The cliffs around Old Harry Rocks are made of chalk. Wave refraction causes erosion of the headland and deposition in the bays either side. The cliffs are eroded when waves undercut the land causing the rock above to collapse. Caves are formed where cracks in the chalk are expanded due to hydraulic action. Caves can open up through the cliff to form arches, and the collapse of arches results in stacks such as Old Harry Rocks.
Students could be given a series of simple images - a steep headland, a gently sloping coast, a cave, an arch, and a stack. They could either annotate these as the clip plays, explaining the changes taking place or they could be given a number of key words and phrases which they simply have to match to the different parts of the proccess. To extend this, they could be given an aerial image of the headland and Old Harry and could be asked to predict how the coastline will look in the future based on their knowledge of the processes.