1. Geological History
Somewhere around 88-90 million years ago, during a time of great turbulence in Earth’s geological history, when continents were splitting and drifting, creating even new oceans like the Atlantic and signaling the end of some others like the Tethys Sea, the Western Ghats were born or got an independent identity, when India separated from the east coast of Madagascar (. The elevated rifted edge of the Indian west coast, rising precipitously parallel to the ocean, the Western Ghats, has an undulating leeward side to the east merging with the Deccan Plateau. The separation of Seychelles, further north from the western margin, around 65 million years ago, was marked with some of the most violent Volcanic eruptions in the geological history, destroying much of life on the Earth and covering about 500,000 km area in land and sea with basaltic lava. The Deccan volcanic province on the land, as evident today from basaltic (lava solidified into rocks) outcrops, covers a vast area from almost northwards of Phonda in Goa and Belgaum in Karnataka, spreading eastwards and northwards stretching into Central India.
Whereas Madagascar, after separation from India, remained to the east of southern African coast, the Indian subcontinent drifted northwards from a position far down the equator, closer to Antarctica, through the ancient Tethys Sea towards Eurasia. Crossing of the equator towards the north, was a process that lasted through several million years, a period, understandably marked with high humidity and heavy rains, almost all over India, including the present Rajasthan desert, even which was covered with rain forests, as indicated by fossil pollen grains. The summits and ridges, the escarpment of the Western Ghats, became a dividing line for the watershed, a major drainage divide, creating numerous short distance rivers running down the escarpment towards the newly developing Arabian Sea; the rivers flowing eastwards, gradually descended the leeward slopes, through the Deccan Plateau to the eastern ocean.
Waters flowing down the west facing escarpment of Western Ghats would naturally erode the rocks, a process that probably went through the last 80-90 million years. Sheets of water rushing seawards would carry huge amounts of soils mixed with pieces and pebbles of rocks of diverse kinds depositing them along the coast. The gorges of the rivers Sharavathi (eg. at Jog waterfalls), Aghanashini (at Unchalli Falls), Gangavali (at Magod Falls), Kali (at Lalguli waterfalls), are mere remnants perhaps of scores of rapids that flowed into the Arabian Sea depositing eroded materials all along the coast causing raised platforms and mounds resulting in certain recession of the Western Ghats. Today, in Uttara Kannada district for eg., much of this ancient deposited materials by palaeo-water courses, constitute compact, flat topped low level hills and plateaus abutting the Western Ghats, and flanking present day rivers like Sharavathi, Aghanashini, Gangavali etc., are in all probability predating the Maharashtra coastal formations heavily influenced by Cretaceous-Tertiary boundary volcanism.
The collision of Indian sub-continent with south China region in Eurasia, marking the end of its northward drift, caused the rise of the Himalayas. India’s placement today in the northern hemisphere, its southern tip at almost 8°N lat, coupled with the Himalayan barrier caused increased aridity and development of monsoonal climate. Rain forests perished everywhere except in North-East and in southern Western Ghats where they survived due to relatively more rainy months. Everywhere in the peninsular India, water levels would have lowered in the rivers, many ancient watercourses drying up or persisting as streams in deep valleys and gorges. dried up or turned seasonal, but the deposits of eroded materials exposed as the low, hilly compacted coastal landscapes, that is for instance the hallmark of Uttara Kannada.
2. What is laterite?
‘Laterite’ is the Latin word for brick. Francis Buchanan (1807) described laterite for the first time in Angadipuram in Kerala, while looking for iron ore occurrence in Malabar region:
“In all the hills of the country there is found forming beds, veins, or detached masses in the substratum of industrial clay …. one of the most valuable materials for building. It is diffused in immense masses, without any appearance or stratification, and is placed over the granite which forms the basis of Malayala. It is full of cavities and pores, and contains a very large quantity of iron in the form of red and yellow ochres. In the mass, while excluded from the air, it is so soft, that an iron instrument readily cuts it, and is dug up in square masses with a pickaxe…. It very soon becomes as hard as a brick, and resists the air and water much better than any bricks I have seen in India”.
The general features of laterites are as shown below:
- Presence of large amounts of hydrated iron, aluminum and manganese oxides
- Depletion of free silica
- Alumina (aluminum oxide) combines with silica to form kaolin, or white clay, that is mainly aluminum silicate
- Negligible or no bases and alkalies
- Hardening on exposure to air; in wet conditions it is softer, and can be cut easily, cured and used as bricks.
- Basically a vesicular rock, having large pores, vesicles and tubular (vermiform) cavities, often filled with kaolin if the laterite is underneath soil and not exposed to severe weathering.
