Scott Pollak, age 13, of Houston, Texas, for his question:

Why do the Southwestern mesas have such flat tops?

Everybody has seen a few pictures of our flat topped mesas but only those who visit the southwest or live there have seen them in three dimensional reality. These lucky people are always astounded by the vast numbers of mesas and by their endless variations of shapes and colors. The name "mesa" is borrowed from a Spanish word for table and no matter what its height or shape, its size or color, the top of the mesa is as flat as a table. The level ground may be SO or more than 100 feet below    at this present time in geological history. But in past ages, the surface ground level was actually where the tabletop now is.

Mesas are land formations of arid regions where the surface levels of loose, sandy soil are at the mercy of breezy winds and swift flowing flash floods. These stirring events cause erosion. Areas of softer deposits are gradually blown and washed away, exposing lower and lower levels. Here and there cores of granite and other harder rocks resist the erosion while the surface of the ground around them becomes lower. These sturdy pillars stand up as mesa formations, and their flat tabletops are level fragments of the old plain that once covered the area.