| MESOLITHIC |
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PLEISTOCENE GEOMORPHOLOGY
The planet earth has a long history of 4.6 billion years or 480 crore years. Throughout this long period the conditions on its face are not stable and same. There occurred great changes in different periods of its past. Earth’s history is divided mainly into four main periods, i.e. Pre-cambrian, Palaeozic, Mesozoic, Cenozoic, with several subdivisions in them . The recent period Cenozoic is again Sub-divided into Tertiary and Quaternary.
For study of human past Quaternary period is important as it corresponds
to human evolution and its various cultural periods.
Quaternary is sub-divided into Pleistocene and Holocene (Recent) starting
10,000 years ago) periods.
Pleistoce period commenced some 30 lakh years ago. The Pleistocene period
corresponds to the long history of
palaeolithic couture. The
conditions during the Pleistocene period are not uniform and there occurred
several drastic changes during this period.
In general pleistocene period was a cold period, during which large areas
of earth were covered by thick sheets if ice, and hence this period is called
“ice age” . Infact there were
not one, but four ice ages during the Pleistocene period.
They are Gunz (600,000), Mindel (400,000), Riss (200,000), and Wurm
(100,000).
There is no unanimity among the scientists regarding the reasons behind
the Ice Ages.
They propounded several theories.
1) ECCENTRICITY OF EARTH ORBIT: According to this theory due to some unknown reasons, the earth changed its orbit around the Sun and drifted away from it causing cold spells.
2) SUN SPOTS: Sun has some unstable dark areas in it. These dark areas the amount of heat radiation received by earth. If , due to some reason, more dark area is towards earth, then earth receives less radiation bringing down temperatures.
3) METEORIC STRIKE: It is postulated that a big meteoric struck the earth , which threw up so much dust and smoke into the atmosphere that, it blocked the sun rays considerably causing fall in temperatures on earth . This theory is not accepted by many.
4) COSMIC DUST: Earth along with the galaxy rotates constantly in the space. Probably the galaxy entered an area with cosmic dust, which blocked sun’s radiation, consequently bringing down temperatures on earth. Whatever the reason for the commencement of “ice ages” the effects of it are clearly visible on the surface of earth. Since the effects of the Ice are same in all the regions we have to study them under the heads, Temperate regions and the Tropical regions.
TEMPERATE REGIONS:
Temperate region is the region 23°30’ north and South of the equator. Since Sun is always at an inclination in this region, it receives leisure radiation compared to the Tropical regain. This results in lower temperature in Temperate regions. Hence during Pleistocen period much of the temperate regions were covered by thick sheets of ice, some times as mulch as two to three miles thick. That is why the ice age is also called glacial period. The formation of ice sheets over extensive areas has brought in several changes. 1 ) Many of the plant and animal forms which could not adapt to the changed conditions perished. 2) Ice sheets reflected much of the suns radiation back into the space, bringing down the temperatures further, 3) As enormous quantity of water is locked in the ice sheets, the sea level has gone considerably low, exposing large coastal areas in tropical regions.
The ice have over the land have altered the landscape. Large boulders were either transported or broken in to pieces. The gaps and splits in the rocks have widened due to wedging effect of the ice. The riverbeds have developed wide ‘U’ shaped channels due to the scouring (erosion) effect of the moving ice and the material embedded in it.
TROPICAL REGIONS:
In tropical regions the situation was different. Though ice formed on the upper reaches of mountains like Nilagiris, in South India, most of the land has experienced wet and dry climatic cycles. Dry climatic conditions during the glacial (cold) periods known as Stadial persisted as there was relatively lesser cloud formation and wetter conditions during antiglacial periods, when due to heat more water evaporated and helped cloud formation. The wetter cycles are called ‘fluvial’ periods and the dryer cycles as anti-fluvial periods.
The effects of altering wetter and dryer cycles are many. The increasing and decreasing flow of water in rivers brought forth different effects on the landscape. These cycles helped terrace formation and altered the face of the earth in a considerably way. The force of the water flowing in the rivers transported large amounts of gravel, boulders, pebbles and silt and deposited them in the valleys and riverbeds.
TERRACE FORMATION:
The effects of Pleistocene changes are visible in the river terraces formed in different periods of Pleistocene. This could be understood in the following way. When a river forms due to tilting of earth during earthquakes or due to erosional activity of water, in the beginning it covers a wide area. Gradually, due to water flow, material is carried away and results in ‘degradational or erosional terrace’. But when the water velocity is not sufficient to carry the material it is carrying, it deposits the material in its bed and it results in ‘agradational terrace’.
During Pleistocene period the increasing rainfall and decreasing rainfall effected the flow and velocity of the rivers, helping in one way the formation of terraces. The changing sea levels also effected the river flows. For example the erosional power of a river is increased due to lowering of sea level. Similarly, depositional activity increases when the sea level rises, due to decreased velocity of the river near the seacoast.
In the upper reaches of the river away from the effects of the rise and fall of sea level, the situation is reversed. Here, aggradation takes place during the glacial phase (lesser rainfall) when material is deposited in the river valleys due to reduced water flow which results in aggradation. During inter-glacial period (wet phase) when there is again sufficient water, erosion or degradation takes place, cutting through the earlier deposited material.
Thus although the material of the terrace is laid down during aggradational period, the actual terrace is formed only when erosion takes place. In this way erosion and aggradation makes several “steps”, “levels” or “terraces” along the river. Generally the highest terrace is the oldest, while the lowest would be the youngest.
The main terraces of the river systems in tropical countries are formed during Pleistocene period and they represent the four glacial periods, i.e., Gunz, Mindel, Riss, and Wurm. The stone tools found on various terraces thus could be dated depending on the study of various terraces.
EFFECTS ON THE LIFE OF ANCIENT MAN:
The changing climatic conditions had influenced flora (plants) and fauna (animals) to considerably extent. The primary result was that the life forms (both plants and animals) which could not adapt to the changing conditions became extinct. Generally speaking, colder climates are not suitable for the survival of life, whereas, warmer climate promotes life. So, during colder climates the availability of food was restricted as fewer species were existing. Similarly, during warmer climates, food availability was plenty since plants grew in abundance, which naturally promoted animal species also. Similarly, the life of man was also effected. During colder periods, he had lesser selection of food, whereas, in warmer climatic periods he had plenty of food material available to him. The effects on the life of man are visible on the types of tools manufactured by him. Towards the end of the Pleistocene period when, warmer climatic conditions were introduced during Holocene period (10,000 years ago), the abundance of food gave him enough leisure to pursue other activit4ies like spiritualism and art. The easy way of life also led to inventions like the domestication , agriculture, pottery, house-building etc. Thus the climatic conditions in the past have played significant role in the way of life of Man.
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K. P. RAO.