Thursday, May 25, 2017

Chasing The South Tibetan Detachment- Panchachuli Glacier Area Kumaon Himalaya

This is a geology travelogue of my recent trek to the Panchachuli Glacier.

The South Tibetan Detachment System (STDS) is an important fault zone in the Himalaya, bringing in to structural contact the Tethyan Sedimentary Sequence (TSS) with the underlying metamorphic rocks of the Greater Himalayan Sequence (GHS). It is a northerly dipping extensional or normal fault. This means that the Tethyan sediments which make up the hanging wall of the fault have moved down relative to the footwall made up of the Greater Himalayan Sequence. As the name suggests the STDS is most prominently developed in the southerly Tibet plateau like physiographic province of the Himalaya, north of the great Himalayan summits.

What is the history of the TSS and the STDS? As is the case with Peninsular India, the foundation of the northern extent of the Indian plate is made up of Proterozoic rocks which were deformed and metamorphosed during the Mesoproterozoic times (1.8 billion to 1 billion years ago). On this "crystalline basement" were deposited a succession of sedimentary rocks ranging in age from the Neoproterozoic to the Eocene.

In the early Cenozoic, when the Indian plate impinged into Asia this sedimentary "cover" was folded, faulted and scraped off to form an early "Tethyan" mountain range. As collision continued and as lower tiers of the Indian crust subducted under Asia, thrust faults moved slices of deeply buried and metamorphosed crust upwards. These slices are the Greater Himalayan Sequence. They are bounded by the Main Central Thrust at its base and by the STDS at the top. Concurrent with the movement of the Main Central Thrust, fault zones developed at the base of   the Tethyan sedimentary cover, perhaps along the same planes of breakages that had earlier uplifted the Tethyan ranges. This fault zone evolved into the STDS.

There are different hypothesis on how important the STDS is to the evolution of the Himalayan orogen. One school of thought suggests that the extention and thinning of the crust along the detachment zone accelerated the exhumation of the deeply buried GHS and brought these deeper levels of the crust in to structural contact with the Tethyan cover sequence. Alternative scenarios argue that the STDS did not play such an active role in the exhumation of the footwall GHS.

Whichever scenario is correct, there is no doubt that the South Tibetan Detachment is a major structural boundary separating two distinct lithologic terrains.

The outcrops around me during my trek where all metamorphic rocks of the Greater Himalayan Sequence. I had hypothesized that the South Tibetan Detachment and Tethyan rocks if they indeed were present in the area would be making up the summits of the ranges around Duktu and in the Tidang area.

After days of observation I was proved right about that. I used three types of indicators to infer the  presence of Tethyan sedimentary rocks high up on the summits and to recognize the fault boundary between them and the underlying Greater Himalayan Sequence metamorphic rocks.

1) Structural discordance between the Greater Himalayan Sequence and the Tethyan Sedimentary Sequence. This could be clearly seen near the summits of the ranges north and east of Duktu.

2) Boulders of sedimentary rocks like conglomerate and planar and cross bedded sandstones in the streams draining these ranges.

3) Dilation fractures in both the phyllite grade metamorphic rocks of the Greater Himalayan Sequence and in sandstones of the Tethyan Sedimentary Sequence. This indicated the presence of an extensional stress regime. The South Tibetan Detachment is a zone of normal faulting. The crust has been broken and pulled apart by tensional forces. These stresses were felt over a broad zone and impacted the footwall and hanging wall rocks.

As I am writing up these three criteria I have to admit that my thinking about these lines of evidence was not at all clear when I started the trek. Rather, my ideas and understanding of the local geology evolved haphazardly over the days as I walked the valley and started noticing structural orientations, stream rubble and fracture patterns.

We began our trek at Nagling village. Our destination was the village of Dukti (Lat 30.2486, Long 80.5460). We walked northwards. As Himalayan thrust sheets dip north, we were going structurally higher and higher up the Greater Himalayan Sequence. At, and ahead of Nagling, we were in a zone of partial melting and granite intrusions. High grade gneiss and migmatites were intruded by dykes and sills of granite. I'll be posting about this section separately. This high grade gneiss zone was overlain by a sequence of phyllite grade metamorphic rocks. These phyllites show tight isoclinal and recumbent folding. The internal structure of the Greater Himalayan Sequence is interesting. There is an increase in metamorphic grade from the base to the higher levels and then a decrease towards the very top.

