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.
Using maps and images, students could produce annotated diagrams explaining how a stack such as Old Harry has been formed. Based on their understanding of the processes involved, students should predict how this stretch of coastline is likely to change in the future. Look back at maps from different time periods and at aerial images to look for changes in the rate of erosion in the area. Students should try to explain any changes they see.