Show and tell: the geology of Kaas

The region of the Western ghats around Satara and Pune are full of large plateaus and oddly shaped peaks. When you travel through them, the first impression you have of the mountains is that they look like a layered cake. I stood at the Thosegarh waterfall (featured photo) and found that even the monsoon-fed vegetation could not hide this appearance. The layers are a succession of lava flows, laid down in a massive burst of volcanism 60 to 100 million years ago. These successive layers of lava are called the Deccan traps. I found it hard to estimate the thickness of the layers by eye, but going by the heights of trees, perhaps they are between 50 and 100 feet thick. Since each layer of lava covers a considerable area, this means that each burst of volcanism would have lasted long and spewed out immense amounts of rock and ash. Not only would this have killed all life where it flowed, it would have dimmed the sunlight reaching the earth, and contributed to a mass dying of vegetation around the world.

Satara valley

In the 60 million years since it contributed to killing off dinosaurs, the traps have weathered. Today we see them as flat topped hills, cut through by deeply eroded valleys. Some of the waterfalls lead down to rapids extremely suitable for white water rafting. In other places there are very wide valleys. The town of Satara, which you see in the photo above, lies in the extreme western end of the rain-shadowed region of the Deccan. As a result it gets sporadic monsoon rains, enough to keep it green. The urban sprawl gets its drinking water from the river Umboli, which arises in the Kaas plateau, about 25 kilometers away. Traveling in this area, I saw many high plateaus. At one point each of them must have been home to the variety of flowering herbs and bushes whose diversity is now mainly visible in Kaas.

The surface of Kaas plateau

When you reach the top of one of these plateaus you see exposed rock everywhere. This is the volcanic rock called laterite, formed by weathering of the traps. You can see the dark porous rock peeping out from the low cover in the photo of the Kaas plateau above. There is hardly any soil. What little there is forms in little depressions in the rock. This area is covered by tiny herbs: mainly the carnivorous bladderwort (Utricularia) and sundew (Drosera) species, and tiny coexisting herbs. Between such rocky outcrops, there are deeper fissures where a little more soil can collect. There are higher bushes such as the Topli Karvi and arrowroot. There are very few trees on the plateau. What little soil forms is constantly removed by rain and wind. It is a marginal environment where extremely specialized plants grow. Twenty meters below the top, soil can accumulate, and the vegetation changes quite dramatically. It becomes characteristic of the rest of the Sahyadris. As a result, these high plateaus are like islands: the flora of each plateau is isolated from those of its neighbours.

When you walk through the Kaas plateau your eyes take in the evidence that geology determines ecology, that life is shaped by the land.

Kaas: a Deccan Plateau

On the road to Thosegarh

Head out of Mumbai, pass the Expressway and its food courts, and you soon come into impressive weathered hills. Strange shapes rise out of the land. Pass Pune, and head South, and you begin to see ranges of hills with a very characteristic feature: they seem to form gigantic layers or steps. If you look carefully at the far, mist covered, hills in the photo alongside you see these characteristic layers. They are volcanic features called “trappen” in Swedish, meaning steps. Successive volcanic eruptions created these layers.

One of the waterfalls at Thosegarh

A closer and clearer view of the steps is in this photo taken at the Thosegarh waterfalls south of the town of Satara. These layers are called the Deccan traps. It is thought that thirty thousand years of continuous volcanic eruptions laid down these layers of basalt over a huge part of the Indian plate. This happened while dinosaurs still roamed the earth. Since then the volcanic rock has worn down to a half million square kilometres area in the Deccan plateau.

The soil of the Kaas Plateau is very poor in parts

This is the geology within which the Kaas and neighbouring kilometer-high plateaus stand. Over millennia, the basalt was weathered down by successive dry and wet seasons until it is a porous rock. You can see the weathered rock, called laterite, in the featured photo. Over this laterite is a thin layer of red iron-rich soil called lateritic soil. You can see in the photo alongside how poor the soil is. The grasses and the low herbs of these plateaus, including Kaas, barely manage to hold the soil together. Of all the things that damage this fragile ecosystem, tourists are the worst, although the construction of windmill farms and extraction of bauxite also harmful. Reading about this area, I discovered the word “inselberg”, meaning island mountain. It is a very apt description of these plateaus: each stands isolated from others. The flora of each of these inselbergs is different from that in the surrounding lowlands. Environmental degradation of a plateau is like killing off the ecology of an island. It seems that of the 850 odd species of herbs identified here (including genera found only here), more than 600 are on the IUCN red list of threatened species.

A monsoon pond in the Kaas plateau

The ministry of the environment lists many benefits that these ecosystems provide, including medicinal plants. Another ecosystem service provided by this fragile system is the recharging of the surrounding water table. As you can see in the photo here, the grasses and herbs trap water into little monsoon pools. This water is then absorbed by the spongy laterite rocks. In the absence of the flora, the water would run off too fast to be soaked up. The average annual rainfall here is between 2 and 2.5 meters, so this is quite a service!

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