Above the phyllite grade rocks separated by the STDS are the Tethyan sediments. I figured I would have traveled north enough, i.e. structurally high enough along the GHS to cross the phyllite zone and into the overlying Tethyans. I had an expectation that at the very least I would notice them capping some of the ranges I was going to encounter between the villages of Duktu and northward towards Sipu.

Here is an interactive map of the area I trekked, which you can use to follow the text and check on the locations of the samples.

Day1 - The northerly walk takes a left turn as we enter the Panchachuli Glacier valley. The river Dhauliganga is a west to east flowing river in this valley near Duktu village. We entered the village of Duktu in pouring rain. Every mountain range was covered in clouds, and in any case the rain was heavy enough to keep us indoors for the evening.

Day 2- More rain! It happened during my last trek in the Munsiari valley too. We go stuck there for two days due to heavy rain and snow. As it happened, the rain stopped by afternoon and we could go for a short walk to the twin village of Dantu across the Dhauliganga river. The river bed was chocked with boulders of a distinct biotite-tourmaline bearing granite (Picture to the right). Both Duktu and Dantu villages are located in phyllite grade rocks. This conspicuous granite does not intrude these rocks. Its source lies in the Panchachuli ranges, lower in the GHS. The Panchachuli Glacier has gouged it from the Panchachuli ranges and transported the debris to this valley. All the summits were still covered by clouds and I was resigned to wait it out for any further observations of the geology.

Day 3- Perfect weather. It was bright and sunny. But I hardly did any geology this day. We took a spectacular 5 kilometer walk westwards to the Panchachuli glacier.  The terrain was covered by forest, shrubs and grass and higher up by ice. We walked along the lateral moraines of glaciers past. The Panchachuli glacier was much bigger during the Pleistocene ice ages and glacial deposits are piled up high in the valley. I'll be posting on these deposits too. If only I had just glanced to the east of Duktu and looked carefully at the ice snow covered ranges!!

Day 4- Great weather again! We took a northerly course towards the village of Tidang. Our original plan was to walk up further north to the village of Sipu. However,  the ITBP (Indo-Tibetan Border Police) were restricting movements of civilians in that area and we got a nod to go only to Tidang on a day trip. This is fantastic terrain. We passed through pine forests and then into a landscape of open woodlands and scrublands.

We were now in the Lassar Yankti valley. This river joins the Dhauliganga near the village of Duktu. There were enormous mountain ranges on both sides of the valley. I was keeping my eyes peeled for anything interesting. And soon I began noticing that phyllite grade rock fragments scattered along scree slopes showed dilation fractures (Pic to the left). These fractures occur when the crust is being subjected to tensional forces. I now strongly suspected that these upper structural levels of the GHS were close enough to the STDS to have experienced extensional stresses. The picture show a phyllite grade rock with foliation displaced along a fault and showing dilation fractures (above) and another phyllite with parallel sets of dilational fractures (below). The fractures have been filled or healed with secondary quartz.

We passed the village of Dakad (Lat 30.2756, Long 80.5291). A few hundred meters ahead I had the first of the big "aa-haa" moments of the trek. A large boulder of sandstone showing planar and cross bedding lay just a few meters aside of the trail. It must have been transported there either during a rock fall or by glaciers from high up on the ranges on the left bank of the Lassar Yankti. A few minutes ahead we came across a stream draining these ranges and joining the Lassar Yankti. In that stream near the bridge connecting to village Tidang I saw a conglomerate boulder (Lat 30.2822,  Long 80.5262). Sedimentary rocks of the TSS were definitely present high up in that range. Here is a picture of the cross bedded sandstone (above) and the conglomerate (below).

Looking up towards the ranges, I could not identify a lithologic or structural boundary, but the presence of dilation fractures and sedimentary debris pointed to the presence of the STDS and the TSS high in those ranges.

