1) Slow Subsidence of Scientific Institutions: As land movement and destruction of homes in Joshimath Uttarakhand became a prominent talking point, the Indian government reacted like it usually does when faced with an awkward situation concerning its own accountability. It imposed a gag order on its scientists, forbidding them from talking to the media until publication of a final report. Dinesh C. Sharma offers a thoughtful perspective on the corrosion of autonomy of India's scientific institutions and the damage this withholding of information does to open and informed debate.
2) Teeth Reveal How Brains Developed In Utero: How fast did our ancestors brain grow before birth? When did patterns of brain growth become more human like? Teeth start developing very early in a fetus at about 20 weeks old and they fossilize well too. Researchers found a relationship between molar length and prenatal brain growth by studying teeth from skeletons of various primate species and comparing them with gestation length and mass at birth of each species. The final conclusion was that rates of pre natal brain development increased during hominid evolution and became more human like about one million years ago. By anthropologist Tesla Monson.
3) Grains of Sand: Too Much and Never Enough. This is a topic of great concern in India too. Unregulated sand mining is stripping river valleys barren of sand, in turn changing river morphology and devastating habitats. Alka Tripathy-Lang writes about the global demand for sand and its impact on environment and livelihoods. What is the future for this resource? Will we learn to use it sustainably? I'll also recommend this Planet Money podcast episode on Peak Sand featuring a stolen beach! - Episode 853: Peak Sand.
The Chalukya era (6th-8th CE) rock cut caves and sculptures at Badami in Karnataka are an archeological wonder. But there is plenty of geology there to admire. In January 2020, I spent some time wandering through Badami. The sandstone layers are 900 million years old river deposits. I wrote a long post about them, explaining the primary sedimentary structures that one can observe in these rocks, and what they tell us about the water depths and currents during deposition of the sediment.
But these primary structures, i.e. sedimentary layer orientations that form during deposition, are not the only interesting features of these rocks. Chemical reactions in these sediments after their burial has overprinted an intriguing fabric on to the rock.
In the picture a very distinct dark and light banding is seen in one of the Badami rock surfaces. This is Liesegang banding.
The dark bands are rich in iron oxide. The lighter bands have little or no iron oxide. Such banding forms by the mobilization of ions from one location in the sediment and their precipitation at another. Ions diffuse along a concentration gradient in the water filled pore spaces. Robert A. Berner's book, Early Diagenesis: A Theoretical Approach, has a good explanation for the formation of Liesegang banding. I am reproducing that below.
"Mobilization of different components of a substance can occur at two or more different locations. The best example of this is the formation of Liesegang banding.In Liesegang banding we have the interdiffusion of two dissolved ions which cab react with one another to form a relatively insoluble solid. The two ions can come from different sources and when their concentrations at a given site build up, via diffusion, to sufficiently high values, precipitation of the insoluble solid occurs. This precipitation suddenly lowers concentration in the neighborhood of the solid, and as a result the diffusion profiles become altered. Continued interdiffusion results in a new build-up in concentration and precipitation at another site. Depending on the geometry of the situation, this process may result in Liesegang rings (3-dimensional), tubes (2-dimensional), or layers (1-dimensional). A common example of Liesegang phenomenoa are rhythmic bands of iron oxides often found in sandstones. In this case precipitation is most likely brought about by the interdiffusion of dissolved Fe++ (from an anoxic) source) and dissolved O2 (from an oxic source). Where the Fe++ and O2 meet, Liesegang banding occurs".