Bourgeon (1989) considers laterites/lateritic soils as characteristic of the coastal hinterland of Western Karnataka. This hinterland is formed by the dissection of old laterized geomorphic levels. In this geological relief closer to the coast are laterite capped mesas (tablelands with steep sides). These are most typical, hardened flat topped hills, more towards the coast (Apsarakonda, Gunavanti, Dhareshwar, Gokarna, Tadadi, Aghanashini fort etc.); but these tablelands may be present inland coast along the river valleys (Eg: Muroor hill and Divgi-Mirjan plateau alongside Aghananshini). These tablelands are not perfectly plain having also elevations and depressions, all laterized on the top. The convex portions have bare laterite and the depressions with pockets of soil as well as crevices have plant growth that is discontinuous clumps (mainly Sapium insigne, Memecylon umbellatum, Flacourtia indica, Ixora coccinia; Plectranthus rheedei etc.). Bigger and flatter depressions having thin layers of soil (>50 cm) are cultivated.
Most of the coastal laterite mesas and other elevations are barren, scorified, hard and black with exposed honey comb structure, with soil remaining only in depressions or crevices. Hills with better vegetation, and slopes of these hills are often strewn with massive boulders of exposed laterite with more of soil pockets between them. Such hill sides are covered with better vegetation, including certain evergreen species, which may include even cinnamon and canes otherwise to be found in interior forests.
According to the Geological Survey of India (2006) majority of the rock sequences in Karnataka are laterised due to their exposure to suitable climatic conditions for prolonged period. These laterites occur as extensive cappings in the Western Ghats and coastal plains. Their thickness ranges from a few cm to as much as 60 m. Based on their elevation level, two types are identified, one at +600 m elevation confined to Western Ghats and the other fringing the coastal lines along the west. The latter type is gravelly to sandy in texture and appears to be transported, whereas those transported to Ghats are homogeneous and less sandy. In the Uttara Kannada district the generalized laterite profile displays a reddish brown lateritic soil on top underlain by hard, concretionary and pisolitic laterite, underneath which is 0.3 m thick aluminous laterite and 0-4 m thick bauxite zone. Kumta, Honavar and Bhatkal laterits are considered good examples.
3. Primary laterite (high level laterite)
Palaeomagnetic studies reveal laterites over the tops of plateaus on the Western Ghats are older than those over the coast. The former are found over the Deccan basalt and even over the older Pre-Cambrian. High level laterite reveals an anticlinal structure and could obviously be developed from the basalt strata below. The coastal low level laterite has a general westward slope, indicating possibly the effect of neotectonic activities (of recent origin) in the region (Ramakrishnan and Vaidyanathan, 2008). Many hills and plateaus in Maharashtra are formed by in situ transformation of the parent rock beneath, the basalt, which is of volcanic origin. As far as Western Ghats covering Maharashtra and North Goa are concerned Widdowson and Cox (1996) documented the high level laterites as capping the high summits forming a trail from Phonda (16°N), Devrukh (17°N), Bamnoli Range (17°30’N), to Mahabaleswar plateau (18°N). These laterites are very similar in composition to the parent rock the basalt, of volcanic origin. Stratigraphic studies reveal their beginnings in Early Tertiary Age, obviously after Deccan volcanism 65 Mya. Laterite formation is essentially due to a process called laterisation, which involves removal of soluble elements by water. In Maharashtra laterites, both high level and the coastal are formed by weathering related to water. The most extremely laterised rocks predominantly consist of aluminium oxides and ferric oxides. These rocks are largely stripped off easily soluble oxides of silicon, calcium, magnesium and sodium. Potassium oxide is much less depleted and is present in the clay of laterite formations. The less mobile elements are Zirconium and Nobelium, which have their concentrations, nearly as much as in basalt, clearly establishing the laterite of Maharashtra as of basaltic origin (Widdowson and Cox, 1996). We may here refer to parent rock (for eg. basalt) transformed by weathering, involving mainly leaching of soluble elements by water, leaving behind, a porous honey-combed structure, capping the parent rock itself as primary laterite. These are in situ formations as seen towards the summits of Maharashtra ghats.
