Echoes Beneath the Western Ghats — A Geoscientific Reflection on the Kaveri Impact Basin and the Charnockite of St. Thomas Mount
© Dhinakar Rajaram, 2025 — All rights reserved.
Preface
This essay is born of both starlight and stone. For years, I travelled across southern India — from Coimbatore to Chennai — tracing landscapes that quietly preserve the Earth’s most ancient memories. Beneath the lush folds of the Western Ghats lies a terrain shaped not merely by time, but by cosmic collision.
The proposed Kaveri Impact Basin, one of the world’s least-known geological enigmas, and the St. Thomas Mount Charnockite, a relic of our planet’s deep crust, together reveal how celestial and terrestrial histories entwine. This work is not a technical paper but a reflection written in the spirit of science — to help students, readers, and wanderers see the land beneath their feet as part of the same universe they gaze upon above.
— Dhinakar Rajaram
Abstract
Southern India’s geological framework preserves some of the most ancient and enigmatic features of Earth’s crust. Recent studies suggest that the region surrounding the Palghat Gap and the Kaveri River basin may represent a large, deeply eroded impact structure — the Kaveri Impact Crater — measuring approximately 120 km across (Subrahmanya & Narasimha, 2017). This paper-style essay presents a synthesis of published evidence and firsthand field observations from Coimbatore, Salem, and Erode, alongside a reflection on the St. Thomas Mount Charnockite near Chennai, another relic of India’s Archaean past. Together, these localities reveal the intertwined story of celestial violence and continental endurance — an astro-geological continuum that connects India’s landforms to planetary evolution.
1. Introduction
Planetary geology reveals that impacts by extraterrestrial bodies have profoundly shaped terrestrial crustal evolution. Earth, however, retains few well-preserved craters due to its active plate tectonics and pervasive erosion. Within this context, the Kaveri Impact Hypothesis provides a rare opportunity to study a potential large ancient impact site within the stable Southern Granulite Terrain (SGT) of India.
As an amateur astronomer with a keen interest in astro-geology, I have traversed this terrain, particularly along the Coimbatore–Salem corridor, documenting topographic, structural, and lithologic features suggestive of a deeply eroded impact basin. These field experiences complement scientific studies and underscore the significance of protecting such sites as geo-heritage resources.
2. Geological Background
2.1 The Southern Granulite Terrain
The SGT represents one of Earth’s oldest crustal provinces, composed of high-grade metamorphic rocks (granulites, charnockites, and gneisses) that record pressures >7 kbar and temperatures >700 °C (GSI, 2021).
2.2 The Palghat–Cauvery Shear Zone
This major lineament separates the northern Dharwar Craton from the southern Madurai Block. It acts as both a tectonic boundary and, possibly, the northern rim or structural control for the Kaveri impact basin.
2.3 Previous Studies
Subrahmanya and Narasimha (2017) identified an elliptical depression bounded by arcuate highlands — the Nilgiri, Anaimalai, and Palani Hills — and reported mineralogical evidence of shock metamorphism such as planar deformation features (PDFs) in quartz, diaplectic glass, and pseudotachylite veins.
Figure 1. Visualisation of the proposed Kaveri Impact Basin showing the elliptical structure east of the Palghat Gap.
Source: The Hindu (Science & Technology, 2019).
Figure 2. Topographic rendering of the Kaveri Basin showing the surrounding highlands — Nilgiri, Anaimalai, and Palani Hills.
Source: Wikimedia Commons, 2018.
3. Field Observations
3.1 Site and Traverse
Multiple traverses were made between Coimbatore, Salem, and Erode (2013–2019). Rock exposures along the national highway reveal steeply tilted and occasionally overturned beds, with local dips between 45° and 50°.
3.2 Morphological Indicators
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Mountain arcs to the north, west, and south delineate possible rim segments.
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Palani Hills, a spur of the Western Ghats, form a prominent southern rim.
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The terrain slopes eastward toward the Kaveri River, which appears to exploit a structural low formed by the impact basin.
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Extensive blasting during highway expansion has destroyed many key outcrops — emphasising the need for documentation.
3.3 Visual Evidence
Figure 3 & 4. Northern rim of the proposed Kaveri Impact Structure — north of Coimbatore. This massif shows steeply projected strata and fault-bounded blocks consistent with rim uplift morphology.
