Hello, students, welcome back. In this module we will gather information about the physiography of Himalayas, that is the North South Division of Himalayas. Now the Himalayas are the world’s highest mountain ranges, and they consist of four parallel ranges from south to north. The southernmost and the lower most range of the Himalayas is the Shiwalik Range. Beyond that we have the Lesser Himalayas which is also called as the Himachal Range. Beyond the Lesser Himalayas we have the Great Himalayas and they are also called as the Himadri Range. Beyond the Great Himalayas we have the Trans Himalayan Ranges. So we can see that Himalayas consist of four parallel ranges from south to north and they are the Shiwalik, the Lesser Himalayas, the Great Himalayas and the Trans Himalayas.

Now how are these parallel ranges formed? We have learnt that when the Indo-Australian Plate collided with the Eurasian plate, the sediments which were there at the bottom of the sea of the Tethys folded to form the Himalayan mountain ranges. But, my dear students, it was not one fold, the folds were multiple.  So when the collision happened, the very first fold of the mountain was the lowermost fold and that is the Shiwalik mountain range. Beyond that the second fold was the Lesser Himalayas of a greater height. Beyond that we had the Great Himalayas which was of a still more height. And beyond that whatever mountain ranges were folded and formed they were together called as the Trans Himalayas. When we say ‘Trans’ it means beyond, so all the mountain ranges beyond the main Himalayan belt that is the Shiwalik, Lesser and Great Himalayas are together grouped and called as Trans Himalayas. So the Himalayan mountain ranges are four parallel ranges, the Shiwalik Range, the southernmost range, then the Lesser Himalayas, then the Great Himalayas and beyond that they are called as the Trans Himalayas.

So let us begin with the very first and the southernmost range which is the Shiwalik Mountain Range. The southernmost range of the Himalayan system is known as the Shiwalik Mountain range. The average altitude of the Shiwalik range varies between 900 to 1100 meters. The area between the Shiwalik and the Lesser Himalayas consist of a number of valleys. Now you will be wondering what do you mean by a valley, a low area of land between hills and mountains is called as a valley. As you can see very clearly in the picture here, the low lying area between two mountain ranges, this area is called as a valley. So we have similar valleys between the Shiwalik and the Lesser Himalayas. These valleys are more or less parallel to the mountain ranges. As you can see here, we have one mountain range and then the second mountain range. We have both the ranges running parallel and we have parallel longitudinal valleys. So such kind of longitudinal valleys exist between the Shiwalik and the Lesser Himalayas. Such valleys are called as Duns in the local language. There are many such examples of Dun valley, for example the Kotli Dun, Dehradun which is a very famous tourist destination, and also Patli Dun, these three are examples of Dun Valleys. The Shiwaliks as you can very clearly see in the map here are very clearly visible in the western side, but as we look towards the east they become less obvious means they are less visible in the Eastern Himalayas. Why, because in the eastern part which you can see the area which is highlighted in red, these mountain ranges almost merge with the Lesser Himalayas.

Now there is one important feature which we find at the foothill of the Shiwalik ranges, and this feature is nothing but the Alluvial Fans. What do you mean by the Alluvial Fans, what are these Alluvial Fans? These are unconsolidated heaps of sediments abruptly deposited by the rivers while entering the plains. Now when we say unconsolidated, it means loose sediments. There are many rivers which originate from the Himalayas and they enter the north Indian plain. While travelling from the Himalayas they do a very important work of erosion, they bring in a lot of sediments, they bring in a lot of alluvium with them. In the mountainous region when the rivers are flowing, they flow in a very restricted area which is called as a gorge. Now what do you mean precisely by a gorge? A deep narrow valley with steep sides, usually formed by a river or stream cutting through the hard rock. So when the rivers originate from the mountain, they break the parts of the mountain, they create a path for themselves. This restricted narrow path created by the river between the mountain is called as a gorge. So the rivers travel in a very restricted area which is a gorge. But as soon as they enter the plain it is a wide area. So the river water spreads abruptly, also the alluvium brought by the river also spreads abruptly here. And these features that we find at the foothills of the Shiwalik are called as Alluvial Fans. Now why do we call it as Alluvial Fans? Because they are created by the deposition of alluvium brought by the rivers and they have a structure or a shape of a fan.

We will continue with Lesser and the Great Himalayas in the next module.