Gary Repovsh, age 10, of Tucson, Arizona, for his question:

How long have the Appalachians been wearing down?

The southern arm of the Appalachians reaches down into Alabama and the northern regions through the Gaspe Peninsula in Canada. The entire chain of mountainous ridges and ranges is about 1,500 miles long. And the Appalachians are the oldest mountains in the United States. The northern arm arose during the Devonian Period of geological history. This period began some 320 million years ago and lasted some 55 million years. Wind and weather began wearing the ridges down as they poked up their peaks    but the mountains outdrew them. The peaks reached their full height perhaps 250 million years ago but no one can say exactly how high they were. Since then, gradually, they have been wearing down.

The Appalachians south of New England arose in the Permian Period that began about 210 million years ago and lasted some 25 million years. Weathering also began wearing down these ridges as they grew. This gradual erosion went on a little faster after the lofty ranges reached their full height about 185 million years ago. We cannot say how high they rose because their lofty peaks have been wearing down gradually ever since. The older ranges in the northern arm of the chain have suffered an extra 65 million years of weathery erosion that has worn them down even more.