Before the rise of the Himalayas, probably the Sahyadri-Aravalli axis was the main water-divide in the then Gondwanaland. According to one hypoth­esis the existing Peninsula is the remaining half of a land mass, which had the Western Ghats very near its centre as its primeval water-shed for two drainage systems, one easterly and the other westerly. Some­times during the early Tertiary period half of the Peninsula lying west of the Western Ghats is pre­sumed to have cracked and sunk beneath the Arabian Sea.

This was due to the formation of a great normal fault along the Western Ghats. This argument is supported by the straight coastline, steep slope, and absence of delta deposits along the Sahyadri coast.

Another view, equally probable, is suggested by the exceptional behavior of the Narmada and the Tapi. It is believed that these rivers do not flow in the valleys of their own eroding, but have usurped for their channels two fault planes, or deep alluvium- filled rifts in the rocks running parallel with the Vindhyas.

These faults are said to have originated with the bending or ‘sagging’ of the northern part of the Peninsula at the time of the upheaval of the Himalayas. During the same time the peninsular block, south of the cracks, tilted slightly eastwards, giving a new orientation to the entire drainage to­wards the Bay of Bengal (Wadia. 1975, pp. 25-26). While this argument well explains the eastward drainage of the Peninsular Rivers it also poses certain unanswered questions. For example, the tilting should have increased the gradient of the river valleys and caused some rejuvenation in the rivers (Mehdiratta, 1962, p. 26) for which no concrete evidence is available.