Most dales are valleys that have been widened and deepened by glaciers. The word dale is used across northern England, and valleys called dale can be found in the Lake District as well as the Pennines. The Yorkshire Dales National Park is mainly in North and West Yorkshire.

In North and West Yorkshire, dales are valleys cut into rock that is made of layers of limestone, sandstone and a clay rock called shale. Each of these has a different hardness, so where limestone and sandstone occur they often make ridges in the valley sides. The highest peaks above the dales are also made of sandstone and limestone.

The Yorkshire Dales region is famous for cave systems that have been formed in the limestone. There are also many places where water sinks down into the limestone. These are known as swallow holes by geographers (popularly called potholes).

The Dales is also a region where many valuable minerals have been found, including lead. The Romans and many others mined lead in the Dales. You can often see the remains of the tunnels and spoil heaps on the hillsides.