Plateaus of Panchgani and Wai and the places located along the Western Ghats in Satara is capped by an interestingly looking reddish black rock called as Durricrust or Ferricretes *(1) also popularly known as Laterites *(2,3). A micro-habitat for diverse creatures and plant life. Alot of work is underway to document little known flora and fauna of these region but I would like to direct my readers attention to the Geological setting of this magnificent landscape which is one of the second highest plateau in Asia after the Tibetan plateau and are the Water buckets of the Western Ghats.
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| Panchgani Ferricretic Table land. (Photo Credit: Aparna Shrivastava) |
Understanding what they are made off, how they are formed, when they were formed, where are they found or the spatial extent of this rock type and why is it important; will help in appreciating the value of protecting mesas and plateau capped by Ferricretes.
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| Reddish black Ferricrete rocks resting on the hard volcanic Basalts in Wai. |
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| Weathered in-situ Lateritic soil,Wai. |
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| Parent rock Basalts below the Ferricrete blanket showing columnar jointing, Wai. |
The first ever laterites being documented and named was in Kerala by a Scottish physician Francis Buchanan- Hamilton during his stay in India *(3), since then on extensive studies were carried out in understanding what they are made of and their origin. The rock looks grippingly reddish with holes, having yellowish halos that has resulted in pisolitic and vermicular structure. These ferricretes are developed by intensive and long-lasting weathering of the underlying parent rock, that have been created due to leaching away off more soluble elements within the Basalts like calcium, magnesium, sodium, potassium and finally silica in rain water solution, leaving behind concentration of insoluble hydroxides of aluminium and iron; which on hardening to exposure to the atmosphere form hardened crust - the Duricrust.
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| Basalt and Ferricretic pebbles for comparison from Konkan Coast. (Photo Credit: Abhijit Gandhi) |
But you will be curious how these Ferricretes ever happened to rest on these hard volcanic rocks, well there is an interesting mechanism put forward by Ollier and Sheth which suggest of inversion of relief which sounds quite intriguing. Well it is all about Geologising it!! Schematic diagram below shows possible mechanisms for formation of Ferricrete/Lateritic Tableland put forward by Ollier and Sheth compared to general accepted theory of formation of Laterites. *2
- Widdowson & Cox explanation: A period of chemical weathering along a widespread area of basalts formed a continuous blanket of ferricrete. Subsequent erosion of most of it left discontinuous patches.
- Oliier & Sheth explanation: The ferricrete formed in shallow flat bottomed river valleys that developed on the basalt surface. The alluvium and colluvium of these basalt valleys got converted to ferricrete. The surrounding highlands got eroded away being softer than the ferricrete. The mesas represent an "inversion of relief". Former valley bottom now standing in higher relief.
These dark patches streching north-south outline a dendritic pattern (shown in the map below), just like a river system. So if the mechanism put forward by Ollier and Sheth is right then this ancient river system might have flowed in a northerly direction depositing iron rich sediments and forming Ferricrete which later on due the differential uplift of the Western Ghats got uplifted and reversed the drainage flow direction to today's existing southeasterly direction.
Ferricretes are chemical weathering products and such surfaces of intense chemical weathering was possible only during times of tectonic stability. The ferricretes around Panchgani and Wai tells us that after the eruption of the Deccan Basalts in the late Cretaceous there was a long period of intense chemical weathering in a tectonically stable regime. An initial period of uplift in early Cenozoic post deccan volcanic eruption lifted the Ghats as well as the coastal plains followed by a period of stability. Weathering on a stable block of crust formed flat surfaces or peneplains or planation surfaces which is what the Panchgani and Wai tableland and other such surfaces further south of Satara are made of. Later episodic uplift alternating with periods of stability resulted in development of such flat weathering surfaces at various altitudes as mountain ranges rise and get weathered and chamfered and then rise again. The younger surfaces will occur at lower altitudes.and that is the the reason we have the presence of ferricretes along the konkan region, west of the Sahyadri ranges.*(1)
The majority of the land areas with Laterite cover occur between the tropics of Cancer and Capricorn. That suggest Tropical climate for formation of Ferricrete. Tropical weathering (laterization) is a prolonged process of chemical weathering which produces a wide variety in the thickness, grade, chemistry and ore mineralogy of the resulting soils. This active laterization period extended from about the mid-Tertiary to the mid-Quaternary periods (35 to 1.5 million years ago) *(1). The spatial extent of these Ferricrete capping over basalt in Satara can be seen in the map shown below. *(3)
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| Spatial extent of Laterite/Ferricrete in Satara district. Courtesy: Grampari |
The thick Ferricrete layer is porous and slightly permeable so the layer functions as an aquifer in rural areas of Wai and Panchgani.
