A mesa is an isolated landform characterized by a flat top and steep, cliff-like sides that rise abruptly above the surrounding terrain. The term comes from the Spanish word for “table,” describing its table-like appearance. These geographical features are most commonly found in arid or semi-arid regions, such as the southwestern United States, where the specific conditions for their formation are prevalent.
Defining Physical Characteristics
The most recognizable feature of a mesa is its expansive, nearly level summit, formed by a layer of hard, erosion-resistant rock called the caprock. This protective layer, often composed of durable materials like basalt, sandstone, or limestone, shields the softer sedimentary layers beneath it. Without the caprock, the underlying materials would quickly erode away, preventing the landform from maintaining its elevation.
The sides of a mesa are defined by steep slopes known as escarpments, which drop sharply to the lower land around them. These slopes frequently exhibit a staircase-like structure called cliff-and-bench topography. The more resistant rock strata form the vertical cliffs, while the less resistant layers erode into gentler, sloping benches. Geographers define a true mesa by its size, requiring the overall width of the flat top to be greater than the remaining height of the structure.
The Geological Process of Formation
The creation of a mesa begins with tectonic uplift, which raises large, horizontally layered sedimentary rock deposits high above sea level. This elevated land, initially a vast plateau, is then subjected to erosion. The process responsible for carving the mesa is differential erosion, where different rock types wear away at different rates.
Water, often flowing through fractures in the plateau, cuts deep valleys into the softer rock layers, like shale or mudstone. Because the hard caprock resists erosion, it remains intact, forming the flat top of the isolated remnant. The steepness of the slopes is maintained by scarp retreat, which involves the progressive undercutting of the mesa’s base, a process known as basal sapping.
Basal sapping occurs when weathering and erosion at the base remove the softer support material. This causes the overlying, harder caprock to break off in large, vertical blocks. This action causes the cliff face to retreat parallel to itself over millions of years, leaving behind the characteristic, nearly vertical walls. A mesa represents an intermediate stage in the long erosional cycle that gradually reduces a vast plateau into smaller isolated structures.
Distinguishing Mesas From Related Landforms
Mesas belong to a family of flat-topped landforms that includes buttes and plateaus, with the primary distinction being size. A plateau is the largest formation, defined as a broad, expansive area of flat-topped land that covers hundreds or thousands of square miles.
A mesa is a remnant of a plateau that has been isolated and reduced by erosion, making it significantly smaller but still wider than it is tall. Continuing erosion eventually reduces the mesa further, turning it into a butte. A butte is distinguishable because its remaining caprock summit is so small that the structure becomes taller than it is wide, marking the final stage before the entire structure is worn down.