The
natural processes of weathering along with scarp retreat and biotic interference
have had a serious deleterious impact on the sculptures and paintings
in the caves and its environs. Unstable slopes and seepage are the two
major causes of concern while applying conservation methods. On request
of the Archaeological Survey of India, the Geological Survey of India
undertook a programme of multi disciplinary geoscientific studies comprising
topographic survey and geological mapping of 0.46 sq. km area, geophysical
surveys, geotechnical studies within and outside the caves; terrestrial
stereophotographic studies of vulnerable slopes, seismic risk assessment
by micro earthquake studies and environmental impact assessment of the
caves and their surroundings. |
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The caves are excavated in a pahoehoe
type of basaltic flow exposed in a 76m high semicircular scarp carved
out by the Waghur River which descends beyond the last cave in a water
fall
with seven leaps known as the 'Sat Kund'.
Three flows are exposed in the scarp where the caves are located. The
lower flow is of AA type and is exposed in the river bed. A prominent
fragmentary zone marks the top of the flow. The middle flow contains
the caves and is of pahoehoe type. It consists of 16- 19 sub horizontal
flow units. The basal parts of individual units are characterised by
pipe amygdules, whereas the top parts exhibit highly vesicular zones
with ropy surfaces. The upper flow is of AA type, the massive part of
which forms a vertical scarp. The basalt in which these caves are excavated
is medium to fine grained, moderately porphyritic, vesicular and is slightly
weathered. The vesicles varying in size from 1 mm to 2 cm are mostly
rounded and filled with silica, zeolites and green earth material.
The
agents of weathering like wind, water, temperature variations, climate,
bioorganisms etc, have affected macro as well as micro structures in
the rocks, to bring about a gradual deterioration in the rock mass quality.
Grade II weathering ( slightly weathered rock) is seen in most
of the exposures outside the caves while the rocks inside the caves are
generally fresh and unweathered. Patches of moderately weathered basalt
are seen in the toe portion of some caves and inter cave zones. The thickness
of the weathered rim is 1- 2 m. Accentuated weathering is seen along
unit contacts which has led to the destruction of many pillars.
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The
process of valley formation has been active and the scarp surface has
been retreating leading to instability
of slopes and widening of the valley: The original entrances to the caves
were from the riverbed through a series of rock cut staircases. None
of these steps now exist (except for a few steps below cave 17). The
outer
portions of a few caves have also been destroyed. Thus it is estimated
that scarp retreat is of the order of 5-7 m in the last millennium and
half, since the caves were excavated. Interior of the caves are relatively
stable while the facades and verandahs show varying degree of instability. Most
of the cracks observed in the caves are parallel to the scarp slopes
except in caves 17,21 & 23 where
they are transverse. These cracks are generally restricted to the outer
2 -3m of the caves. The
entire scarp between caves 1- 28 forms an unstable slope. The rock
mass in the slopes was characterised and zoned using terrestrial stereophotos.
The rock mass was classified on the basis of three parameters like relative
strength, joint volume / degree of jointing and relative weathering.
Approximately
40% of the slope area in and around caves needs to be treated.
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