We have already considered two types of joints. In Chapter 9 we learned that columnar joints form when igneous rocks cool and develop shrinkage fractures that produce elongated, pillarlike columns(see Figure 9.28). Also recall from Chapter 4 that sheeting produces a pattern of gently curved joints that develop more or less parallel to the surface of large exposed igneous bodies such as batholiths. Here the jointing results from the gradual expansion that occurs when erosion removes the overlying load(see Figure 4.5, p. 88).