Susan Rowley, age 10, of Woodlake, California, for her question:

When were the Grand Tetons formed?

About 150 million years or so in the past, the patient earth began to clear the ground for this stupendous scenery. At that time, the place where the proud Rockies now stand was a long ditch of sea water, reaching from the Gulf of Mexico to the Arctic. Streams flowed down the sides and gradually the old ditch filled with thick layers of mud and sandy silt. About 100 million years ago, these massive layers began to rise in a long hump. This restless region of the earth's crust quaked and erupted as it formed the growing Rocky Mountains.

In Wyoming, this mighty mountain chain bent and folded to form the scenic wonderlands of Yellowstone and the neighboring Grand Tetons. The enormous building job took more than 50 million years and the grand Tetons are still fairly young mountains. Then, in the last million years, immense ice age glaciers invaded the Northern Rockies. They gouged valleys and lakes, scraped the steep rocky slopes  ¬and created the stupendous scenery of the Grand Tetons