Jim Keeports, age 10, of Lancaster, Pennsylvania, for his question:

How old is Grand Canyon?

Scientists think that Grand Canyon started to form at least 50 million years ago. In the dim distant past, this part of the world was rather flat and the ancient Colorado River flowed along to empty itself into what is now the Gulf of California. Then the crust of the earth began to heave up to form the great western mountains. As the ground rose higher, the busy river dug deeper and deeper. After a few million years, the river flowed through a ditch with steep cliffs on either side.

As the growing mountains rose higher, the mighty little river kept on digging deeper and deeper. Rock slides and landslides wore down its steep sides and at last the river at the bottom was a stile below the top of its banks. It took the Colorado River at least 50 million years to do all this digging. The result is Grand Canyon and several other steep, terraced canyons along the old river banks