Despite their homogenous topography and simple relief the mystery about the origin of the plains has not yet been fully resolved. The puzzling questions are related to the enormous thickness of the alluvium, nature of the depression, mode of its formation and the subterranean rock-beds and geological structure. Following is the summary of some of the prevalent views which have tried to resolve these mysteries in different ways:
1. Alluviation of the Foredeep-According to the eminent Austrian geologist Edward Seuss a ‘for deep’ was formed in front of the high crust-waves of the Himalayas as they were checked in their southward advance by the inflexible solid land-mass of the Peninsula.
This for deep was like a large syncline giving the impression of a synclinorium (a large syncline with a number of small anticlines and synclines) owing to the unevenness of its bottom. According to Suess the bed of this for deep had gentle slope towards north whereas the peninsular side depicted steep gradient.
This bed rests on the basement of hard and crystalline peninsular rocks through which the region is connected to the Himalayan and the Peninsular blocks. The alluviation of this for deep led to the formation of the Great Plains by the rivers descending from the Himalayan region.
ADVERTISEMENTS:
One of the serious gaps in Suess’ theory is related to his inability to give suitable explanation to the uneven slope of the bed of the Plain. Similarly so far no conclusive evidence is available through which it could be inferred that there exists crustal connection between the Himalayan region and the peninsular block through the Great Plains.
2. Infilling of the Rift Valley-According to Col. Sir G. Burrard during the time of the formation of the Himalayas a rift valley was formed due to the sinking of a portion of land between two parallel faults (one along the Siwalik range and another along the northern boundary of the Peninsula). It was filled up by the detritus brought by the rivers from the Himalayan region.
To support his argument Burrard has cited examples of the formation of similar rift valleys in the Himalayas and the peninsular region especially the Narmada rift valley. But this concept does not find approval of the modern geologists. According to them nowhere in the world such giant rift valley with a length of 2400 km and a depth of 500 m has ever been formed by crustal down warping. Also there is no geological evidence of formation of such graben in the northern part of the peninsular foreland.
3. Recession of the Sea-According to Blanford during the Eocene period the Peninsular India was joined together with Africa. During that time there was one sea extending from Assam valley to the
ADVERTISEMENTS:
Irrawaddy river in the east and another from Iran and Baluchistan to Ladakh (Indus valley) in the west. During the last part of the Eocene period arms of the western sea extended up to the Punjab. Due to the rise of the Himalayas during the Miocene period these seas started receding by gradual deposits of sediments from the rejuvenated Himalayan Rivers.
After a prolonged period of sedimentation and subsidence these gulfs (Gulf of Sind in the west and Eastern Gulf up to Shillong Plateau) were filled up yielding place to the Great Plains. The evidences cited in favour of the recession of the sea include the occurrence of limestone rocks in Kumaun-Garhwal region, presence of saline water lakes in Rajasthan region, joining of the islands of the Gulf of Kachchh with the mainland, and seaward extension of the Ganga Delta, emmer.