A flat-topped mountain is called a mesa, from the Spanish word for "table." Mesas get their characteristic shape from differential erosion, usually due to running water.
Mesas are most often found in arid regions such as the Southwest United States. They are usually the remains of rocks raised by tectonic activity. Mesas often consist of horizontally layered sedimentary rocks with a harder top layer, often hardened lava, called cap rock. Over time, water seeps into weak spots and wears away stone to form channels. These weak spots grow wider and deeper, allowing softer rock beneath the top surface to be worn away into canyons, ultimately forming mesas. Over time, extensive erosion around the sides and base can cause further collapse. When most of a mesa has eroded, it is called a butte.