Tuesday, June 9, 2009

Erosional features on the top of the hill near Ranchi city of India.

Evidence of lost river is also seen.
Dr. Nitish Priyadarshi

Few days ago one of my friend from National TV News channel requested me to accompany him to the hill which is 20 kms south of Ranchi city in Jharkhand State of India. His purpose was to do a story on Hindu Mythology which is related to that hill. It was about the saint Valmiki who wrote Ramayana. According to the belief Valmiki stayed here. Interestingly the name of the hill is MARASREE. If you read it in hindi from opposite direction it will spell SHREERAM (name of the lord Rama of Ayodhya).

But my interest was not related to such findings. As being a geologist I was more interested on the geological aspect of the area. After climbing on the top of the hill I saw different geological erosion and weathering. This hill is dome shaped popularly known as dome –gneiss. This low gentle dome like feature is a residual phenomenon and in this ancient part of the crust it represents the end stage of the degradation of a higher hill. Such a dome –gneiss, however, is due to the highly massive non-fractured and non-jointed nature of the mass.

There is one small water pond and different potholes, and small circular depressions on the top. Probable source of water to this pond and other potholes is from inside the hill. In the rocky beds of streams we have kettle-like depressions, called potholes. They may be seen both in long and cross profiles. The stream beds in Jharkhand Plateaus are frequently dotted with potholes. A pothole is formed by the constant swirl of an eddy, which carries pebbles or sand round and round in one spot. Gradually a hole is bored downward into the rock. The sand and pebbles that served as the tools may often be found at the bottom of the depression. Whether the vortex be one in a current of wind or water or ice, the action is the same, although the rate of abrasion may vary. Potholes are most commonly of fluviatile origin. So presence of potholes on the top of the hill clearly indicates that water used to flow in geological past.

Writer found a shallow valley eroding the hill from the top of the hill to the base. From the satellite image evidence of abandoned flow path of water in form of different rivulets in the valley are seen. It may also be the evidence of the flow path of the lost river. In earlier days water filled in the pond on the top of the hill may have been the regular source of water to this river. Pond still exist on the top but river has vanished or it may be flowing only in rainy season. Depletion of groundwater level due to extreme long heat season in Ranchi plateau is affecting such rivers which are dependent only on groundwater flow. Many such small rivers originating from the top of the hills in Jharkhand state has now dried up or flow only during rainy season.

Pond water of the studied hill is highly polluted which is evident with its green colour. pH value is 12.

Many stone minings are going around this hill posing threat to this hill. State government should take care to preserve such hills which has kept the evidences of earlier geological history of more than 2 billion years.

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