Mesas usually are portions of larger plateaus that have been detached by the formation and widening of canyons or arroyos. An arroyo is the Spanish name applied to flat-bottomed, steep-sided valleys.
A resistant layer of rock, such as sand-stone or some form of solidified lava, usually forms the top part of a mesa. The more rapid erosion of a less resistant rock beneath is responsible for the table-like appearance of this plateau feature. The resistant layer remains and forms the mesa. Formations of similar origin but smaller in size are called buttes.
Mesas are quite characteristic of New Mexico and Arizona. Buttes are found usually in Montana, the Dakotas, and Wyoming.
