Lori Peterson, age 10, of Sioux City, Iowa, for her question:

Which are the olddst'mountains in North America?

Mountains, of course, tend to grow in ropey ranges    sometimes hun¬dreds of miles long and sometimes thousands of miles long. The immense job of growing up from ground level takes many millions of years. They grow inch by inch or foot by foot, and we do not notice much change in a human lifetime. You might think that the oldest mountains must be the tallest ones. If this were so, our western mountains would be the oldest. But actually most of them are youngsters. The real old timers are not especial¬ly tall for a very good reason. As they grow tall, the blustery weather wears down their lofty peaks.

When mountains reach their full height they stop growing and they weardown to gentle little hills. The oldest mountains in North America are thelovely Laurentians of eastern Canada. About 2,000 million years ago theyreached their full height. For all we know, they may have been as tall as the proud Rockies are today. But wind and rain, ice and gushing streams have worn them down to sweet rolling hills. And the plant world has cloaked their sloping sides with woodsy greenery.