Day 5- The weather Gods were kind again. We trekked westwards from Duktu along the left bank of the Dhauliganga river towards the terminal moraine of the Panchachuli glacier. The rock walls on the left bank of the river were phyllite grade rocks. Again, I found dilation fractures in them. And in a small stream draining those ranges... another conglomerate (Pic to the right) ! (Lat 30.2471, Long 80.5181). I looked up to the summits carefully. Perhaps my viewing angle was just right or perhaps my mind was now better prepared but... there it was... a clear structural discordance between steep northwesterly-dipping rocks and the overlying more gently northeasterly-dipping rocks. I was looking at the South Tibetan Detachment Fault that had placed Tethyan sediments over the Greater Himalayan Sequence (picture below; join the tips of the arrows to trace the detachment fault).

I then looked through the valley straight towards the ranges to the east of Duktu. Again, that same structural discordance was clearly visible in the snow capped summits. The picture below (photo credit: Swati Pednekar )  shows this eastern range, the detachment fault (join the tips of the arrows to trace the fault) and the lithologic units.

Day 6- A trek to villages of Goe, Philam and Bon. We walked north from Duktu, crossed the Lassar Yankti river a little ahead of Dantu village and entered village Goe (Lat 30.2602, Long 80.5411) and then walked southwards. That morning I had confidently predicted that we would find sedimentary rock with dilation fractures on this trail. These villages are at the base of the ranges shown in the picture above. Although not diagnostic, there was another strong hint that these ranges had sedimentary rocks at the summits. The summit rocks have weathered into a blocky square edged pattern typical of jointed sandstones and quartzites.

And I was right! Sandstones along with low grade phyllite rocks (from the lower levels of the mountain) were being used to build walls and pavements in all the three villages. Picture on the left (above) shows a cross bedded sandstone block making up part of a wall in village Goe. And a cross bedded sandstone slab (left, below) is being used as a pavement stone for a village trail between Goe and Philam. Further south ahead of village Bon, a large stream draining these mountains contained boulders of bedded sandstones. And at a small bridge at the bottom of the valley  (Lat 30.2370, Long 80.5450) I came across a sandstone block (picture below) with slickensides (black arrows) and dilation fractures (red arrows). Slickensides are striations on rock surfaces formed by frictional movement of rocks along a fault. This was a strong indicator that these sandstones were sourced from an extentional fault zone high up near the summit.

We continued walking southwards, into lower levels of the GHS. Soon, we were back in the Nagling area, in the zone of partial melting and granite intrusions.

This ended our trek in the Panchachuli Glacier area. To date, it was the most satisfying trek I had done in the Himalaya. Although the STDS was high up and I could not actually walk across it, I had hypothesized, made observations and validated my expectations of the presence of the detachment faults and Tethyan sedimentary rocks. This would be a good field exercise for students! And I am hoping this post will be used by trekkers wanting to explore and understand the geology of this area.

Day 7- We trekked to the Nagling Glacier which has carved a perfect U shaped valley. Certainly one of the most beautiful sites I have been to.

... more geology posts on glacial deposits and granite intrusions... coming soon.. !

Tuesday, May 16, 2017

Landscapes: Panchachuli Glacier And Lassar Yankti River Valley Kumaon Himalaya

I'm back. It was epic. There was geology. I saw the South Tibetan Detachment fault zone. I saw rock deformation. I saw Pleistocene -Holocene glacial deposits. I saw glaciers... I trekked, I photographed, I lived with the local nomads and farmers.

I need a little time to write more on the geology. Meanwhile, here is a glimpse of the absolutely wonderful landscape I wandered through for the past couple of weeks.

Here is an interactive map of the area I traveled through.

and these lands...

1) The crown jewels of the region- The Panchachuli Range seen from village Dantu. There are five peaks. From this angle, the fifth is hidden behind the peak on the left.

2) Sunrise at the village of Nagling.

3) Himalayan valleys, forested slopes and snowy peaks. En route from Nagling to Duktu. View looking south towards Nagling.

4) Village Baaling with northerly dipping metamorphic rocks of the Greater Himalayan Crystalline Sequence

5) Climbing towards the Panchachuli Glacier. This is a superb 2 hour walk from village Duktu passing through birch and pine forests, scrubland, meadows and finally glacial moraines and ice.

6) On the Glacier! About 13,500 feet ASL.

