The first Spanish explorers to the area called it Mesa Verde, or "green table." This expression is actually a misnomer. The correct geological term for the area is a cuesta, not a mesa. Mesas are isolated, flat-topped highlands with steeply sloping sides or cliffs, and are topped by a cap of much harder rocks that are resistant to erosion. The cap protects the softer underlying slopes or cliffs from being quickly weathered away. The only difference between a cuesta and a mesa is that a cuesta gently dips in one direction. Mesa Verde is inclined slightly to the south at about a seven degree angle. This cuesta is made up of many separate, smaller "mesas" situated between the canyons. Although technically we should call the park "Cuesta Verde," convention dictates that we use the term "mesa" when describing the area.
The seven degree angle of Mesa Verde is essential to the formation of the alcoves in which most of the cliff dwellings are found. The alcoves provided the spectacular preservation of this architecture. Alcoves are large, arched recessions formed in a cliff wall.
An alcove is not the same as a cave. Caves are underground chambers, not found in Mesa Verde. Alcove formation is caused by water that seeps into cracks, freezing and thawing in them, eventually expanding and slowly pushing the rock apart. These portions fall off in blocks, creating the alcoves you now see. These blocks of sandstone were shaped and used by the Ancestral Puebloans in the construction of their homes.
Alcove formation is assisted by water that is absorbed into and percolates through pores in the sandstone. The water eventually reaches a layer of shale, which is much less porous, or absorbent, than the sandstone. The water cannot easily pass through the shale, and so gravity guides it along the top of this layer to the cliff face. Seep springs are found where the water emerges from the cliff face, directly above the shale layer. These seeps provided a continuous source of water for the residents of the alcoves.
Places to view seep springs: Active seep springs are located along the trail to Spruce Tree House, on the trail to and within Balcony House, as well as in Long House.