The iron (Fe++) would already have been present in the sediment perhaps in discrete grains of pyrite (FeS2), or trapped in carbonaceous plant debris. Rainfall fed groundwater is the common source of oxygen. As pyrite gets oxidized it releases Fe++ and sulphur ions. The ferrous ions get oxidized to ferric ions (Fe+++). These then nucleate to form iron oxide or hydroxides. Rapid diffusion of ions towards a growing crystal will eventually lower the concentration of ferric ions in the region surrounding the grain to below the nucleation threshold, at which point crystal growth stops. This threshold is reached at a different location where pyrite oxidation is releasing a fresh supply of Fe++. At this new location the concentration of ferric ions build up again to levels where they start nucleating into iron oxide. This migration of zones of dissolution (of pyrite) , diffusion, and nucleation results in the distinct banding. I've summarized this explanation from a paper by P. Ortoleva and colleagues on redox (reduction-oxidation) front propagation and formation of mineral banding.
Formation of redox fronts during the burial of a sedimentary rock can be economically important. For example, a certain type of sandstone hosted uranium deposit known as 'roll-front' occur where oxidizing fluids containing dissolved uranium meet reduced components such as pyrite or organic matter.
Here is another close up of these Liesegang bands. They have a ring or a tube like geometry. The cross bedding indicated by the arrow is a primary structure formed by the movement of sand sculpted into ripples or waves on the river bed. The Liesegang bands have been imprinted over the cross beds subsequently.
The chemical reactions that occur in sediment after their deposition are of great interest to geologists. They play a large role in the reorganization of porosity and permeability through the dissolution and re-precipitation of minerals.Throughout the history of a sedimentary basin, fluids move through these pore networks mobilizing elements, and under favorable conditions, enriching them at particular locations. Geologists prospecting for metal and hydrocarbon deposits want to understand this process.
Is this sandstone slab in its original geological orientation (as when the sedimentary layers were deposited) or is it upside down? I'll answer this a little later, but first some background.
I recently visited the Chalukya style temples and rock cut monuments at Aihole, Pattadakal and Badami (6th -8th CE) in northern Karnataka and noticed some great sedimentary structures in the building stones. The term sedimentary structures refers to the shape and form sedimentary layers get sculpted into by the action of waves, currents, tides and wind during deposition of the sediment. The size of the deposited sedimentary particles and the orientation of layers are a reflection of both the vigor of the currents and waves and the direction of flow of water or wind.
These monuments are made up of Neoproterozoic age (900-800 million year old) sandstones. Geologists have recognized using detailed sedimentological analysis that the sandstones formed mostly in a large braided river system that flowed in a northwesterly direction.
Between roughly 1800 -800 million years ago, over the course of a billion years, the Indian continental crust sagged due to various tectonic forces to form several long lasting sedimentary basins. The Kaladgi Basin in which the Badami area sandstones were deposited is one such basin. The paleogeographic reconstruction below shows the position of the Indian continent at about one billion years ago and the location of the various sedimentary basins within it.
Much of this deposition took place in inland or epeiric seas that flooded the Indian continent. During intervals of sea level fall, rivers carved valleys and deposited coarse sediment. The Badami Cave sandstones are river deposits of the Kaladgi Basin. The stratigraphic column shows various sedimentary deposits of the Kaladgi Basin and their inferred environments of deposition.
The Badami braided river system was receiving sediment eroded from Archean age (>2.5 billion year old) rocks situated SE of the basin. These were granites, granodiorites, and low to medium grade metamorphic rocks of the Dharwar craton (a large block of stable old continental crust).
Land plants did not exist then. Weathered debris was moved quickly by surface flow into streams. Large sediment load, moving by traction i.e. by rolling and sliding on the stream bed, repeatedly choked the channels, forcing bifurcation of streams and formation of braids. Very broad braided rivers formed since there were no plants to stabilize banks. The Badami sandstones (Cave Temple Formation) are technically known as arenites. This term indicates that the rock is made up of mostly coarse sand with very little finer sized mud. Accumulation of mostly coarser sand size and pebbly particles reflects a locale of repeated high discharges and vigorous currents which winnowed away the finer sized mud. The braided river shown below as an example is from the Canterbury Plains of New Zealand.
