The Sussex Border Path begins at Thorney Island, now effectively a peninsular that juts into Chichester Harbour. The path encircles the island and then extends across the South Coast Plain to Emsworth on the Hampshire side of the River Ems, the river which forms the Sussex-Hampshire border at this location. The path continues over the chalk ridge of the South Downs and onto the Greensand Ridge of the western Weald. Here the trail ascends Blackdown, the highest point in Sussex at and the highest point on the Sussex Border Path.