We've all seen pictures of them - a very flat surface surrounded by a very steep drop-off. It occurs to me that I have no idea how they were formed. Got to go search for that. Unless stales beats me to it.

Here's URL with picture of mesa:http://www.mines.utah.edu/~wmgg/Geology/DeadHorseGIFS/DeadHorsePt.html

Here is another URL with a lot of text describing mesa formation:
http://www.us-parks.com/US_National_Parks/mesa_verde/mesa_verde_geology.shtml