West Sussex is bordered by Hampshire to the west, Surrey to the north and East Sussex to the east. The English Channel lies to the south. The area has been formed from Upper Jurassic and Lower Cretaceous rock strata, part of the Weald-Artois Anticline. The eastern part of this ridge, the Weald of Kent, Sussex and Surrey has been greatly eroded, with the chalk surface removed to expose older Lower Cretaceous rocks of the Wealden Group. In West Sussex the exposed rock becomes older towards the north of the county with Lower Greensand ridges along the border with Surrey including the highest point of the county at Blackdown. Erosion of softer sand and clay strata has hollowed out the basin of the Weald leaving a north facing scarp slope of the chalk which runs east and west across the whole county, broken only by the valleys of the River Arun and River Adur. In addition to these two rivers which drain most of the county a winterbourne, the River Lavant, flows intermittently from springs on the dip slope of the chalk downs north of Chichester.