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Sandstones of Central India

Across India, the artistry, audacity and ambition of ancient empires, have been carved into sandstone. Numerous monuments bear testimony to its versatility and durability; Delhi's Red Fort and Humayun's tomb, the Buland Darwaza of Agra's Fatehpur Sikri, the Bhutanath temples in Badami, Odisha's Lingaraja Temple, Rajasthan's Amer and Mehrangarh Forts...the list goes on.

In Central India, Khajuraho's intricately carved shikaras and spires, Orchha's Jahangir Mahal and the cenotaphs, Sanchi's stupa and the Ashoka pillar (with the four lions and wheel of law), or Gwalior's stupendous Jain colossi and the fort, all showcase the diversity of hew and hue that is possible in sandstone. Sandstone remains a popular building material, with many varieties quarried and sold across the country.

Yet what makes sandstone so interesting? Why does it occur in many colours or bear many patterns — stripes, ripples, dunes, scrapes, honeycombs? What processes have led to its formation that made it easy to sculpt and shape yet resilient enough to stand the test of time?

This excerpt traces the sedimentary origins of some of central India's sandstone landscapes and monuments, to scrape away at the layers of geology and human history beneath them.

SANDSTONE - sedimentary, siliclastic, stratified

I love alliterations, and this string of descriptors is how one can classify sandstone. Sedimentary rocks are formed when sediments accumulate and are compacted together or bonded by chemical cements, with some spaces in between. Sandstone, as the name suggests, forms when sand-sized grains of rock, minerals like quartz, feldspar or clay, or organic materials are cemented together by silica, calcium carbonate or iron oxide. 'Siliclastic' means that sandstones consist of fragments of minerals and rocks derived from pre-existing rocks, that is, they were formed after the processes of weathering (breaking down of rocks), and erosion (transport of weathered material by water, wind, or gravity). As weathered and eroded sediments accumulate in a basin or depression, they build up in layers — newer layers over older layers, adding more weight, heat and pressure — this results in stratified or layered rocks.

Sandstones often form in terrestrial or marine environments where sediments deposit; river channels, alluvial fans (where rivers descend from mountain ranges and spread out over the plains), lakes, deserts, deltas, beaches, and tidal flats, among other environments. Sandstone offers some hints to the past; the presence of feldspar indicates that the sandstone was formed close to the mountains, the degree of roundness indicates how far the sediments travelled before they were compacted, ripples indicate the direction of the water or wind currents that shaped the rocks.

Sandstones get their colours from the different minerals compacted or the cementing mineral. Potassium feldspar imparts a pinkish colour, and minerals like slate, chert or andesite result in a salt-and-pepper look. The more quartz in sandstone, the glassier and whiter its appearance. If the rock is cemented by iron oxide, it is often yellow, orange, brown or red-hued, calcite imparts a greyish hue, and chlorite makes it greenish-black, and extremely hard.

Textures in sandstone are formed by the environment. You'll find ripples and dunes if the sandstone was formed underwater. Honeycomb or pitted patterns, if water continued to erode away at the surface. Scores and scratches, if the surface was further shaped by the wind.

Most sandstones are soft enough to be carved or sculpted. If they contain lots of quartz or feldspar (which do not react easily with other chemicals), they will be more resistant to chemical weathering. If sandstones are cemented with clay or iron oxides, they will be more susceptible to weathering. The mineral content of the sandstone dictates how much it can withstand the tests and trials of time. Fossil remains, however, especially soft-bodied creatures, are not often preserved in sandstone. Yet sometimes, ancient hard-shelled marine creatures and fragments of wood may be fossilised, as are trace fossils like footprints or claw marks.

PACHMARHI - the faults lie in the sediments

The weathered, rounded crags at Dhupgarh, the highest point of the Satpura Range, overlook the forested valley. These sandstone crags are not sharp or angular, as they have been worn down over time by rainwater. Image credits: Pradip Krishen

Located in the Mahadeo hills of the Satpura range in Madhya Pradesh lies Pachmarhi (also spelt as Pachmadhi), a forested hill station best known for its waterfalls and caves. Its touristic appeal hinges on its incredible geomorphology; the scenic views over an escarpment or steep slope, the waterfalls that plummet down jagged cliffs, the maze of gorges, potholed river channels, springs, and caves formed by the interplay between porous sandstone and groundwater.

Gorges, waterfalls and cliffs are quite common across India, yet what makes Pachmarhi unique is the formation of the Satpura range. When the earth's crust is pulled apart or extended, normal faults occur, causing some sections to be lifted up and others to dip, resulting in a landscape of alternating ranges and valleys. The uplifted block is known as a 'horst', and the valley section is known as a 'graben' — in this case, the Satpura range forms the horst, and the adjacent Narmada and Tapi valleys form the grabens.

A diagrammatic representation of the 'horst' and 'graben', or ridge-and-valley landscape, and the crust movements that shape it. Image credits: public domain.