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| Spring water oozing out from the contact of Ferricrete and Basalts. This water is used by the village communities for drinking and other domestic use, Wai. |
Recent anthroprogenic or human interest in such so called 'barren lands' has lead to decrease in water table at these plateaus due to sinking of bore-wells that intercept spring flow and reducing the overall water table of the ferrcretic plateau. It may seem mostly barren, but there has been various articles that have metioned such plateau environs are teeming with plant and animal life, specialized to live in crevices and along the slopes and depressions and hollows and water seeps that have formed by the action of physical and chemical weathering. *(4)
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Expected stages of changes in the ground-water outflow at springs. (A) Stage I—before sinking of borewells in Lateritic aquifer, groundwater comes out as springs from the contact of Ferricrete and Basalts filling the spring box. (B) Stage II— initiation of drawdown due to pumping at the borewell has caused movement of water away from the spring, but the spring has become losing or possibly intermittent. (C) Stage III—after a substantial period of pumping in excess of rate of ground-water recharge from the Lateritic aquifer, the spring may be disconnected if groundwater flow cannot provide enough recharge to maintain the water table. The spring has become ephemeral. (D) Stage IV
— the spring source becomes disconnected due to no water in the lateritic aquifer pushing the groundwater table further down. (© Grampari)
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These rocks has considerable water-holding capacity, depending on the depth of the formation. These Ferricrete aquifers recharges rapidly with the first two months of the monsoon, and continues to fill with the monsoon rains. The water table recedes slowly and is recharged several times during the rest of the year but this natural annual recharge can be affected if there is an overdraft from these aquifers.
The springs, streams and rivers originating from the Western Ghats upstream in Satara are capped by Ferricretic rocks needs due protection as these rock formations absorb and retain monsoon rainfall and slowly release this water throughout the year. This not only feeds the springs but also groundwater and base flow in nallas and in adjacent valleys supporting unique vegetation and endemic flora and fauna of the region and providing drinking water to million of vulnerable communities situated up-slope. These upstream areas are a treasure trove of the Western Ghats and for the rural people.
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| Water percolating slowly down through the Ferricrete. |
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| Criss-crossing pipes that direct spring water downslope to villages under gravity. |
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| Traditional spring water protection cum collection tank carved out into Ferricrete rock during the reign of King Shivaji at Kamalgad fort, Wai. |
Recent decades have seen a boom in building of second holiday homes, tourist resorts housed in plantations and new hill stations. Rapid spurt in growth of roads as well as railway lines across the Ghats with resultant disruption of connectivity between natural habitats. With rapid increase in means of communication and transport, emergence of a large wealthy middle class and availability of powerful earth-moving machinery, the Western Ghats are beginning to be urbanized with a proliferation of holiday homes and resorts. These tend to be accompanied by a total decimation of natural biological communities and displacement of local people. The people of the Western Ghats traditionally depend heavily on natural vegetation for meeting their requirement of shelter, fodder and fuel *5. And it is our responsibility that we protect these areas and not be driven by our greed.
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| Excavation of 4 m depth of Ferricrete for foundation pillars, upsetting the delicate groundwater flow at Yeruli, Wai. |
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| Tourist resort being built and bore-wells being sunk, sucking all the groundwater on the ferricrete outcrops at Yeruli, Wai. |
I hope my small writing endeavour and my personal experience while working with these people and in the region highlight the concerns and threat to the ferricrete outcrops, which will help in keeping these geological heritage intact and help in protecting the rich biodiversity of these region. Such plateau capped by Ferricrete should be also included under the ecosensitive zones apart from other already declared and minimal anthropogenic interventions is essential to preserve this natural heritage and biodiversity of the Western Ghats.
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| Protected forest at the base of Kamalgad fort which is made of ferrricrete rocks, Wai. |
References:
*1 Ollier, Cliff D., H.C. Sheth. (2008): The High Deccan duricrusts of India and their significance for the ‘laterite’ issue. Journal of Earth System Science. Volume 117, Issue 5, pp 537-551.
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