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
Born of starlight and stone
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
When Earth remembered the stars
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 reflection synthesises published evidence and firsthand field observations from Coimbatore, Salem, and Erode, alongside an interpretive discussion of 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 connecting India’s landforms to planetary evolution.
1. Introduction
Where planetary scars meet continental memory
Planetary geology reveals that impacts by extraterrestrial bodies have profoundly shaped the evolution of terrestrial crusts. Earth, however, retains few well-preserved craters, their traces largely erased by plate tectonics and erosion. Within this context, the Kaveri Impact Hypothesis presents a rare opportunity to study a potential large, ancient impact structure within the stable Southern Granulite Terrain (SGT) of India.
As an amateur astronomer and student of 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 published research and underscore the importance of preserving such landscapes as geo-heritage resources, where science and wonder coexist.
2. Geological Background
2.1 The Southern Granulite Terrain
An archive of Earth’s oldest metamorphic symphony
The Southern Granulite Terrain (SGT) represents one of Earth’s oldest crustal provinces, composed of high-grade metamorphic rocks — granulites, charnockites, and gneisses — that record pressures exceeding 7 kbar and temperatures above 700 °C (GSI, 2021). These rocks, forged deep within the crust, are the crystalline witnesses of the planet’s formative epochs.
2.2 The Palghat–Cauvery Shear Zone
The invisible frontier beneath the mountains
This major east–west lineament separates the northern Dharwar Craton from the southern Madurai Block. It acts both as a tectonic boundary and, possibly, as the northern structural rim of the hypothesised Kaveri Impact Basin. The zone’s recurrent reactivation through geologic time has influenced drainage, metamorphism, and crustal architecture across southern India.
2.3 Previous Studies
Tracing the first clues of a buried scar
Subrahmanya & Narasimha (2017) identified an elliptical depression bounded by arcuate highlands — the Nilgiri, Anaimalai, and Palani Hills — and reported mineralogical evidence of shock metamorphism, including planar deformation features (PDFs) in quartz, diaplectic glass, and pseudotachylite veins. These features, if verified in situ, provide strong indicators of impact-related deformation.
Figure 1. Visualisation of the proposed Kaveri Impact Basin showing the elliptical structure east of the Palghat Gap.
Source: The Hindu, Science & Technology (2019).
Source: Wikimedia Commons (2018).
3. Field Observations
3.1 Site and Traverse
Walking the rim of a forgotten crater
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°, consistent with rim-uplift morphologies observed in ancient multi-ring impact basins.
3.2 Morphological Indicators
Mountains that remember an ancient fall
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Mountain arcs to the north, west, and south delineate possible rim segments.
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The 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 — underscoring the urgent need for documentation and protection.
3.3 Visual Evidence
Photograph © Dhinakar Rajaram (2015).
Figures 5, 6 & 7. South-western and western rim highlands near the Anaimalai Range, forming part of the crater’s western arc.
Photographs © Dhinakar Rajaram (2015).
4. Discussion
4.1 Interpreting the Structure
An argument written in arcs and anomalies
The arcuate disposition of the Nilgiri–Anaimalai–Palani massifs, coupled with gravity anomalies and tilted strata, supports an impact-related origin rather than a purely tectonic basin. Numerical models of multi-ring craters of comparable scale predict rim collapse, central uplift, and differential erosion consistent with the present-day morphology of the Kaveri Basin.
4.2 Post-Impact Modifications
When time remodels a catastrophe
Following impact, the basin likely underwent:
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Rapid erosion and sediment infill.
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Reactivation of pre-existing shear zones (notably the Moyar–Bhavani–Attur system).
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Differential uplift during later tectonic phases, rejuvenating rim sectors and exposing deep crustal levels.
4.3 The Kaveri as a Geomorphic Historian
The river that remembers
The Kaveri River flows eastward through the basin’s axis, carving its course along the ancient crater floor — a textbook case of fluvial adaptation to impact-generated weakness zones. The river thus becomes both a hydrological witness and geological historian, tracing through time the contours of an event that once reshaped this corner of the Earth.
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
Where celestial scars mirror across worlds
Earth shares its impact history with the Moon and Mars, yet only a fraction of its ancient craters endure — the rest erased by plate tectonics, erosion, and the restless breathing of our planet’s crust.
For perspective, the great survivors of planetary trauma stand as geological monuments to deep time:
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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 rarefied league — one of the five largest known impact basins on Earth, and among the few that bridge the disciplines of planetary science and regional geology.
6. The Charnockite Beneath St. Thomas Mount
" Where the Earth’s interior finds its voice at the surface ... "
6.1 Lithology and OriginWhere the ancient crust rose and froze in silence
The St. Thomas Mount charnockite in Chennai (Madras) represents Archaean granulite-facies metamorphism (2.6–2.8 Ga). It consists of orthopyroxene, feldspar, quartz, and iron oxides exhibiting NE–SW foliation produced by 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
Where faith, nomenclature, and geology converge
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) — a place where the disciplines of geology, history, and faith intersect upon a single hill of ancient stone.
6.3 Metamorphic Conditions
From the depths of Earth’s crust to the language of its folds
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 saga 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
Where deep-time surfaces and stone breathes again
The outcrop 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 certain seasons the surfaces appear to bleed rust — thin films of iron oxide seeping along micro-fractures, a vivid reminder that these deep-crustal rocks still interact with the atmosphere billions of years after their birth.
