I first wrote about this topic in 2011 in response to a question by a reader. I thought I would update my post with some new maps and explanations. Why is there such a pronounced pattern of easterly flow of the rivers in the Indian Peninsula. I keep getting asked this question. It was time for an update on this interesting topic on geology and landscapes.
The region south of the Tapi river covering the Deccan basalts and the southern Indian peninsula exhibits an easterly drainage with the rivers flowing into the Bay of Bengal. The map below shows the Indian peninsular region with easterly drainage. The Deccan Plateau is mostly but not entirely covered by the Deccan basalts. South of this region is the Karnataka Plateau with a Precambrian geology. Along the east coast there are Permian-Triassic and Cretaceous basins.
Source: Hetu C. Sheth: Deccan Beyond the Plume Hypothesis
The question posed to me was - What is the relationship between the Deccan volcanics and the easterly tilt of the Indian plateau (i.e. the plateau covering the Deccan volcanics and the southern Indian peninsular region)?
The easier more intuitive answer would have been that the western ghats provide the topography and Deccan volcanism created a lava pile that is thicker to the west and which thins to the east, thus generating an east sloping surface. Rivers follow the slope to the Bay of Bengal.
There are some geologic age inconsistency in this answer and this also does also not fully explain why the region south of the Deccan Volcanics too has an easterly drainage. Clearly, something more is going on.
To understand the evolution of the Peninsular drainage patterns let us look back to the time when the Peninsula didn't exist. In early Mesozoic, India was part of Gondwanaland and was joined to Antarctica and Australia to the east, and Africa to the west. The triangular shape of south India with characteristic eastern and western coastlines had not formed yet.
How can we find out the direction rivers were flowing back then? Geologists look to clues in the sedimentary basins of that age. The composition of sand in sandstone is matched to the most likely source terrain. And current directions can be inferred from studying ripples preserved on the surface of ancient sand.
The paleo geographic maps below shows Gondwanaland and the location of the Pranhita Godavari basin in the Mesozoic.
Source: Sankar Kumar Nahak and Coworkers 2024.
West North West flowing rivers originating in the highlands of the future Antarctica and in the Eastern Ghats were funneling sediment to the basin. Much of the interior of the region that would become the southern Peninsular India was a peneplain. There wasn't much topography towards the west for an easterly drainage network to develop.
India broke away from Antarctica beginning about 140 million years ago. A distinct eastern continental margin formed. Several NE- SW oriented basins developed along the edge of the Indian continent. Since by this time an expanding Indian Ocean lay to the east, the orientation of a natural drainage system would have been from the west towards the east.
We can say with some confidence that by 90 to 80 million years ago, east flowing rivers originating in the interior of the Indian continent were depositing sediment along the eastern Indian margin. See this map of sediment distribution along the Indian east coast.
Source: K.S. Krishna and Coworkers 2016.
It shows the thickness of Mid- Late Cretaceous sediment, ranging in age from about 100 million years ago to 65 million years ago. The sediment lobes coincide with the mouths of the Godavari, Krishna rivers and other southern rivers, indicating that the paleo Godavari and the paleo Krishna system had begun building deltas from that time. Since there were no Western Ghats then, these rivers may have been shorter, with their source somewhere in the Archean and Proterozoic terrain of Peninsular India.
Further to the south, geologists find a similar story with the ancient Cauvery. The Cauvery basin formed when Sri Lanka detached from the Indian continent. Its delta and marine deposits too contains sediment from the Late Cretaceous.
The easterly drainage pattern of Peninsular India developed before Deccan Volcanism and the formation of the Western Ghats.
India broke away from Madagascar about 88 million years ago and subsequently from the Seychelles about 66-64 million year ago. The latter separation coincided with Deccan Volcanism and the eventual formation of the western Indian continental margin. Block faulting that accompanies continental breakup would have created a north south oriented high area, which would eventually evolve into the present day Western Ghats. The thinning of the lava pile to the east also would have created an easterly slope. Rivers originating in the western highland now would flow across the length of the Peninsula.
New streams would have incised the fresh volcanic surface as lava buried the older etched landscape. But the regional pattern of easterly flow persisted.
Some geologists maintain that there has been some fairly recent Cenozoic age (past 15-20 million years) uplift of the Western Ghats which has accentuated relief and produced the youthful looking topography of scarps, waterfalls, and deep canyons. These earth movements would have certainly given new energy to the drainage system, but there is some geologic evidence to suggest that the streams originating in the western ghat region are antecedent to the uplift of the ranges.
For example, in the Mahabaleshwar area easterly drainage cuts across the axis of a north south oriented gentle anticlinal structure, implying that the drainage predates the uplift and warping of lava flows. Evidence from sedimentation patterns of the eastern river deltas also show that the easterly drainage originated much earlier than the formation of the Western Ghats.
What then created that initial slope to the east that imprinted the drainage network that continues today?
One reason is that the eastern margin formed first. Basin formation along the eastern edge of the continent would have created a relief difference between the western interior and the eastern depressions, resulting in stream networks flowing eastwards. Secondly, the new oceanic crust made of lava that formed when India and Antarctica separated in the Late Jurassic and Early Cretaceous would have cooled by Late Cretaceous times. Becoming colder and denser it has been sinking and dragging the Peninsular region with it.
Earlier eastern basin formation and a tug from the floor of the Bay of Bengal may have been enough to impress an east flowing drainage. Later, the east sloping lava surface and the rise of the Western Ghats reinforced this distinction between the west and the east perpetuating the direction of river flow initiated since Cretaceous times.
