Sunken roads

Places and routes linked to sunken roads. A (natural) sunken road indicates that it has existed for centuries, because such a road is literally eroded hollow by the action of walking. Sunken roads are formed when the clay minerals in the ground are literally flattened (horizontally) by walking and the weight of the cart wheels. This makes the soil less permeable at that point. You see the same effect of "treading down clay minerals" on a football field where the goalkeeper normally paces. Usually, there's a puddle there. But on a slope like this, the water drains away, eroding a section of the road. Year after year, century after century, this erosion causes the road to cut into itself, flanked by steep walls.