(ii) The evidence furnished by the depositional history in the Ganga delta and in the Assam does not fit well with the concept of Indobrahma stream. The gap between the Rajmahal Hills and the Shillong Plateau has been filled by a very thick mantle of alluvium. The time taken to deposit such a large quantity of alluvium should be much longer than suggested by the Indobrahma theory.
(iii) The evidence of the Tipam sand stones of Assam which were deposited in an estuary situated so close to the source of the Indobrahma also speaks against the theory.
The drainage patterns of Indian sub-continent are mostly influenced by the evolution of three physiographic divisions. This drainage pattern is broadly divided into the Bay of Bengal drainage and the Arabian Sea drainage with distinct water partings lying approximately along the Sahyadri, Amarkantak, Aravallis and the Satluj Yamuna divide.
It is also distinguished as Himalayan drainage and peninsular drainage though several of the Peninsular streams like the Chambal, the Betwa, the Sone etc. much older in age and origin form part of the Himalayan drainage system.
On the basis of origin two broad drainage systems of India are recognised (i) The Himalayan Rivers and (ii) The Peninsular Rivers.