Modern Calcutta rests atop a grand geological phenomenon spanning hundreds of millions of years. In the Jurassic age (190–136 million years ago), the Indian subcontinent broke free from the southern supercontinent Gondwana and drifted northward. By the Eocene (54–38 million years ago), it rammed into the Eurasian and East Asian plates, thrusting up the Himalayas and Arakan Yomas. These titanic collisions triggered not only mountain‑building but also the gradual subsidence of Bengal’s northeastern flank—creating the vast Bengal Basin that would cradle future Calcutta.
The basin’s shape was sculpted by two great faults: a subduction zone beneath the Himalayas and a transform fault along the Arakan Yomas. Between these lay the Garo‑Rajmahal gap, splitting the basin into a fore‑deep to the north and a tectonic trough to the south. Over the Pliocene (7–3 million years ago), sediments from the newly uplifted Himalayas—carried by the Ganges and Brahmaputra—filled the trough, building up the Indo‑Gangetic plain layer by layer. By the late Pleistocene, repeated sea incursions and regressions deposited alternating marine and freshwater layers, leaving a complex stratigraphy of gravel, sand, silt, and clay.
Evidence for Calcutta’s ongoing descent comes from 19th‑century boreholes and excavations. A well dug near the Hooghly in 1814 revealed 32 feet of subsidence beneath ancient peat beds; similar tests in Fort William (1836–40) recorded up to 50 feet of sinking, with buried Sundri trunks and fish‑scale limestones marking former shorelines . Even today, minor earthquakes in the eastern Himalayas hint that the Bengal Basin—and its urban crown—has not yet reached equilibrium.
Understanding this subterranean story reframes Calcutta not simply as a colonial city but as a deltaic marvel born of tectonic drama and riverine artistry. Each monsoon, when the rivers swell, they echo the movements of plates far to the north; each tremor carries memory of the subducted Indian craton. Beneath our feet lies a living archive of earth’s restless past, revealing how Calcutta emerged from the interplay of continents, oceans, and time itself.
Source: Biswas, Oneil. (1992). Calcutta and Calcuttans. Calcutta: Firma KLM Pvt Ltd.
Arup K. Chatterjee 