Because they are composed of softer materials. B. Jointing patterns . Joints are small fractures in rocks. Once joints form, water seeps into these cracks. Joints become the main sites of chemical and physical weathering.
Some rocks are more jointed than others.
Stone Mountain is a nice example of a solid hunk of granite with few joints, so it stands above the more easily eroded rocks of the surrounding landscape. C. Rock resistance and differential erosion . Some minerals and rock types are harder, more strongly cemented, or otherwise resistant to weathering and erosion. Metamorphosed rocks, exposed to higher temperature and pressure, are usually much more resistant to erosion than sedimentary rocks. Where soft and hard rocks are exposed at the surface adjacent to one other, the softer rock units will be more deeply eroded, becoming valleys.