South of the Shawnee Hills, the land flattens, the drainage is poor, and frequent flooding occurs. Only knolls and ridges of the Cretaceous Hills break the broad plain of alluvium from the Cache, Ohio, and Mississippi River bottoms that make up this division. The coastal plain of Illinois resembles lands that surround the present-day Gulf of Mexico. This division is divided into the Cretaceous Hills and the Bottomlands sections. The Cretaceous Hills extend in a narrow band from the Mississippi to the Ohio rivers. They are low hills made of gravel, sand, and clay and are remnants of the more broadly spread Cretaceous deposits in Kentucky and Tennessee. The hills are low and contain many seep springs. Plants associated with northern bogs can be found here, including sphagnum moss and a profusion of ferns. The Bottomlands Section contains southern swamps of bald cypress and water tupelo at their northern most limits. The trees may be surrounded by a thick green blanket of duckweed, its surface broken only by the ribbon of a swimming cottonmouth. Heron Pond State Natural Area, with its long boardwalk, is an excellent place to discover this division.