The region consists of a coastal plain, narrow in the north and wider in the south, bounded by the Tasman Sea on the east and the mountainous, almost impassable Illawarra escarpment (forming the eastern edge of the Southern Highlands plateau) to the west. In the middle of the region is Lake Illawarra, a shallow lake formed when sediment built up at the entrance to a bay. The district extends from the southern hills of the Royal National Park in the north to the Shoalhaven River in the south, and contains the city of Wollongong, the fourth largest urban area in New South Wales. North of Wollongong the plain narrows to a small strip of land between the coast and the escarpment. At Coalcliff and Stanwell Park small valleys are formed allowing further settlement. To the south it widens, and becomes increasingly hillier before reaching Stockyard Mountain, a long divide between the main plain and the Jamberoo Valley, which stretches until it reaches Kiama. South of Kiama is Saddleback Mountain and south of that the Shoalhaven plains and the outcrop of Coolangatta Mountain.