Urban encroachment and quarrying threaten its preservation, yet 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
Where knowledge turns stone into story
From the metamorphic depths of St. Thomas Mount to the celestial imprint of the Kaveri Basin, these two terrains together illuminate the full arc of Earth’s memory — one born of heat and pressure, the other of impact and aftermath.
7.1 Geoheritage
Guardians of Earth’s ancient chronicles
Both the Kaveri Basin and St. Thomas Mount warrant formal Geo-heritage recognition. Each represents a distinct expression of planetary evolution: the Kaveri as a possible relic of impact modification, and St. Thomas Mount as a testimony to metamorphic reconstruction. Together, they frame the continuum of Earth’s geological narrative — from cosmic collision to crustal renewal — offering an unparalleled natural archive within the Indian peninsula.
7.2 Educational Potential
Where the classroom meets the cosmos
For students of Earth science, these sites offer living laboratories that bridge planetary geology and terrestrial metamorphism. Through guided study, learners can:
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Identify impact signatures — breccias, planar deformation features (PDFs), and circular drainage patterns that reveal ancient trauma.
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Recognise deep-crustal processes — visible in the orthopyroxene-bearing charnockites of St. Thomas Mount.
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Synthesize planetary and terrestrial perspectives — understanding how the same physical laws shape both craters on Mars and granulites beneath Chennai.
In uniting these disciplines, the region becomes not merely a field site but a classroom of the cosmos, where the stories of starlight and stone merge into one continuum of learning.
8. The Cosmic Continuum
Where cataclysm and endurance complete the circle
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 journey from Coimbatore to Chennai, we traverse not merely the geography of Tamil Nadu, but nearly two billion years of planetary evolution — from the fiery violence of a meteor strike to the silent resilience of deep-crustal rock. The river that now nourishes life once traced the scars of collision, while the hill that watches over the city rose from the depths in response to pressure and heat. Between them lies the grand continuum of cosmic memory: impact and recovery, destruction and renewal, written in stone.
9. Evidence and Institutional Recognition
From hypothesis to heritage
🔍 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) document structural alignments that reinforce 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 hill that gave a name to a rock
The Geological Survey of India formally recognises the St. Thomas Mount charnockite as a National Geological Monument, acknowledging its dual historical and scientific importance. The naming of the rock traces back to Sir Thomas Holland of the GSI, 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 holds 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 urban encroachment, the hill endures as a protected educational landmark — a living archive of deep time.
9.2 Mapping and Survey Work in the Kaveri Basin Region
Reading the landscape through gravity and stone
Although the Kaveri Impact Basin has not yet been formally recognised by the GSI as a confirmed impact structure, much of its supporting evidence arises 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 — which correspond closely with the inferred crater boundaries (Ministry of Mines 2023).
Further, the peer-reviewed research of Subrahmanya & Narasimha (2017) integrates GSI’s gravity and magnetic anomaly datasets with field petrography, proposing that the region’s arcuate topography and brecciated lithologies may indeed reflect an ancient impact origin.
9.3 Implications for Geo-heritage and Scientific Recognition
Preserving the dialogue between stone and sky
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 example of successful institutional recognition and protection. The Kaveri Basin, by contrast, awaits similar acknowledgement. Its inclusion in future GSI Geo-heritage inventories would not only validate a growing body of scientific research but also safeguard field sites essential 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 hill to Coimbatore’s uplifted arcs — are celebrated as integral to our scientific and cultural inheritance.
10. Conclusion
Where astronomy and geology meet in memory
The convergence of astronomy and geology — or astro-geology — offers 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 interconnected chapters of a single cosmic epic.
Every tilted ridge north of Coimbatore and every bronzed stone beneath St. Thomas Mount speaks in the same ancient dialect — a story of impact and endurance, of fire transformed into form. Recognising and preserving them enriches not only science, but also our cultural understanding of Earth as a dynamic celestial body, born of both cataclysm and calm.
Glossary of Key Terms
Understanding the language of deep time:
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 (GSI).
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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Springer Nature 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, photographed by American military photographer Clyde Waddell, 1942–45
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Image sourced from Wikimedia Commons: St. Thomas Mount
Acknowledgment
To those who listened when the Earth spoke softly
The author extends gratitude to the geoscientific community whose prior research — notably the work of Subrahmanya & 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 remain celebrated as living classrooms of deep time.
Author’s Note
Listening to the land between stars and stone
This journey 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 reflections, drawn from my travels across the Coimbatore–Salem–Erode–Chennai corridor (2013–2019), attempt to bridge that gap — to tell how Earth’s deep-time narratives intertwine 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 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
When the stones remember the stars
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 reveal how cosmic and terrestrial histories converge, showing that the stones beneath our feet are, in truth, fragments of the universe itself.
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#AstroGeology #PlanetaryGeology #ImpactCraterResearch #StThomasMountCharnockite
#IndianGeoHeritage #CoimbatoreGeology #WesternGhatsScience #EarthHistory
#GeoEducation #CosmicContinuum #DeepTime #CraterToCrust #ScienceOfIndia
#GeologyMeetsAstronomy #CelestialAndTerrestrial #DhinakarRajaramWrites
