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Shri Ashtapad Maha Tirth - II
The Ice Age A distinct cooling all across the earth began about 3 million years ago. Though there have been many ice ages in the distant past of our planet, this most recent event is what is usually meant by the "Ice Age." It was during this era that humans arose, and culture had its beginnings (see chapter 5). The Ice Age lasted until 14,000 years ago, but was punctuated by many warmer periods called interglacials that might last 10,000 years or more. As the climate shifted between glacial and interglacial, the vegetation on the Tibetan plateau shifted back and forth between forest and steppeland. In glacial times, the forests receded from mountain slopes sometimes sparse spruce and fir forests would survive in sheltered valleys, but much of the landscape was treeless, shrubby grassland and steppe. During warmer periods, the temperature increased rapidly even in the northern areas. As glaciers melted new lakes formed across the plateau, and older lakes were replenished, while marshland increased. Trees returned to the valleys and lower mountain slopes, and alpine forests of pine, oak, birch, elm, and cedar flourished again. Since 1960 scientific expeditions have gathered information on glaciers at over twenty locations including Mount Everest. This data can be connected to well-known worldwide climatic changes. About thirty-four cold periods occurred between 3.25 million years ago and 128,000 years ago, the ice grew deeper about 2.4 million years ago. In this early part of the Ice Age, about 1.8 million years ago, glaciers formed on the highest peaks of the Himalayas, but not on the other Tibetan mountain ranges. About 700,000 years ago, the ice pack on the Arctic began to last through the summer months. In this middle period of the Ice Age, the climate in Tibet during the glacial periods was extremely severe, resembling that in polar regions today. The worldwide decrease in temperature was intensified in Tibet because of the continued rising of the plateau, which was now happening at an accelerated pace. Some investigators believe the plateau had now reached 10,000 feet (3000 meters), and the Himalayas were 15,000 feet (4500 meters). Many mountains in Tibet were covered with glaciers by this time. On the Himalayas, huge glaciers, some of them 24 miles (40 kilometers) across, grew down into the foothills. Most of the slopes of Gangs-ti-se were glaciated as fingers of ice 12-15 miles (20-30 kilometers) long reached down the mountains. Valleys in Khams were filled with extensive glaciers nearly 80 miles (130 kilometers) long, and IDang-la glaciers also increased as ice came down the mountainsides, it pushed huge amounts of rock and gravel in front of it. When the climate warmed and the glaciers retreated, they left behind these new hills, called moraines. As the plateau rose higher and higher, the Himalayas became so high they began to block moisture coming from the Indian Ocean. At higher elevations, the air above the plateau was thinner and less able to hold moisture, and solar radiation became more intense. Thus, the climate became less favorable to glaciers, which need moisture to form ice, now snowlines retreated, moving higher up on the mountainsides. From the middle of the last Ice Age, the glaciers on the interior of the plateau were diminished even in glacial periods. Those in the Himalayas and in the southeast, however, still received moisture from monsoons and remained extensive.
Ancient Tibet
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