As river channels episodically migrated sideways and the basin floor subsided to accommodate more sediment, channel deposits and adjacent sand bars got stacked to form thick 'multi-story' sandstones. Each bed tells a story of a discrete depositional episode.
The arrangement of sand layers within each bed tells us about the subenvironments in which it formed and the energy and direction of water flow during deposition. I came across many types of these internal structures. I recognized tabular cross beds, trough cross beds, planar lamination and rippled beds. Water (or wind) can move & shape sand into piles or waves. Sand grains roll along the direction of flow, then avalanche down the steeper side (lee side) of the wave forming a layer inclined (cross) to the orientation of the main sand body. Successive avalanches form a set of cross beds. The graphic shows the formation of a set of cross beds.
Here is an example of cross beds from near the town of Badami.
And this one is from a building stone from Pattadakal temple.
Such cross beds were built by sediment avalanching on the lee side of migrating sand bars during high flow.
This picture show trough cross bedding from near the Badami cave complex. These represent the internal structure of migrating sinuous sand dunes on a channel floor.
See this elegant explanation by Dawn Sumner, a sedimentologist at the University of California at Davis, of how trough cross beds form.
Email subscribers who may not be able to see the embedded video, click on this link: Trough Cross Bedding Video.
And here is a beautiful example of trough cross bedding found in a Pattadakal temple building stone.
This is planar lamination on a slab at Pattadakal. The bed is constructed of parallel layers of coarse sand. It is interpreted to have been deposited in a high flow regime from sheets of water flowing over mid channel sand bars.
Ripples on a slab at Pattadakal. This is a rare preservation of a bedding surface showing rippled sand. Erosion usually cuts off the wavy upper part. These ripples indicate migration of small sand waves in a quieter flow regime on the channel floor.
Remember, cross beds are the inclined layers that form on the lee side of a ripple or wave or dune. Here are small cross sets on the floor of Aihole rock cut temple! These represent the cross beds formed by migration of small ripples. The ripples themselves have been eroded away. Arrows indicate the direction of water flow and cross bed accretion as ripples migrated.
Okay, let's go back to my first question. Is the slab I showed in the picture geologically upside down?
Yes it is. But how to tell?
As sand avalanches down the lee slope it forms a tail at the toe of the slope resulting in cross beds which become tangential to the floor. In picture the cross beds are tangential towards the top of slab i.e. that is actually the base.
Lets see at how the cross bed contact with the top and bottom bedding plane looks in an outcrop. Here is the original depositional orientation of cross beds manifest in this outcrop near Badami caves. They show a tail or tangential contact of the cross beds with the base. Since top of cross beds are not usually preserved they show a high angle contact truncated by upper bedding plane.
This slab is upside down too! Notice again the tangential contact of the cross beds (white arrow) is towards the top, which means that must have been the base. Yellow arrow points to high angle contact with the upper bedding surface.
Towards the top of the exposed section of sandstone around Badami I came across some truly impressive examples of cross bedding. These particular exposures were on the crags opposite the four main Badami temples. There is a narrow passage past the archaeological museum and a short climb to the top. Take a look at these beauties!
These large cross beds reminded me of the inclined beds of wind blown sand dunes. Is it possible that abandoned sand bars were sculpted by wind in to big dunes? Or does this upper level sandstone represent, as a recent study suggests, the beginning of a marine incursion in to the basin? In this scenario, deposition of sand took place in high-energy shallow waters near the shore. These cross beds represent large migrating sand waves which were eventually shaped in to beach ridges and tidal bars. The outcrops and building stones of these monuments mostly record the processes within the Badami braided paleo-river. 900 million yrs ago a complex of channels and bars, quieter pools and rippled sand beds existed where these temples stand today.
Do visit Aihole, Pattadakal and Badami and gaze at its splendid architecture and sculptures. But spare some time to appreciate the magnificent record of our natural history that these monuments preserve.
Quiz- Is this slab upside down or in its true depositional orientation? 😉