7) Terminal Moraine and the place of origin of the river Dhauliganga.

 8) The Dhauliganga river with biotite-tourmanline granite boulders sourced from the Panchachuli massifs. This is a Miocene granite intrusive into the Greater Himalayan Crystalline Sequence metamorphics.

9) The Lassar Yankti river valley with village Goe at a distance.

10) View from village Tidang of the surrounding rock massifs. The northerly dipping rock slabs are phyllite to medium grade metamorphic rocks of the Greater Himalayan Crystalline Sequence.

11) Another view of the Lassar Yankti river from village Tidang

12) View from village Philam looking east towards some impressive mountains. These are mostly made up of phyllite grade metamorphic rocks of the Greater Himalayan Crystalline Sequence... but with mystery rocks at the very top! 

13) A little piece of heaven. Nagling Glacier over the Pleistocene ice ages has carved a perfect U shaped valley

 14) Village Duktu. We were close to ten and half thousand feet ASL here. Most of these villages were still uninhabited. People who had migrated to lower altitudes the previous November had locked up by placing wooden shafts and thorny scrub branches against their doors to ward of evil spirits...  and I suspect the occasional Himalayan bear who might fancy hibernating in their home. When we reached here, villagers were just beginning to return with their livestock for their summer stay.

 15)  Relaxing at village Dantu with my friends.

 16) Mystery solved. That's me pointing to the South Tibetan Detachment Zone.

How did I figure that out? What were the geological indicators?.. Coming soon!

Sunday, April 30, 2017

Gone Hiking! Panchchuli Glacier And Beyond- Kumaon Himalaya

I'm leaving today for a trek in the Kumaon Himalaya, Uttarakhand. The destination is Panchchuli Glacier in the Darma Valley. We will also be going over on to the next ridge to the east and hiking toward the village of Tidang and finally Sipu in the Lasser Yankti river valley, gateway to Ralam Glacier.

I've embedded below an interactive map of the area.

The Panchchuli Glacier base camp is around 13,900 feet ASL. From what I've heard from friends and the pictures I have seen, the trek offers some pretty stunning views of the Himalaya. Hopefully I'll come across some interesting geology too. This time I made a decision not to read up on the geology. My recent Himalaya trips have given me some familiarity with the lithology and structure of the region. I am guessing that most of the early part of the trek will be in the hanging wall of the Main Central Thrust. High grade metamorphic rocks of the Greater Himalaya Crystalline Sequence are exposed here. Towards the village of Sipu I am hoping to get a glimpse (even at the distance will do!) of the Southern Tibetan Detachment, a fault zone that separates the Greater Himalaya metamorphic rocks from the overlying Tethyan sedimentary sequence.

Let's see.

I'll be posting on my trip later in the month of May. Depending on connectivity I may be able to send a few field dispatches via Twitter.

Stay tuned.


Friday, April 28, 2017

Himalayan Gravel Flux And Flood Risk

Why should an understanding of sediment transport distance and whether that sediment gets broken down into coarser gravel or finer sand be of any practical use?

Here is a good example from the Himalaya.

Abrasion-set limits on Himalayan gravel flux- Elizabeth H. Dingle, Mikaƫl Attal & Hugh D. Sinclair

Rivers sourced in the Himalayan mountain range carry some of the largest sediment loads on the planet, yet coarse gravel in these rivers vanishes within approximately 10–40 kilometres on entering the Ganga Plain (the part of the North Indian River Plain containing the Ganges River). Understanding the fate of gravel is important for forecasting the response of rivers to large influxes of sediment triggered by earthquakes or storms. Rapid increase in gravel flux and subsequent channel bed aggradation (that is, sediment deposition by a river) following the 1999 Chi-Chi and 2008 Wenchuan earthquakes reduced channel capacity and increased flood inundation. Here we present an analysis of fan geometry, sediment grain size and lithology in the Ganga Basin. We find that the gravel fluxes from rivers draining the central Himalayan mountains, with upstream catchment areas ranging from about 350 to 50,000 square kilometres, are comparable. Our results show that abrasion of gravel during fluvial transport can explain this observation; most of the gravel sourced more than 100 kilometres upstream is converted into sand by the time it reaches the Ganga Plain. These findings indicate that earthquake-induced sediment pulses sourced from the Greater Himalayas, such as that following the 2015 Gorkha earthquake, are unlikely to drive increased gravel aggradation at the mountain front. Instead, we suggest that the sediment influx should result in an elevated sand flux, leading to distinct patterns of aggradation and flood risk in the densely populated, low-relief Ganga Plain.

