Gwen Gallimore, age 11, of New Brunswick, N.J., for her question:

IS THERE REALLY A PETRIFIED FOREST?

The word petrified means turned to stone  or frozen with fear. Forests, of course, do not become frozen with fear, but it is possible for old tree trunks to turn to stone. Whole petrified forests are quite possible, and there is a remarkable one in the Painted Desert of Arizona. In the Petrified Forest National Park the sand is strewn with scattered and broken logs and trunks  many of them made of colorful semiprecious stones.

Millions of years ago this was a moist region where forests flourished. Later the climate changed and the trees were wiped out. Fallen trunks were partly buried in silty sand. Rains and ground water seeped through the sand and woody cells, dissolving minerals. Gradually, molecule by molecule, the seeping water replaced the wood with hard, stony minerals.

Gradually the woody trunks were changed to stone  and there were enough of them to form a whole petrified forest.