Ronnie Melton, age 12, of Gastonia, N.C., For his question:

What are tar pits?

A tar pit is a natural deposit of dark, tangy tar or asphalt. There are likely to be central puddles where the tar is soft and sticky, and the areas around the edges of the deposit may be layers of dry and hardened asphalt. The tar pit forms where petroleum is present in the rocks near the surface of the earth.

Some of the chemicals of petroleum are volatile and tend to evaporate in the open air. This happens when the oil oozes out frown cracks in the ground. As the lighter chemicals evaporate, the heavier, tacky materials in the petroleum are left behind as puddles of tar.