Jeanie Glockneth, age 10, of Greenacres, Washington, for her question:

How fast do mountains wear down?

The Rockies northeast of Spokane started to rise about 200 million years ago. In their kindergarten days, they were hills and the rains ran down their gentle slopes, washing away dirt from the tops. But even though the mountains grew slowly, they grew faster than the weather could wear them down. After many millions of years the mountains reached their full height. They stopped growing, but the weather went right on wearing them down. Every year the lofty peaks lost an inch or maybe several inches. Sometimes a landslide tumbled several feet from the top.

It takes millions of years of weathering to wear down a lofty range. The oldest mountains in North America rose in eastern Canada. The weather has been trying to wear them down through 500 million years. Even now they are not worn down to sea level. They are the Laurentian Mountains of Quebec, gentle hills about 2,000 to 3,000 feet high. But nobody knows how tall they were half a billion years ago or how many feet have been worn down in all that time.