Welcome to You Ask Andy

Rieta Nydam, age 13, of Peterboro, Ont., for her question:

How old are the Rockies?

The story of the Rocky Mountains is a long tale of ups and downs. Several times the whole region was a deep trough filled by the sea. Time after time the old earth rallied her forces and pushed up a spine of mountains where the Rockies now stand. Violent volcanoes and cruel glaciers also played a part in the making of the proud mountains.

The age‑long war which made the Rockies is between the weather and the earths crust. The earth's crust reaches to a depth of about 30 miles. It seems important that the weight of the crust be evenly balanced all over the earth. Weight has little to do with thickness. For a bulky mountain made of light weight rocks may weigh less than a valley of dense, heavy rocks.

Where a region of heavy rocks borders a region of lighter rocks there are stresses and strains. The earth's crust bonds and buckles into mountains and valleys. This evens out the weight of the crust. But only for a while. The weather gets busy blowing the dust and washing the mountain tops down into the valleys. After millions of years of this washing and dusting, the weight of the crust is out of balance again. The old earth must shrug her shoulders in another upheaval.

The Rockies have been through ups and downs of this kind through the past billion years. Some 250 million years ago, the western side of North America was a series of islands. Where the Rockies now stand there was sea. Sediments collected, adding their weight to the sea bottom. Rocks formed from these sedimentary layers are now part of the uplifted Rockies.

Over 100 million years ago the Arctic Ocean joined hands with the Gulf of Mexico. The region of the Rockies was again swamped. About 60 million years ago, a vast region down the western continent began to uplift. The old earth was again shifting the weight of her crust and the Rocky Mountains began to rise.

Thick layers of the earth's crust were forced to hump upward. Very often they bent and bro", One side of the fractur"~ight rise high above the other, tilted at a steep angle. One side might be pushed to overlap the other, forming a crusty sandwich.

For several million years, the mountainous upheaval was very intense. Then came long ages of quiet. The weather wore down the high peaks and smoothed the area into a high plain.

The present day Rocky Mountains began to uplift only a few million years ago. But we cannot give this as the total age of the Rockies. For our Rockies rest on the worn shoulders of older mountains. And for a billion years, the whole region has been a mountain‑making area of ups and downs.

Perhaps our Rockies are the highest mountains that ever stood down the Western continent. In a hundred lifetimes, we would not notice much change, in them. But we can be sure that day by day, year by year, the wind, the rain , the snow and the ice are busy wearing down those proud peaks. In a dozen million years a high plain, a valley, a sea or even a new range of mountains may occupy the site of our beautiful Rockies.

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