In the mid‑19th century Fort William boreholes (1836-40), engineers encountered a 15‑foot peat bed at about 120 feet below ground—distinct from marine sediments, this dark, fibrous layer preserved pollen grains and freshwater gastropod shells. Subsequent studies interpret it as the residue of a Late Quaternary lake or marsh that predated the Hooghly’s current course. The peat’s woody fragments testify to massive inputs of decayed riverine vegetation, trapped as the land slowly subsided. Far from being geological trivia, it marks the first stage of coalification and offers a snapshot of Calcutta’s vegetational history before human settlement. Even modern metro borings (1970) have confirmed pockets of peat, highlighting the city’s hidden wetlands under concrete.
Deep drilling beneath Fort William yielded surprising finds: at 350 feet a canine bone; at 360 feet a tortoise shell; and at 392 feet small coal pebbles. These fossils, identified by Prinsep and later researchers, demonstrate that Calcutta’s subsurface once hosted terrestrial mammals and freshwater turtles—and that ancient rivers like the hypothesized Indobrahm may have carried coal fragments from Meghalaya into the basin. Combined with other excavations (Dum Dum tank and Matla river works), these remnants illustrate shifting estuarine and freshwater conditions over millions of years, long before the city’s founding. Each bone and shell fragment is a relic of the “animal world” that occupied the Bengal floodplain, now entombed beneath colonial ramparts.
Calcutta sits in a subsiding trough of the Bengal Basin, with recorded land‑level drops of 30–50 feet over the past two centuries. Well borings in 1814 and 1836–40 found peat and buried Sundri stumps, proving that portions of the city were once tens of feet lower relative to the Hooghly. Tank excavations near Chowringhee and Sealdah confirmed similar subsidence. Meanwhile, early 20th‑century tests at King George Dock recorded 40 feet of sinking. These data, seen in conjunction with current Himalayan tectonics and delta‑building, reveal Calcutta to be a city in constant vertical flux, whose foundations are set upon soft, young alluvium, still adjusting to the colossal forces that uplifted the Himalayan ranges.
Source: Biswas, Oneil. (1992). Calcutta and Calcuttans. Calcutta: Firma KLM Pvt Ltd.
Arup K. Chatterjee 