How do mesas form?
anguilla answer is mostly correct. However, he has some misconceptions that come out at the end of this explanation. If you look at a mesa, you will see that it is made of flat-lying layers of rock, not tilted rocks as implied by anguilla. The layer on top of the mesa is a rock that is more resistant to erosion than the layers under it. So, once a stream erodes down through the resistant cap rock, it can more quickly erode down through the underlying layers. The cap rock itself helps to protect the rocks right below it from erosion. Thus, stream channels slice these sections off of a plateau to create mesas. The softer rocks beneath the cap rock will be eroded away. This process will slowly undermine the cap rock and eventually whittle away at the mesa until it disappears completely.
