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The Jurassic Forest

Tensing Rodrigues

From the Kalinadi estuary at Karwar, today we travel 356 kilometres north to Dapoli, Ratnagiri to listen to the story told by a well in Kamgavai, a tiny village with less than 1,000 inhabitants, 15 kilometres from the shoreline. The entire district is hilly, with a number of small rivers flowing down into the sea. Kamgavai is surrounded by three of these rivulets – Bhalja, Ilne and Jog. [Kumaran et al, 2013: Vegetation Response To South Asian Monsoon Variations In Konkan …, in Quaternary International, 286; Srivastava et al, 2016: Monsoon variability over Peninsular India … terrestrial archive from the corridors of Western Ghats, in Palaeogeography, Palaeoclimatology, Palaeoecology, 443]

The diameter of the Kamgavai well is 8 metres and it is 12.0 metres deep; the top portion was examined for the study. Below the top layer of soil lay a hard crust of erosion-resistant sedimentary rock cemented together by percolating iron oxides (ferricrete)). Below that lay soft, porous rock,thoroughly decomposed by humid, tropical climate (saprolite). These two layers constituted the laterite cover, and extended to about 12 metres to 15 metres; below this lay the basaltic rocks. Between the two was the underground water. There were no channel gravels between basalt and overlying saprolite, indicating that there has been no flowing water. So the deposits of the sediments with plant fossils have happened in situ – that is, the sediments are made of matter from the site itself. The digital elevation model of the area suggests the possibility of a basin-like depression at the site which has accumulated the deposits over several thousands of years – an ancient lake (palaeolake) that has been filled with and is now fully covered over with subsequent sedimentary deposits.

The 8 metres sedimentary column studied by Kumaran et al at Kamgavai showed five clear layers, corresponding roughly to the period 44,000 BCE. The base layers V and IV, 8 metres to 5.7 metres, corresponding roughly to the period 44,000 to 1,500 BCE, were rich in organic carbon content and were of lake origin. They bore abundant fossils and contained well-preserved compressions of plant objects like leaves, pieces of wood and rootlets. These fossils were of wet evergreen forest types, mainly belonging to freshwater myristica swamps, found today largely in the Kollam district of Kerala, the Uttara Kannada district of Karnataka and Goa; myristica is some sort of a wild nutmeg, found in Asia and Western Pacific. The dominant tree type was syzygium, a family to which belongs the jambhull, zambh (rose apple/java apple) and clove. [Kumaran et al, 2013: 9] Myristica swamps are freshwater swamps (unlike mangrove swamps) that probably are one of the most ancient ecosystems, which survived till about 42,000 thousand years back as relic forests; today only some refugia patches exist. The palaeo lake referred to above was most probably a myristica swamp. Relative abundance of fossil pollen grains and cuticles of reed bamboo in the sediments further confirms that the area was swampy. Today there exist no myristica swamps, reed bamboo stands or any allied vegetation in the region.

The three layers that lay above these, layers III, II and I, at 5 metres to 3.70 metres, corresponding roughly to the period 1,500 BCE, were very different from the layers V and IV. The very long time gap is intriguing – 44,000 to 1,500 BCE. Was nothing deposited during the long interval? What happened to the myristica swamp and its environment? The younger three layers are made of silty clays, with different coloured streaks and patches, and rich in wood fragments but without compaction. The deposits are not as in a lake; they seem to have been left behind by a flowing seasonal stream. The topmost layer seems to have been alluvium deposited by flood during monsoon. The layer below that, contains gravel typical of a flowing stream; this layer is less than 2,000 years old. The layer III shows transition between the two neighbouring layers – layers IV and II, and spans roughly the period 1,500 to 0 BCE; it shows features of both stagnant as well as flowing water. Most important of all, the plant fossils indicate that by this time the luxuriant tropical wet evergreen rain forests had completely disappeared and were replaced by moist deciduous forests, with only some patchy semi-evergreen forests along the streams and rivulets.

This is not a story of forests lost or forests gained. It seeks to answer the question: What type of landscape did our ancestors find as they descended the Sahyadri onto the coast around 1,500 BCE? As we hinted earlier [The Story That Kalinadi Wrote,  August 5, 2018] they had to penetrate and clear moist deciduous forests, with may be some patches of semi-evergreen forests; or rather, they must have encountered semi-evergreen forests in the valleys of the foothills of the Sahyadri, and then entered the moist deciduous forests as they neared the coast. Thank God they did not have to wade through the myristica swamps. Or did they really escape that ordeal?

Before we try to answer that question, let us see the climatic macro picture that lay underneath the drastic shift between the layers IV and III of the Kamgavai well. The study by Srivastava et al reconstructs the rainfall around 44,000 BCE on the basis of the requirements of the vegetation whose fossils have been found in the deposits of that period and the geological data. A comparison between the reconstructed rainfall and the current actual rainfall in the region explains the cause for the drastic shift in vegetation. According to this reconstruction the average annual rainfall in the area has actually increased significantly, during the last 44,000 years or so – from about 1,900 mm to about 2,800 mm. But, at the same time the pattern of rainfall over the year has changed. 44,000 years back S W monsoon contributed just about 64 per cent of the total annual rainfall as against about 93 per cent now; SE monsoon contributed about 18 per cent of the total annual rainfall as against less than 5 per cent now.  Pre-monsoon rainfall contributed about 15 per cent of the total, as against 2 per cent now; the same was true of the post-monsoon rainfall. Besides all this there was a dry season rainfall contributing about 3 per cent of the total. So, though the total rainfall was lesser, it was distributed over a longer time – as much as nine months of the year; the fossil locality now experiences only 4 months of rainfall. The wet evergreen forests in the Western Ghats require less than 4 months of dry season to thrive; where such conditions are met, one still finds myristica swamps. [Srivastava et al, 2016: 64]

Categories: Panorama
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