The middle section of the Atlantic plain in the United States is distinguished by three deep indentations,--Chesapeake, Delaware, and New York bays; each of these is the expanded mouth of a comprehensive river system, and furnishes abundant anchorage,--New York bay being the finest harbor on the continent. Along the coast south of New York is a low, level base-plain of sand and clay, from twenty-five to fifty miles in width, the larger towns being generally situated on the uplands beyond. The Appalachian mountains extend in several ridges across the middle district from southwest to northeast, the highest elevations being those of the Catskill group in southeastern New York, where Slide Mountain towers 4,205 feet above sea-level. New Jersey is largely occupied by the base-plain, with hills in the northwest. From the eastern range of mountains, the surface of New York slopes gently down, with great diversity, to Lake Ontario; the mountains are rent by the interesting and important water-gap of the Mohawk valley, which in an earlier geological age connected the lake basin with the trough of the Hudson. Pennsylvania has three distinct topographical divisions: (1) the highly fertile district between the Blue Mountains and the sea,--including Delaware; (2) the middle belt of elevated valleys, separated by low parallel ridges of mountains rich in anthracite coal and iron ore; (3) the upland north and west of the mountain walls, sloping down to the tributaries of the Ohio with a wealth of bituminous coal, oil, and natural gas. Sidenote: Intermingling river-systems. In the New York and Pennsylvania hills the numerous rivers of the region have their rise. These rivers either flow westward into the Mississippi basin, northward into the Great Lakes, eastward into the deep cleft cut through the mountains by the Hudson, or southward into the estuaries of the Delaware and Chesapeake. Within a short distance of each other are waters which will reach the Atlantic ocean by three divergent routes,--through the Gulf of Mexico, the Gulf of St. Lawrence, and the bays we have mentioned. This fact has had a potent influence on the course of American settlement and trade, which have persistently followed the water highways into the interior of the continent; and along those rivers were fought two great wars. Sidenote: Their historical significance.