4. Secondary laterite (Low level laterites)
In Maharashtra, the Konkan coast low level laterites, often below 100 m, are extensively present filling areas between volcanic basaltic rocks. There has been a severe erosion of mountain tops and deposition along the Konkan coast, happening through millions of years. This created indeed the coastal stretches and the recession of the Western Ghats eastwards. The weathering of these eroded and re-deposited materials created the coastal laterites of Maharashtra. The elemental composition is more similar to existing rocks, much of it is basaltic, but need not show as much correlation with basalt as in the high level laterites. We may therefore conclude that the coastal laterites of Konkan region are more of origin from deposited materials from erosion of Western Ghats and in situ formation from low level basalts, mingled together.
The Indian south-west coast laterites are considered to be of secondary nature, being formed from sedimentary deposits such as gravels and pebbles by sesquioxide impregnation and cementation. They are pellet type and quite different from the underlying soil or bedrock. These laterites have a continuous softening effect with depth. The laterites are normally subjected to alternate wet and dry climates (Ranjan and Rao, 1991).
5. Laterites of Uttara Kannada coast
Anybody who visits Uttara Kannada coastal taluks, particularly Kumta, Honavar and Bhatkal, and Ankola to some extent, will not fail to note the extensive stretches of darkish, honey comb like rocks covering hills and plateaus. Many hills are almost fully covered by a thick and hard mantle of this rock with very little or no soil. The vegetation on such hills is sparse and confined to crevices and depressions having trapped soils. Only during the incessant rains of July to September we find these rocks getting covered with slimy algae and several kinds of tiny herbs, including many insectivorous ones (Utricularia spp., and Drosera indica). The rock pools during the rains get colonized by several water plants, including the rare endemics. Clinging to the rocks grow woody species like Memecylon umbellatum, Syzygium caryophyllata, Sapium insigne, Ficus arnottiana and several thorny bushes. The coastal hills were barren and denuded from the dawn of history. Human impact in the form of shifting cultivation, biomass removal and cattle grazing were cited as the reason for soil erosion and laterisation of the hills.
6. Structure of laterite
Laterite hills and plateaus towards the coast may be viewed as mounts of soil covered with iron rich carapace or shell of laterite rock. The outer laterite, exposed to the atmosphere, is dark and gives the appearance of solidified and eroded lava. The inner laterite has varied colours, different hues of red, yellow of purple. The outer laterite has honey-comb appearance, full of empty pores. The inner laterite has vermiform tubes and vesicles or cavities coated by deposits iron rich substance. The cavities are often filled with loose, light coloured material. Below the mantle of laterite, good soil, yellow to orange or red in colour, is found. In the exposed outcrops the filling material is often missing being washed away by rains. The hills of the coastal zone further interior are higher; their sides are more eroded and their tops are not as wide as the tablelands towards the coast. Under forest vegetation there is good soil cover on the surface of ferruginous lateritic layer below. If the forest is cleared, easy leaching of soluble substances and erosion of fine soil particles exposed to heavy rains leaves behind bare laterite rich in iron with honey-combed structure.
The collision of the north drifting India with Eurasia, towards the south of China, resulted in the rise of the Himalayas. The northward drift, away from the humid equatorial regime and the great wall of Himalayas created major climatic changes bringing in monsoonal rains and greater dryness. The rain forests perished everywhere except in South Indian western Ghats and North-Eastern states. The Rajaputana Desert was born and Deccan became a semi-arid territory. As rains would decline even in the Western Ghats the rivers would shrink and valleys, especially along the west through which sheets of water rushed towards the sea, depositing eroded materials along the coast, would vanish or get reduced to smaller streams. These ancient deposits by palaeo-rivers, were in the form of plateaus and flat topped hills, hardly exceeding 100 m in height anywhere.
These coastal highlands formed a distinct geological territory along many parts of the South Indian west coast, most noticeably in the districts of Uttara Kannada (North Kanara), Udupi, Dakshina Kannada in Karnataka and Kasargod and Kannur in Kerala, and to lesser extent in other southern districts. These novo-formed low hills and plateaus would also have had rain forests once, just like the western face of Western Ghats.
7. Case for declaration of Laterite Conservation Reserves
Conservation Reserves are declared for the purpose of protecting landscapes, seascapes, flora and fauna and their habitat. The rights of people living inside a Conservation Reserve are not affected. Conservation Reserves can be declared by the State Governments in any area owned by the Government. Particularly stressed are areas adjacent to National Parks and Sanctuaries and those areas which link one Protected Area with another. Such declaration should be made after having consultations with local communities.
In the cases proposed here of the two lateritic plateaus as Conservation Areas, it needs to be admitted that these are not adjacent to National Parks and Sanctuaries, nor they link one PA with another. We recommend these areas strongly for protection because of their uniqueness in flora, and to some extent in fauna, their ancient geological ages, the formation beginning 88-90 Mya.