Photograph © Dhinakar Rajaram (2015).
Figures 5,6 &7 . South West & Western rim highlands near Anaimalai Range, forming part of the crater’s western arc. © Dhinakar Rajaram (2015).
4. Discussion
4.1 Interpreting the Structure
The arcuate disposition of the Nilgiri–Anaimalai–Palani massifs, the gravity anomalies, and tilted strata together support an impact-related origin rather than a purely tectonic basin. Numerical models of multi-ring craters of comparable scale predict rim collapse and differential erosion consistent with present morphology.
4.2 Post-Impact Modifications
Following impact, the basin likely experienced:
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Rapid erosion and sediment infill.
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Reactivation of pre-existing shear zones (Moyar–Bhavani–Attur).
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Differential uplift during later tectonic phases that rejuvenated rim sectors.
4.3 The Kaveri as a Geomorphic Historian
The Kaveri River flows eastward through the basin’s axis, carving its course along the ancient crater floor — a textbook example of fluvial adaptation to impact-generated weakness zones.
Figure 8. Geological cross-section and gravity model of the proposed Kaveri Impact Structure.
Source: Springer Nature (Journal of the Geological Society of India, 2017).
5. Comparative Planetology — Impact Legacy on Earth
Earth shares its impact history with the Moon and Mars, yet only a fraction of craters survive due to resurfacing. For context:
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Vredefort (South Africa, ~2.0 Ga, 300 km)
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Sudbury (Canada, ~1.85 Ga, 250 km)
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Chicxulub (Mexico, 66 Ma, 180 km)
If validated, the Kaveri Structure (~120 km) would join this elite class, ranking among the top five largest on Earth.
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6. The Charnockite Beneath St. Thomas Mount:
6.1 Lithology and Origin
The St. Thomas Mount Charnockite in Chennai represents Archaean granulite-facies metamorphism (2.6–2.8 Ga). It consists of orthopyroxene, feldspar, quartz, and iron oxides exhibiting NE–SW foliation due to deep-crustal shearing during the Eastern Ghats Orogeny.
St. Thomas Mount Photo credit : https://commons.wikimedia.org/wiki/File:St_Thomas_Mount.jpg
Photographs of St. Thomas Mount presumably taken by American military photographer Clyde Waddell in 1942/45
6.2 Cultural and Scientific Significance
Historically, the rock gave rise to the term “charnockite” after Job Charnock, whose tombstone slab was cut from this very exposure. The site is today recognised by the Geological Survey of India as a geo-heritage monument (GSI, 2021).
6.3 Metamorphic Conditions
Formation under CO₂-rich, dry conditions (> 750 °C) points to lower-crustal metamorphism, where heat and pressure shaped the rock deep within the Earth. The “strained-flow” textures preserved in the charnockite record plastic deformation, subtle echoes of ancient deep-earth dynamics.
At first glance, the St. Thomas Mount charnockite might seem to “flow” from the southwest toward the northeast. However, modern research paints a more complex picture. Studies of the Madras Block charnockites describe them as plutonic granulite-facies bodies, ranging from monzonite to granite, with orthopyroxene-bearing varieties. Their composition and structure reveal a deep-crustal magmatic history of crystallisation and differentiation rather than a simple directional flow. Foliation trends roughly NE–SW, overprinted by high-grade metamorphism and shearing, and though the SW and NE flanks contain noritic masses surrounding a central charnockite core, no clear evidence confirms a unidirectional magma flow.
No publicly available PhD thesis or recent study focuses exclusively on the structural fabric of St. Thomas Mount, making this blog among the few sources to blend observational insight with scholarly context. In this way, even a single modest hill allows us to read billions of years of Earth’s hidden history, where deep crustal processes quietly meet the surface, waiting for attentive eyes to witness their story.
As the deep-time story of formation, flow, and deformation concludes beneath the surface, the modern exposures of St. Thomas Mount reveal these processes in tangible form — the dark grey-green charnockite, the bronzed patina of weathering, and the subtle fractures that whisper of Earth’s ancient inner workings.
