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| Sunday, 7 November 2004 |
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Junior Observer |  |
| News Business Features Editorial
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The changing land We may not have noticed, but the face of the earth has changed extensively over the years!
Changing of the land around us happens very slowly, which is why we don't see it happening. However, a high, rocky mountain can become a low, grassy hill over millions of years. Present day scientists who study the land surface can get an idea of how the earth looked years ago and can predict how it will look in the future. Boulders (large rocks) in low fields and large scratches on rock surfaces have puzzled people in many countries. They have wondered how the boulders, which were too large to be carried by water or wind, ended up in fields where no mountains are close by. They also wondered how straight lines could have been carved on many rock surfaces. Meanwhile, scientists studying glaciers in the snow-capped Alps mountain range in Europe discovered large boulders left on surfaces where the snow had melted years ago. They also noticed the scratches on the rocky surfaces touched by the glacier. They discovered that boulders were moved and the rock surfaces were scratched by the shifting glaciers. They realised that the land had been changed by ice.
Much of the earth is believed to have been covered by ice or glaciers at one time. Such a period is known as an ice age and many scientists believe that the earth had experienced several such ice ages. The last ice age is thought to have occurred about a million years ago. The climate of the earth became cooler during an ice age and the snow that fell during winter would not melt due to the extremely cold temperatures. This build-up of snow over the years would form huge sheets of ice which were, in some places, more than three kilometres thick. These ice sheets would move very slowly over the land, digging up and into the ground and scraping the sides of hills and mountains. Rocks and soil would move along with this ice and the sharp boulders moving on the bottom of this ice would scratch the surface of the earth. When this ice finally melted, the rocks and other particles which were embedded in the ice would drop to the ground, forming mounds and hills. If a glacier moves into the sea and break off into huge blocks of ice known as icebergs, they would float away in the sea, acting in the same way when they came upon other surfaces. The grinding action of these shifting ice sheets created shallow holes and valleys across the land. These holes got filled with water and became small lakes. The Great Lakes of the USA used to be large river valleys before the last ice age. The shifting ice sheets eroded the valleys, making them wider and deeper. The ice started melting about 15,000 years ago and this water filled the valleys, leading to the formation of the Great Lakes. The surface land of the earth is presently shaped in many different ways. The feature which gives land its shape is known as landform and could be a plain, plateau, hill or mountain. Plains are large, flat and level areas found mostly along coastal areas. However, the Great Plains, the largest plains in the USA, are in the middle of the country. A large sea is said to have covered this area at one time. It is a very fertile area and is used for cultivation by farmers. Plateaus are large areas of level land which are found at higher altitudes than plains. Rivers often flow through them carving out steep valleys or canyons such as the Grand Canyon in the USA, which had been cut by the Colorado River. Mountains and hills are two other important landforms. A group of mountains situated in a line or row is called a mountain range. A hill is a mountain which is less than 300 metres in height. Some mountains which used to be tall and rugged millions of years ago, have been eroded by water and ice over the years and are now only hills. |
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