While it is not very difficult to explain the origin of two northerly ranges of the Himalaya, Siwalik poses a number of problems. That is why different opinions have been expressed by the scholars to explain its formation. Here three facts are worthy of mention : (a) the thickness of sediments in Siwalik ranges between 4500 m to 6100 m, (b) the sediments are newer in age, and (c) the deposits also contain marine fossils.
Two major processes, viz., deposition and marked uplift have played significant role in the formation of this outer range. The considerable thickness of Siwalik sediments is attributed to tectonic downwarping. It is worth mentioning that
Siwalik is not a continuous range instead it depicts discontinuity which is called “Doon’ in the west and ‘Duar’ in the east. Following are some of the important views throwing light on the origin of these hills.
1. Natural Levees of the Siwalik River-According to this view Siwalik range is nothing but the natural levees of the Siwalik River which have been subjected to folding by the compressional forces.
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According to the exponents of this concept after the formation of two northern ranges (Greater Himalaya and Lesser Himalaya) the Tethys sea was reduced to a narrow river channel which was named as ‘Siwalik River’ by G.E. Pilgrim and ‘Indobrahma River’ by E.H Pascoe (1919). This river was flowing in northwesterly direction from the east taking a long course from Assam to Aravallis.
This river deposited huge quantity of debris along its banks in the form of natural levees. During the Pliocene period due to the impact of the earth movements the bed of the Siwalik River started rising and its natural levees were compressed into folds. But critics of this theory argue that mere folding of the levees cannot produce a gigantic ridge of altitude ranging between 1200- 1800 m.
To counter this criticism it is argued that uplift did not only affect the levees but the whole river valley so as to give considerable height to Siwalik Hills.
2. Coalescence of the Alluvial Fans of the Himalayan Rivers-According to this theory after the upheaval of the Himalayas many rivers descended from the mountain and discharged their waters into the Siwalik river. Due to break of slope these rivers accumulated huge debris of sand and gravel in the piedmont area forming a number of alluvial fans. Later on individual alluvial fan joined together to form bigger fans.
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During Pliocene period due to earth movements the material of these fans was compressed to form the Siwalik range. This theory poses a number of serious problems. (1) The coalescence of the alluvial fans will not form such a long range as the Siwalik. (2) The material of alluvial fans cannot assume the height of 1500m and a thickness of 4500-6000 m as is noticed in case of Siwalik hills. No such gigantic fan has so far been found in any part of the world. (3) The range formed through the coalescence of different fans will not exhibit structural similarity as is witnessed in these hills. Hence, this theory does not seem to be scientifically tenable.
3. Silting of Lakes-According to this hypotiiesis alter the formation of two northern ranges of die Himalayas the remaining part of die Tethys Sea was broken into a number of lakes of different size and shape. Rivers descending from the Himalayan region deposited their debris in these lakes due to which diesel became shallower and shallower.
The sudden geological movements of die Pliocene times uplifted the beds of these lakes overwhich folds were formed. Thus a new range was formed along the southern margin of the Lesser Himalaya. Since die range has come into being due to the infilling of different lakes it is not a continuous. All those objections mentioned earlier may also be raised against the tenability of this theory.
4. Plate Tectonics-In fact Siwalik range like its osier two northern counterpart has been formed by die northward movement of die Indian plate and its subduction under the Asian plate. The stress so generated produces fracture and down warping of the crust leading to the formation of foredeep as noticed in case of the Ganga Plain.
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Its alluviadon and folding are responsible for the upheaval of the Siwalik Hills. One more such fracture (Himalayan Front Fault) has appeared south of the Siwalik which, according to the geologists, will not only raise the height of die earlier ranges of the Himalayas but may form a new range soda of the Siwalik in distant future.