Behind paywall, but I thought this is a good illustration of how insights into very fundamental earth processes can potentially help save lives.

Wednesday, April 19, 2017

Evolution Of The Konkan-Kanara Coastal Plain

The Konkan coastal plains is a beautiful getaway from west coast city life. Palm fringed beaches, quiet rivers and estuaries, betel nut plantations and forest tracts. Small villages and settlements dot the landscape. To the east, the coastal plains abut against the imposing Western Ghat escarpment.

How did this coastal plain of Maharashtra form? (Kanara refers to the stretch south of Maharashtra in the state of Karnataka).  I came across a paper by Mike Widdowson on the evolution of laterite in Goa. It also has a broader discussion on the conditions that led to the formation of geomorphology of the coastal lowlands extending all along the west coast of India.

Here it is summarized nicely in this figure below:

Source: Evolution of Laterite in Goa: Mike Widdowson  2009

After Deccan Volcanism ended, rifting of the Indian west coast and down faulting of the western side led to the formation of a west facing fault scarp. Erosion of this scarp over the early mid Cenozoic (from about 60 million years ago) has caused it to retreat eastwards. The Western Ghat escarpment is this retreated scarpThe coastal plain formed as an erosional surface that became broader and broader with the progressive eastward retreat of this cliff to the current location. The fault which caused the western side to subside thus lies in the Arabian Sea along the west coast.

In Mid-Late Miocene (~10 million years ago), a phase of humid climate resulted in intense chemical weathering of the basalts and pediment (rock debris) exposed along the coastal plains. This alteration of the basalts formed thick iron rich soils. The reddened and indurated crust of this soil is commonly termed laterite. In the Western coastal lowlands this laterite may be a few meters thick.

Subsequent uplift of the west coast and concomitant down cutting by west flowing rivers formed a dissected landscape composed of laterite capped mesas (table lands) and entrenched meandering streams. These mesas reach altitudes of 150-200 m in the eastern parts of the coastal plain. Nearer the coast they are about 50 -100 m above sea level. 

The western margin of India has seen multiple episodes of extensive laterite formation. The famous table lands of the hill stations of Panchgani and Mahabaleshwar are also made up of laterite. They occur at altitudes of around 1200 m to 1500 m.  However, this upland or high altitude laterite is much older, having formed about 60- 50 million years ago in the early Cenozoic, soon after Deccan volcanism ended. The Konkan and Goa lowland laterites point to another younger phase of laterization. Sheila Mishra and colleagues have identified two more surfaces in the Deccan Traps at 650 m ASL and 850 m ASL that preserve remnants of laterite cover. This suggests a complex polyphase history of denudation and chemical weathering and tectonic stability of the Sahaydri ranges of the Western Ghats.

The sea cliffs that one encounters as you travel along the Konkan and Goa coastline are a result of a late Cenozoic uplift. I remember with fondness a trek I did during my college days from the town of Ratnagiri south to the town of Malvan. There were absolutely majestic sections where we walked on the edge of laterite capped sea cliffs with the Arabian Sea heaving and thundering below us. Little coves and beaches of sparkling white sand lay between the cliffs. Here and there local fisherman had kept their fish catch to dry out in the sun. The pungent smell urged us on!

The satellite imagery below shows a section of the coastal plains from Ratnagiri in the north to Devgarh in the south. White arrows point to the laterite capped table lands dissected by stream networks. Orange arrows point to sea cliffs. Black arrows shows the Western Ghat escarpment.

This is a very interesting paper. Open Access.

Thursday, April 13, 2017

Oceanic Crustal Thickness Since The Breakup Of Pangea

Of interest:

Decrease in oceanic crustal thickness since the breakup of Pangaea - Harm J. A. Van Avendonk, Joshua K. Davis, Jennifer L. Harding and Lawrence A. