The Piedmont Plateau is characterized by the presence of red sandstone and shale. Its surface is very irregular. In its eastern part it is undulating or rolling. In the western, it contains higher lands which may be called plateaus. It contains also irregular ridges such as the Watchung Mountains, the Palisades, and the mountains in Hunterdon County. The Watchung Mountains and the Palisades are composed of trap rock, which in a molten condition was forced up through overlying rock strata. The eastern sides of these trap ridges are steep slopes. The underlying sandstone has been worn away while the harder trap rock has resisted erosion. [Illustration: The Palisades of the Hudson] The Falls of the Passaic River at Paterson are located in this section. These falls, which are seventy feet high, were formed by the water passing over the hard trap rock and wearing away the softer rock below. Because of the nearness of the Piedmont Plateau to the great commercial centers of the country and the excellent facilities for transportation, most of the larger cities of the state are located in this zone. [Illustration: Barnegat lighthouse] The fourth zone, the Coastal Plain, includes the remainder of the state south and east of the Fall Line. The land of this zone is highest in the interior along a range of elevations extending from Atlantic Highlands in the northeast to Mount Holly in the southwest, and slopes outward from these elevations towards the margins of the zone. The greatest elevation is 390 feet, which is found in Monmouth County. About three fourths of this portion of the state has an elevation of less than 100 feet and about one third has an elevation of less than 50 feet. The Coastal Plain is bordered along Delaware Bay and the Atlantic Coast by salt marshes, or tidal marshes, often called salt meadows, whose elevation above sea level is rarely as much as four feet. The total area of these marshes, including those of the valley of the Hackensack River, is almost 660 square miles. Along the Atlantic coast the marshes are separated from the ocean by beaches. These are low ridges of sand, constructed in the shallow water by the action of the waves and wind. These beaches are absent along the shore of Delaware Bay and along the coast from Manasquan to Monmouth Beach. The water between the beaches and the mainland is shallow and is becoming more shallow because of the deposit of sediment which is washed down from the land by streams. Although the coast line of the state along the Atlantic is 120 miles long, there are no good harbors on this line, because of the barrier beaches and the shallowness of the water between the beaches and the mainland.