6.4 Modern Exposure and Weathering
The outcrop at St. Thomas Mount lies partly within the church precincts overlooking the Chennai airport. When freshly fractured, the charnockite appears dark grey-green; yet on exposure to air and moisture, the iron-bearing minerals oxidise rapidly, giving the rock a bronzed, rust-red patina. In some seasons the surfaces appear to bleed rust — thin films of iron oxide seeping along micro-fractures, a vivid reminder that these deep-crustal rocks are still interacting with the atmosphere billions of years after their formation.
Urban encroachment and quarrying threaten its preservation, but several boulders around the hill retain their characteristic hue. This small hill, combining geology, history, and faith, stands as a living geological classroom — a place where the Earth’s deep interior literally meets the open air.
7. Geoscientific and Educational Significance
7.1 Geoheritage
Both the Kaveri Basin and St. Thomas Mount warrant formal geo-heritage recognition. They encapsulate two complementary extremes of Earth’s history — impact modification and metamorphic reconstruction.
7.2 Educational Potential
Students studying Earth science can learn to:
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Identify impact signatures (breccias, PDFs, circular drainage).
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Recognise deep-crustal processes through charnockite exposures.
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Connect planetary and terrestrial geology under one scientific framework.
8. The Cosmic Continuum
The Kaveri basin narrates cataclysm; the St. Thomas Mount charnockite narrates endurance. Together, they embody a planetary truth — that the Earth we inhabit was sculpted as much by celestial impacts as by internal metamorphism.
When we traverse from Coimbatore to Chennai, we move not merely across Tamil Nadu but across two billion years of planetary evolution — from the fiery violence of a meteor strike to the silent resilience of deep-crustal rock.
9. Evidence and Institutional Recognition
🔍 Key Evidence Highlights
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The St. Thomas Mount charnockite was formally recognised by the Geological Survey of India (GSI) as a National Geological Monument / geo-heritage site (GSA Conference 2018; Wikipedia 2021).
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The term “charnockite” was coined by Sir Thomas Holland of the GSI in 1893, referring to the hypersthene granite from this very hill (GSI Records 1893; Wikipedia 2021).
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For the proposed Kaveri Impact Basin, while not yet officially listed by the GSI, the peer-reviewed work of Subrahmanya & Narasimha (2017) draws upon GSI terrain maps, gravity and magnetic anomaly data, and field-petrographic evidence (Geoscience World 2017; Semantic Scholar 2017).
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The GSI’s thematic mapping and crustal studies of the Coimbatore–Salem region (Ministry of Mines 2023) demonstrate structural alignments that support the region’s geologic significance.
(Sources: Geological Survey of India; Geological Society of America Conference Archive 2018; Ministry of Mines 2023; Subrahmanya & Narasimha 2017.)
9.1 GSI Recognition of the St. Thomas Mount Charnockite
The Geological Survey of India (GSI) formally recognises the St. Thomas Mount Charnockite as a National Geological Monument, acknowledging its dual historical and scientific importance. The rock’s naming history traces to Sir Thomas Holland, a GSI geologist who in 1893 coined the term “charnockite” after identifying the distinctive hypersthene granite from this very site (GSI, 1893; GSA Conference Archive, 2018).
This outcrop thus occupies a unique position in both Indian and global geology — serving as the type locality for an entire suite of rocks within the Southern Granulite Terrain. The GSI’s heritage listing ensures that, despite modern encroachment, the hill remains a protected educational resource.
9.2 Mapping and Survey Work in the Kaveri Basin Region
While the proposed Kaveri Impact Basin has not yet been officially listed by the GSI as a confirmed impact structure, much of the supporting data stems from GSI’s regional mapping programmes across Tamil Nadu’s Precambrian shield.
GSI crustal studies and structural maps of the Coimbatore–Salem–Erode corridor document multiple shear zones — notably the Moyar–Bhavani–Attur lineament and the Palghat–Cauvery shear zone — that correspond closely with the boundaries of the hypothesised crater (Ministry of Mines, 2023).
Furthermore, the peer-reviewed work by Subrahmanya & Narasimha (2017) drew on GSI’s gravity and magnetic anomaly datasets, integrating them with field petrography to propose that the region’s arcuate topography and brecciated lithologies may reflect an ancient impact origin.
9.3 Implications for Geo-heritage and Scientific Recognition
These two sites — one officially enshrined in India’s geological heritage, the other awaiting confirmation — illustrate the continuum of discovery, verification, and preservation in Indian earth science.
