The Pliocene and the Pleistocene
The last division of the Tertiary system is known as the Pliocene period. It was during this period that the great land masses of the world took on forms much like those of the present day. Although the seas still covered many areas which are now dry land, the great bodies of water were gradually becoming less and less extensive. This is shown by the limited number of remains of Pliocene sea life which we find stored as fossils in the rocks of the period.
Vast mountain-making movements, which began in the Eocene period, continued during the Pliocene. In Europe, for example, the mighty Alps were still in the process of being formed. In North America the Rocky Mountain region and the eastern part of the continent were being elevated. Many of the famous volcanoes about which we have read, such as Mount Etna in Sicily, show signs of great activity during the Pliocene period. The geysers and bubbling hot springs of Yellowstone National Park are the result of the violent volcanic disturbances that occurred during the Pliocene period.
In a great many ways the animal life of the Pliocene period closely resembled that of to-day. In fact, some of our present-day animals have come down to us from that far-off age almost completely unchanged in form and structure. Hyenas, wolves, bears and bison roamed the countryside. Fleet- footed deer, the hipparion (the three-toed horse), and long-necked camels fled before the ferocious attacks of the sabre-tooth tiger. The hippopotamus, tapir, wild boar and rhinoceros were also present during the Pliocene period. The mastodon, the great elephant-like beast that appeared in the Miocene period, lived on into the Pliocene of Europe and disappeared only late in that age. In America he continued to exist on into the Pleistocene period that followed.
Pliocene deposits have revealed a few scant indications that man may have existed at that time. Among the best known of these remains are those of Pit Izecant hro pus erectus, found in Java in 1891, and Sinanthro pus pekinensis, or Peking man, found in China in 1929. These early men knew how to make fire; and certain stones found with their remains may have been formed into implements of various sort by hand. These stones are known as eoliths, or “dawn stones.” It is not altogether certain that they were worked by hand. They may have been given their shape by purely natural action.
Toward the close of the Pliocene period the climate gradually turned colder and colder, a sign that the next age of the earth’s history was approaching.
This next period, called the Pleistocene, was the first section of the Quaternary, iii which we are now living. It is distinguished by the remarkable climatic changes which occurred, transforming whole continents into fields of ice. For this reason it is often called the Ice Age.
The climate grew colder than it ever had before. The snow fell in great quantities and became packed into vast ice-sheets a mile or more thick. From Canada, Greenland and northern Europe these ice-sheets, or glaciers, slowly spread south. A great ice-sheet covered England, filled the basins of the North Sea and the Baltic, and extended through Middle Europe as far as Silesia, Galicia and Poland. Enormous glaciers filled the Alpine valleys, and descended from the Pyrenees into Spain, while glaciers penetrated even into the heart of France. Practically all of Canada was under an ice-sheet, at least two miles thick in places. In the United States the ice covered New England, New York, northern New Jersey and Pennsylvania, and most of the region between the Ohio and Missouri rivers. In the Far West there was another ice-sheet. The mountainous regions of South America and Asia were also covered.
All Living Things Fled before the Advancing Ice
Animal and plant life was unable to stand the bitter cold brought on by the glaciers and began to move towards milder climates in the south. It is difficult to imagine plants moving, but they really can if they are given sufficient time. However, it is an extremely slow process.
These arctic conditions passed away and the climate grew warmer. The glaciers ceased to spread over the world.
Plants began to move northward to those lands in which they had once flourished. They were followed by the animals who were dependent upon the plants for their food supply. This climatic change was only temporary, however, and again a period of ice and snow set in. It seems indeed likely that there were several glacial periods separated by intervals of warmer climate; certainly there are indications that the ice and snow alternately advanced and retreated more than once before they finally retired to the North. They left behind them many signs of their great force.
Ice Sheets, Thousands of Feet Thick, Marched Over Europe
In Europe the great distributing centre of the ice and snow seems to have been Scandinavia. From the high table-lands there the ice advanced over Europe in all directions, but the ice-sheet covering Great Britain, though joined with the Scandinavian ice- sheet, was independent, and slid down its own mountains and valleys. In the Scottish Highlands the ice-sheet was tremendous, and filled up valleys and lakes and fords, accumulating to a height of 4,000 or 5,000 feet. Mountains 3,000 feet high, with deep lakes at their bases, show marks of ice up to their summits.
