a mean elevation of about , becomes lower and more level toward the southwest, and in the extreme south is flat and but slightly elevated above the sea. The Cumberland Plateau region is drained to the west-northwest by the Tennessee River and its tributaries; all other parts of the state are drained to the southwest. In the Appalachian Valley region the Coosa River is the principal river; and in the Piedmont Plateau, the Tallapoosa River. In the Coastal Plain are the Tombigbee River in the west, the Alabama River (formed by the Coosa and Tallapoosa) in the western central, and in the east the Chattahoochee River, which forms almost half of the Georgia boundary. The Tombigbee and Alabama rivers unite near the southwest corner of the state, their waters discharging into Mobile Bay by the Mobile and Tensas rivers. The Black Warrior River is a considerable stream which joins the Tombigbee from the east. The valleys in the north and northeast are usually deep and narrow, but in the Coastal Plain they are broad and in most cases rise in three successive terraces above the stream. The harbour of Mobile was formed by the drowning of the lower part of the valley of the Alabama and Tombigbee rivers as a result of the sinking of the land here, such sinking having occurred on other parts of the Gulf coast. Flora and fauna The fauna and flora of Alabama are similar to those of the Gulf states in general and have no distinctive characteristics. However, the Mobile River system has a high incidence of endemism among freshwater mollusks and biodiversity is high. In Alabama, vast forests of pine constitute the largest proportion of the state's forest growth. There is also an abundance of cypress, hickory, oak, populus, and eastern redcedar trees. In other areas, hemlock growths in the north and southern white cedar in the southwest. Other native trees include ash, hackberry, and holly. In the Gulf region of the state grow various species of palmetto and palm. In Alabama there are more than 150 shrubs, including mountain laurel and rhododendron. Among cultivated plants are wisteria and camellia. While