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