The St. Thomas Mount charnockite stands as a textbook case of successful institutional recognition and protection. The Kaveri Basin, by contrast, invites the same level of attention and safeguarding. Its inclusion in future GSI heritage inventories would not only validate a growing body of research but also preserve field sites vital for academic study.
Such recognition bridges the space between professional geology and public awareness — ensuring that India’s landscapes of deep time, from Chennai’s rust-red rocks to Coimbatore’s uplifted arcs, are seen as integral parts of our scientific and cultural inheritance.
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10. Conclusion
The convergence of astronomy and geology — astro-geology — provides a profound lens through which to view our planet. The proposed Kaveri Impact Structure and the St. Thomas Mount Charnockite are not isolated curiosities; they are chapters of the same cosmic epic.
Every tilted ridge north of Coimbatore and every bronzed stone beneath St. Thomas Mount tells a part of this story. Recognising and preserving them enriches not just science, but also our cultural understanding of Earth as a dynamic celestial body.
GLOSSARY OF KEY TERMS
Astro-geology (Planetary Geology) — The interdisciplinary science studying geological processes and landforms on celestial bodies such as planets, moons, and asteroids. It bridges astronomy and Earth geology, revealing shared planetary histories.
Astronomy — The study of celestial objects, cosmic phenomena, and the wider universe. In this essay, astronomy provides the cosmic context for understanding how Earth’s geology records extraterrestrial influences.
Charnockite — A coarse-grained, orthopyroxene-bearing metamorphic rock typical of the Southern Granulite Terrain (SGT). Named after Job Charnock, whose tombstone was carved from the St. Thomas Mount outcrop in Chennai. When exposed to air, its iron minerals oxidise, producing a bronzed, rust-red sheen — described as “bleeding rust.”
Shock Metamorphism — Alteration of rock minerals under extreme pressures and temperatures during a meteorite impact, producing diagnostic microstructures such as planar deformation features (PDFs).
Impact Crater — A circular depression formed when a meteorite or asteroid collides with a planetary surface, characterised by raised rims, central uplifts, and brecciated rocks.
Breccia — A rock made of angular fragments cemented together; in impact settings, formed from shattered crust re-welded by melt or debris.
Pseudotachylite — A dark, glassy rock created by frictional melting during impact or fault movement, often seen as veins within crater floors.
Planar Deformation Features (PDFs) — Microscopic lamellae in quartz or feldspar formed only under shock pressures exceeding several GPa — conclusive evidence of impact origin.
Central Uplift — The rebound dome at the centre of a large impact crater, formed when the compressed crust springs back upward after impact.
Foliation — The planar alignment of minerals within metamorphic rocks due to directional pressure. In the St. Thomas Mount charnockite, foliation trends NE–SW, recording ancient crustal shearing.
Neoproterozoic Era — Geological era from about 1,000 to 541 million years ago, the probable time of the hypothesised Kaveri impact.
Archaean Era — The earliest stable era of Earth’s crust (4.0–2.5 billion years ago) when the first continental nuclei, including the charnockites of southern India, formed.
Palghat Gap — A deep, east–west corridor in the Western Ghats between Tamil Nadu and Kerala, marking a crustal discontinuity aligned with the northern rim of the proposed Kaveri Impact Basin.
Southern Granulite Terrain (SGT) — A high-grade metamorphic province in southern India composed of charnockites, gneisses, and granulites — some of Earth’s oldest exposed crust.
Geo-heritage Site — A natural location officially recognised for outstanding geological or educational significance. St. Thomas Mount is one such site under the Geological Survey of India.
Moyar–Bhavani–Attur Lineament — A major shear zone in southern India representing deep crustal faulting, coinciding with the structural boundary of the proposed Kaveri Impact Basin.
Impact Breccia — A chaotic rock of fragmented and melted material produced during a meteorite impact, found near crater rims or central uplifts.
Granulite-facies Metamorphism — High-temperature (>700 °C), low-water metamorphism deep in the crust that produces orthopyroxene-bearing rocks like charnockite.
Geo-heritage Conservation — The preservation of significant geological sites for education, research, and public awareness — protecting ancient rocks and landforms as records of deep time.
