 About 38 million years ago, when the first large mammals were living on Earth, the climate underwent a great change. In the Oligocene Period, 38 to 27 million years ago, it was still warm and temperate but in the Miocene which followed it grew colder, affecting the spread of the plants. Many tropical or subtropical types of plant disappeared and even the woods shrank, to be replaced by immense grassy plains.
The effect of this change on animal evolution were far-reaching. Many mammals who had been used to feeding on the shoots of trees and bushes had to adapt to feeding on grass. This led to their gradual transformation, which was particularly noticeable in the shape of their teeth and the structure of their feet and necks.
In the Miocene Period, the mountain chains which has begun to emerge in the earlier periods continued to be lifted up. New land surfaced above the sea, with result that the oceans shrank. These movements were completed in the Pliocene Period (the last in the Cenozoic Era), which lasted 8 million years. By the end of it, the shape of the continents was it is today. The fossils discovered in Pliocene soil show animal form which are in many ways similar to those of the present. Other fossils belong to species which are now extinct but it is easy to imagine the exact shape of their bodies.
 The strangest of these were certain forms of proboscidea, such as the Amebolodon, with long, flexible noses.
Animal-life had already attained the variety of modern times when, about a million years ago, new changes in climate occurred, upsetting the pattern of life on the continents. The cold became intense and large ice-caps covered the globe as far down as our latitudes, driving nearly all the living beings towards the tropics.
Some animals, however, managed to adapt to the new climatic conditions and continued to live in the regions of the North. A typical example of this is the mammoth, a large woolly elephant, remains of which have even recently been found in the frozen sands of Siberia.
The severe cold froze vast stretches of water, particularly around the North and South Poles. Water vapour froze in the clouds and fell as snow, covering the continents so thickly that it did not melt. This made the flow of the rivers dwindle so much that many of them disappeared and the seas were no longer fed as they used to be. Yet the oceans continued to evaporate, although the vapour did not return to them as rain. The level of the oceans thus gradually dropped and the floors of the seas were revealed. Even the deep trench of the Bering Strait was left dry, after several million years under water.
 The effect of Great Ice Age and the retreat of the ice made a deep impression on the surface of the Earth. As they moved slowly along, the glaciers eroded and scoured, digging out valleys and gradually stripping the walls of rocks. The trails of waste they removed piled up at the base and sides of the glaciers, where it eventually formed huge hills of rock debris.
Vast deposits of rock waste, left behind by the glaciers, are to be found around lakes, such as the Italian Lake Maggiore and Lake Garda.
There is another, more important aspect of the retreat of the glaciers. The enormous quantities of water, collected on hill-tops in the form of snow, rushed down into the valleys as the temperature rose. There they formed raging torrents and rivers which swept away all that came in their path. The mountains were eroded away but, at the same time, vast heaps of debris built up in the valleys. This action was particularly strong in the Alps.
The Po River Basin, which had been covered by the sea up until then, was formed in the Quaternary Period (which covered the last two million years) by the loads of debris carried down into the valley by the rivers.
|
Print
TOP