Most ancient sediments occur in deep river valleys but the Satpura range offers an isolated, uplifted exposure of sandstone dating back to the ~237-251 million years ago. These Gondwana sediments — formed when India was still part of the fragmented supercontinent — were deposited in a lake millennia ago. While these sediments were being uplifted in the shape of a dome, the nearby Denwa river may have continued to flow and cut its way downwards, even as the terrain warped.

Dhupgarh, the highest point of the Satpura range, shows interesting features in sandstone: some natural sculpturing controlled by the joints and fractures in the rock, distinct tower-like structures created by the weathering and surface water erosion, lots of rock shelters and alcoves carved out by groundwater, and some tafoni or honeycomb weathering due to chemical or mechanical weathering.

The flaky, sheet-like formations in sandstone at Dhupgarh, in the Satpura range. The layer of rounded, embedded pebbles indicates the bottom level of the lake or basin where the rock was formed.

Image credits: Pradip Krishen.

Pachmarhi's caves also host stunning and sometimes vandalised, examples of Central India's prehistoric rock paintings representing scenes of hunting, dancing, domestic life and religious rituals. Legends from the Mahabharata are also narrated at certain sites around Pachmarhi, as it is said that the Pandavas traversed the landscape during their thirteen-year exile.

BHIMBETKA - a legacy writ ages before the Mahabharata

A war scene painting from Rock Shelter 5 at the Bhimbetka site, Madhya Pradesh.

Image credits: Bernard Gagnon, via Wikimedia Commons (CC BY-SA)

Prehistoric art can be divided into two categories: rock or cave paintings, and rock carvings or petroglyphs (I'd written about South Goa's petroglyphs earlier). Bhimbetka's rock shelters, along with Pachmarhi, form Central India's astonishing collection of prehistoric (dated back to ~30,000 years) paintings. There's a lot written about the historic and archaeological significance of Bhimbetka, but as a geological feature — over 750 shelters created by groundwater rather than rainwater runoff. The protruding or overhanging sandstone beds provided refuge from rainwater runoff, and instead, caused water to percolate through the joints and fissures. This led to weathering of the rock in alcove or tafoni (honeycomb) formations, which were ideal as early human shelters. Later, the groundwater undermined and sapped at the weathered material, to deepen the caves and cavities. The pronounced sandstone overhangs also explain why humans chose to settle there, and how their paintings have survived over millennia.

Apart from fascinating evidence of early humans, last year, a group of researchers discovered three fossils of the 550-million-year-old 'Dickinsonia', the earliest known living animal. As I mentioned earlier, it is rare to find fossils in sandstone, hence this discovery is quite thrilling. Here's a deep dive blog by Suvrat Kher (@rapiduplift) on the implications of this discovery and the secrets held in other sedimentary sequences, or a lighter read on the fossil discovery below:

GWALIOR - monumental undertakings in sandstone

The staggering Jain colossi stand sentinel along the base of the Gwalior plateau. Sculpted between the 7th-15th CE, these figures pay mute homage to the resilience of sandstone. Image credits: Pradip Krishen.

In Gwalior, the plateau is a must-visit for tourists - after all, it has an astonishing number of monuments built by different faiths and empires: the impressive Gwalior fort encircling temples like the Saas-Bahu, the Sun Temple, the Chaturbhuj temple, Man Mandir Palace, and Teli ka Mandir, and the Data Bandi Chhor Gurudwara. So why does the Gwalior plateau have so many monuments? The answer lies in the sandstone plateau below. The ochre-pink sandstone was easy to sculpt & cut, allowing layers of history to be written upon it. The Jain colossi were hewn into the cliffs — the overhangs protecting the statues from rain, whereas the palaces & the temples have used blocks carved in-situ (from the same site). Rich alluvial plains around the plateau allowed for good agriculture that supported the empires. Throughout history, the plateau proved a geostrategic vantage point for empires that ruled central India — it afforded views across the plains below and made it easy to keep an eye out for enemy troops. Numerous 'talabs' or tanks also ensured a perennial supply of water. Like most sandstones, Gwalior's immense plateau was also formed by the accumulation of sediments (some sources say, alluvial sediments carried by the Chambal river in the past) in the Gwalior basin. The sheer height of the plateau indicates that the basin must have been deep, and the sediments extensive. Around Gwalior, other processes have continued to shape the landscape, from the action of moving water, and deposition of more alluvium, to deep ravines formed by the lack of sediment flowing through.

In April 2021, a tiny green sandstone found 1,300 kilometres from where it originated is said to offer clues into the severity of climate change. Later that year, research into the Singhbhum sandstones in Jharkhand, revealed when the earliest landmasses rose from ancient oceans. In June this year, NASA's Perseverance rover discovered sandstones on Mars which may have formed in fast-flowing rivers. Secrets in sandstone continue to be unravelled, and this excerpt can only cover so much. Hopefully, it offers a starting point for your future travels across India.



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