From the southern end of the main mass of the Dorset chalk hills an extension runs eastward. This, tapering as it proceeds, forms the narrow ridge of the Purbeck Hills, and, after a break caused by the waters of Poole Bay, is continued by the downs of the Isle of Wight. To the north it is bounded by the valley of the River Frome and Poole Harbour. To the south the sea is nowhere more than a few miles away, and in places it has eaten deeply into the chalk ridge itself.