Friday, September 5, 2014

Geology of Pithoria hills near Ranchi city, India.


The hills of Ranchi are generally small isolated residual hills. 
By
Dr. Nitish Priyadarshi.

 Fig.1. A small hill in Pithoria.



The hills of Ranchi are generally small isolated residual hills. In Ranchi hills are generally steepsided made of massive granite-gneiss. We have also found the hills which are not steepsided but a mere irregular pile of huge boulders were found on the hills especially in Pithoria area of Ranchi district. This is the result of highly jointed elements of granite gneiss.

According to climatic geomorphologist like Tricart qualify such hills as inselbergs. They are steepsided residual hills made of massive granite-gneiss. But close by within a few kilometers or a few hundred meters we have residuals which are not steepsided inselbergs but a mere irregular pile of huge boulders.




The rocks disintegrate along these joints under the hydrothermal and atmospheric effects thus leading to the formation of tors or rounded shapes of these ancient granite rocks are the result of cracking and erosion from exposure to sun, wind and rain. The hill slopes are subjected to complex attack by a variety of erosive weapons, water being the most active agent for the removal of waste material from most of the slopes. 


A tor commonly appears as a pile of rock slabs or a series of slabs standing on end, according to whether the dominant joint system is horizontal or vertical. Weathering proceeds most actively along joint planes, thus reducing an originally solid mass first to piles of slabs and ultimately to a heap of loose boulders. Tors usually overlie unaltered bedrock and are thought to be formed either by freeze–thaw weathering or by groundwater weathering before exposure. There is often evidence of spheroidal weathering of the squared joint blocks. Tors are seldom more than 15 metres (50 feet) high and often occur as residues at the summits of inselbergs and at the highest points of pediments.

Fig.2.  This is a picture of small granite tor near Ranchi city.

Fig.3. Tor like structure on top of Pithoria hill.


  Fig.4. Irregular pile of huge boulders on the top of Pithoria hill.



Fig.5.Irregular pile of huge boulders. They are the result of highly jointed elements of granite gneiss. 



Fig.6. A balancing rock was also seen on the top of the hill. 
  
A balancing rock, also called balanced rock or precarious boulder, is a naturally occurring geological formation featuring a large rock or boulder, sometimes of substantial size, resting on other rocks, bedrock or on glacial till. Some formations known by this name only appear to be balancing but are in fact firmly connected to a base rock by a pedestal or stem.

Its an erosional remnant rock formation that remains after extensive wind, water and/or chemical erosion. To the untrained eye it may appear to be visually like a glacial erratic, but instead of being transported and deposited it was carved from the local bedrock.  

 

 
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