23. Formation of Caves, Arches and Stacks A - Waves erode weaknesses in the rock and erode a cave along the greatest line of weakness. B - Cave size increased by further erosion and opens headland at both sides. Other caves increase in size. C - A stack is separated from rest of land and continued erosion of the headland results in an enlarged cave and leads to the collapse of the overhanging rock 'roof'. The next cave is eroded and becomes an arch