The Coastal Plain region is the largest and includes portions of the Atlantic coastal plain and the Gulf of Mexico Coastal Plain. The Atlantic Coastal Plain and Gulf of Mexico Coastal Plain consist of sediments and sedimentary rocks that range in age from Cretaceous to Present. The boundary between the Piedmont Region and the Coastal Plain Region is the Fall Line. The Sandhills or Carolina Sandhills is a 10–35 mi (16–56 km) wide region within the Atlantic Coastal Plain province, along the inland margin of this province. The Satilla River is in the Atlantic Coastal Plain. Alapaha River, Suwanee River, and the Flint and Chattahoochee rivers flow through the Gulf Coastal Plain and the Florida Panhandle into the Gulf of Mexico. Lake Seminole is a reservoir at the confluence of the Chattahoochee and Flint rivers on the Georgia–Florida border.