Geology
The Regional Geology in the Eastern Ghats mobile belt, where the caves are located, is represented by the Khondalite suite of rocks (garnetiferrous sillimanite gneisses, quartzo-feldsphatic garnet gneisses) of Archaen age. Quaternary deposits consist of red bed sediments, laterites, pediment fans, colluvium, alluvium and coastal sands. Particularly, the Borra caves are stated to be one of the largest caves in the Indian subcontinent. The caves, in the reserved forest area consisting of 14 villages inhabited by tribals, basically host a variety of speleothems ranging from very small to big and irregularly shaped stalactites and stalagmites. The carbonate rocks are pure white, and coarsely crystalline and the deformed and banded marbles cover a triangular area of 2 km2 (0.8 sq mi); surrounded by Diopside–scapolite–feldspar calc-granulites. The pyroxenite outcrops are dark and massive and include discontinuous calc-silicate bands, some of brown mica and others with calcite. The Gosthani River, which originates from these caves and flows between the solidified stalactites and stalagmites in the Karstic lime stones formation, is the cause for the development of the odd shapes of structures. Water percolating from the roof of the caves dissolve limestone and trickle drop by drop to form stalactite at the roof of the cave and then dripping down to the ground form stalagmite. (see picture). Stalactites are calcium carbonate deposits that hang from the top of the cave. Stalagmites are deposits that form at the bottom of the cave and grow upward. These deposits have developed into interesting forms and structures inside the caves such as Shiva –Parvati, Mother–Child, Rishi’s beard, Human brain, mushrooms, crocodile, temple, church, etc. These shapes have captured the imagination of tourists, while some have been given religious interpretations. The Caves are deep and totally aphotic. There is a twilight zone in the caves with limited light penetration. The Stalactites seen in the caves are about 0.1 m (0.3 ft) to 3.5 m (11.5 ft) in length while the Stalagmites are 1.2 m (3.9 ft) long and columns are 6 m (19.7 ft) in height and 0.75 m (2.5 ft) in width. The height of the cave is 12 m (39.4 ft) and the length is about 200 m (656.2 ft). The average temperature of the inner cave wall is reported to be about 16 °C (60.8 °F). Sulphur springs discharge into the cave passages causing corrosion of limestone. The spring waters display floating Mucus-like biofilms. These are thick orange microbial mats (2.5 cm (1.0 in)–3 cm (1.2 in) thick) with patches of yellow biofilms extending 3 m (9.8 ft) from the aphotic deep cave orifice. While the caves are basically limestone formations, the area surrounding these are of mica formations which are prospected for precious stones like rubies. The geological features of these caves are stated to be found only in Borra in India. Archeological artifacts (Paleolithic implements) have been found in the caves. The excavations carried out in the caves by the Archeologists of the Andhra University, have unearthed stone tools of middle Paleolithic culture dating back to 30,000 to 50,000 years, which confirm human habitation.
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