Peninsular Gneiss EarthCache
Hidden : 2/11/2011
Difficulty:
2 out of 5
Terrain:
2 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:



Layered gneiss at Lalbagh, Source: Wikimedia



The Indian Subcontinent is a peninsular region in South Central Asia. The older gneissic complex of the abundant rock type is called peninsular gneiss, a term defined by W.F.Smeeth of the Mysore Geological Department in 1916 after the first studies of this old rock, one of the oldest on Earth, took place. Here you find one of the best exposures of it, and a rare special feature in form of an abrupt rise.

Bugle Rock together with the nearby Lalbagh rock where the Kempe Gowda Tower is placed on is one of the 26 national geological landmarks of India (each one worth an Earthcache I believe ;-) ) defined by the Geological Survey of India and spreads over an area of ca. 16 acres. Bugle rock in particular is a little bit further down the road in Bugle Rock Park which is built around the rock formations, in Basavanagudi, Bangalore, Karnataka. The name is derived from it being used as a lookout. From the watchtower on top a guard blew a bugle (Kahale) to alert the citizens in case of danger or to indicate sunset.

Alike the Lalbagh monument, here you find 2nd generation peninsular gneiss which is around 3.3 - 3.2 bln years old and one of the oldest rocks on earth which formed the Gondwanaland supercontinent. The spot you visit lies in the archaean stratigraphic division of the Indian Shield. Bugle Rock represents an interesting, rare abrupt rise on the peninsular gneiss foundation which caused the attention of scientist researchers.

It is the basement on which the supracrustal rocks of the Dharwar Group were deposited. The composite gneiss is formed by migmatization of pre-existing metasedimentary and meta-igneous rocks. These gneisses show the same style and sequence of superposed deformation as those in the enclaves of metamorphic rocks and in the linear Dharwar schist belts outside. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding that affected the Dharwar supracrustal rocks. An even earlier phase of migmatization and deformation is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. The main migmatization is broadly coeval with the isoclinal first folding, which is followed by near-coaxial refolding and non-coaxial upright folding. Small inclusions of migmatized amphibolite and granodioritic to dioritic gneiss, with a fabric athwart to, and overprinted by, the earliest deformation affecting the Dharwar Group of rocks in a large part of the gneissic terrane, point to at least one deformation, a metamorphic event and one episode of migmatization antedating the isoclinal first folds in the rocks of the Dharwar Group. Pressure and temperature levels reached 5-6kbar and 700-750° Celsius during this late archaic to early proterozoic metamorphosis. The Peninsular Gneiss in its present state, therefore, represents an extensively remobilized basement.



Geological schema of the Southern Indian Peninsular (JAYANANDA & PEUCAT (1996) and CHETTY (1996))

CT - Chitradurga Boundary Thrust
CE - Cuddapah Eastern Margin Shear
N - Nallamalai-Shear
M - Moyar-Shear
B - Bhavani-Shear
P - Palghat-Shear
C - Cauvery-Shear
A - Achankovil-Shear


How to log this Earthcache:
1) Send me a short description of the stone in which you find carved drawings, in particular estimate its size and explain where the carvings come from. It is on top of Bugle Rock, just near and slightly below the watch tower.
2) Estimate or measure the altitude of the rise while standing in the watch tower on top of the rock above ground.
3) Explain the nature of the deformation / tonalities shown on your picture (or if you do not want to post a picture, pick a tonality and explain it).
4) Due to current Earthcache guidelines, a picture is not mandatory, but it is very welcome still. You might take it either at Bugle Rock or the Lalbagh site nearby (ca. 1km away from here) where you might want to try the Garden City Parks cache #1: Red Garden (Lalbagh).

If you've got enough time you might also want to visit the nearby Bull Temple, too, which is in the neighbourhood of the Bugle Rock park.

Enjoy and take care of our planet!

This cache is a contribution to the Earth Day 2011 campaign for a better environment. It is listed there as one of the Billion Acts Of Green.

Further Sources: Craton Seminar Papers
Structural Evolution of Peninsular Gneiss
Chemical Evolution of Peninsular Gneiss

Additional Hints (No hints available.)



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