This is an image from Lindisfarne, or Holy Island, which is a tidal island; the island is cut of from the mainland at high tide. The view is towards the mainland, and the posts you see on the right mark the Pilgrim's Way leading from the mainland running across the sands, exposed by low tide, over to the island. The posts are about 4 meters tall, and the difference between high and low tide may exceed 4 meters.