Looking up the Cuckmere river from the beach, Sussex, UK. The river separates the cliffs of Seaford Head on the west from those of the Seven Sisters to the east. This is at low tide, when the river can be crossed by wading. At high tide it becomes an impassable barrier and the only way forward is a walk upriver to the nearest bridge, a detour that adds at least a mile and a half to the hike.