Cosmic Chronology — The timeline connecting celestial events such as meteor impacts and stellar evolution with Earth’s geological and biological history.
Erosion and Tectonic Rejuvenation — Processes that gradually erode ancient craters and uplift older crustal blocks, reshaping the surface over millions of years.
Crater Morphology — The structural form of an impact crater, including rims, terraces, and central uplifts, which reveals the impact’s energy and age.
Planetary Memory — A poetic yet scientific concept denoting how Earth’s landscapes preserve the imprints of cosmic and geological events through deep time — the central theme of When Earth Remembered the Stars.
References
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Subrahmanya, K. R., & Prakash Narasimha, K. N. (2017). Kaveri Crater – An Impact Structure in the Precambrian Terrain of Southern India. Journal of the Geological Society of India, 90(4), 387–398.
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Geological Survey of India (GSI). (1893).
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Geological Survey of India (GSI). (2021).
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The Hindu. (2019).
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Wikimedia Commons. (2018). Topographic visualisation of the Kaveri Crater.
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Springer Nature. (2017). Journal cover image used for educational reference.
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Geological Society of America Conference Archive. (2018).
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Ministry of Mines & Geological Survey of India. (2023). Annual Report on Geoscientific Mapping in Tamil Nadu.
Figure Credits
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Visualisation of the Kaveri Crater — The Hindu (2019).
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Topographic visualisation of the Kaveri Crater — Wikimedia Commons (2018).
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SpringerNature journal cover (2017) — used for educational reference.
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Northern Rim of the Kaveri Crater, north of Coimbatore — © Dhinakar Rajaram (2015).
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Poster — Echoes Beneath the Western Ghats © Dhinakar Rajaram (2025).
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St. Thomas Mount, presumably photographed by American military photographer Clyde Waddell, 1942–45.
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Image sourced from Wikimedia Commons: St Thomas Mount
Acknowledgment
The author extends gratitude to the geoscientific community whose prior research — notably the work of Subrahmanya and Narasimha (2017) — laid the foundation for renewed inquiry into the Kaveri Impact Hypothesis. Sincere thanks are also due to the Geological Survey of India for recognising and preserving St. Thomas Mount as a geo-heritage site, and to the science journalists and photographers of The Hindu, Wikimedia Commons, and Springer Nature whose visual materials aided this educational synthesis.
Special appreciation is offered to readers, students, and fellow enthusiasts of astronomy and geology who continue to explore the silent narratives of our planet. Their curiosity ensures that landscapes like the Kaveri Basin and the charnockite hills of Chennai will not fade into obscurity, but remain celebrated as living classrooms of deep time.
Author’s Note
The journey that led to this work began not in laboratories or libraries, but along highways, ridges, and riverbanks — watching the land and listening to its silences. The Kaveri Impact Basin has long remained hidden in plain sight; the Charnockite of St. Thomas Mount, though world-renowned in geology, is scarcely known to the very city that shelters it.
These observations, drawn from my travels across the Coimbatore–Salem–Erode–Chennai corridor between 2013 and 2019, aim to bridge that gap — to tell the story of how Earth’s deep-time narratives are interwoven with cosmic history.
All field photographs reproduced here were taken by me unless otherwise credited. Scientific diagrams and reference images are used under fair academic citation from The Hindu, Wikipedia Commons, and Springer Nature. The poster titled “Echoes Beneath the Western Ghats” was created to visually summarise this study and to encourage geoscientific curiosity among students and enthusiasts.
If this essay helps even one reader to see a mountain or a river with new wonder — as a remnant of the stars — it would have fulfilled its purpose.
— Dhinakar Rajaram, 2025
Epigraph:
“Every rock is a fossil of fire — a quiet memory of the stars that built our world.”
— Dhinakar Rajaram
End Note:
This essay forms part of an ongoing series of reflective science writings exploring the intersection of astronomy, geology, and human understanding. Through these narratives, the author seeks to illuminate how cosmic and terrestrial histories intertwine — revealing that the stones beneath our feet are, in truth, fragments of the universe itself.
#AstroGeology #IndianGeoheritage #StThomasMountCharnockite #CoimbatoreGeology
#WesternGhatsScience #PlanetaryGeology #ImpactCraterResearch #EarthHistory
#GeoEducation #CosmicContinuum #DhinakarRajaramWrites