In Canada there seem to have been three separate sheets which ran together, called the Labrador, the Keewatin, and the Cordilleran sheets. All of them came down into the United States.
As all these enormous ice-fields and glaciers moved along they brought about considerable geological changes. Huge masses of moving ice ground up the rocks over which they passed, and converted their surface layers into a mixture of clay, sand and stones known as boulder clay. As they carried the boulder clay along with them the sand and stones acted like great sheets of emery paper, and still further ground down the rocks. The glaciers even scoured over the floors of seas and lakes, and carried shells of shellfish in their further journey overland.
How the Glaciers Carved the Face of the Earth
There can be no doubt that the moving ice-sheets wore down the rocks to a considerable extent. We find everywhere rocks scratched, eaten away, ad polished, and the great masses of boulder clay sugest that enormous destruction occurred. Not only were rocks worn down, but they were often broken off, crumpled and bent by the weight and pressure of the ice. Thousands of lakes in Canada and the United States owe their origin to the ice-sheets. On the other hand, many lakes which existed during the period have now disappeared. The Great Lakes, once larger than at present, are of glacial origin.
When the ice-fields and the glaciers melted they naturally gave rise to streams and torrents whose rushing water continued the work of destruction. The direction of many streams was changed. Many of the waterfalls which now furnish power go back to glacial times. How long since the ice-sheet receded we can only guess. Few scientists think it was less than twenty-five thousand years.
Man and His Struggle to Exist in a Hostile World
Through all these changes man persisted. Flints, tools and skeletal remains are found in Pleistocene deposits, showing that man made great progress during that period. He must have had a hard fight for his life, but he managed to survive. He had to fight not only the glaciers but also the many formidable animals that then roamed the world. In this period the English Channel and the Irish Sea and the North Sea were all dry land and Britain was joined to the continent. The whole of northwestern Europe was overrun with elephants, mammoths, the wooly rhinoceros and the hippopotamus. Bears and hyenas were particularly plentiful in England. There were also great herds of reindeer, elk, bison, red deer and wild horses. All these man must have fought and hunted, and in the caves of the IJordogne Valley in France are found the bones of reindeer, elk, bison and other animals killed and eaten by the cave-men of those days. Even mammoths and bears fell a victim to these early hunters, for their bones are also found in the cave larders.
As man became better acquainted with the world about him, he developed a more advanced culture. Great strides were made in the fields of religion, social organization and the arts. In some of the caves of Spain and France are found remarkably life-like drawings of the animals hunted by early man.
Other drawings hint at religious ceremonies and rituals. On pages 192 and 193 some of these drawings are reproduced.
For tools, weapons and other implements to help him in his daily life, man first used naturally-formed stones found in stream- beds and along the shore. Later he discovered that these stones could be made into more serviceable implements by shaping them artificially. To be sure, his first attempts were crude, but as time went on he learned to become quite expert at chipping beautiful knives, arrow-points, axes and many other useful articles from stone. Still later he learned to polish these implements until they almost looked as though they bad been shaped and finished by modern machinery.
With each forward step, man slowly but surely became master oxer the other animals that had ruled the earth before him for millions and millions of years.
The Pleistocene period shades gradually into what we call the Recent or Human Age, and all the time there have been changes and redistributions of life. The mammoth has disappeared from America, the sabre-tooth tiger has gone, the elephants have departed from Europe. Such changes are still slowly going on. The wolf, among other animals, is doomed to extinction; even races of men are disappearing. Some animals are increasing in numbers and others are changing their haunts and habits. In many cases the march of civilization, more than nature, is responsible for these changes.
The surface of the earth, though in its larger features much the same through the Quaternary, has also been changing. It is changing still; the mountains are still being worn down into the sea, and under the sea new mountain ranges may still be in the process of formation.
