Almost all types of rocks develop cracks, but there is no displacement of rocks on either side. In igneous rocks joints are formed during the period of their cooling and contraction. In coarse-grained granite sets of joints at right angles are common.

In fine-grained basalts the joins often produce hexagonal columns. Sedimentary rocks also develop joints as they contract. Folding involving stretching and flexing of rock layers produce joints in them.

Jointing is an important process because it allows easier movement of underground or sub­surface water and makes the work of gradational agents easier.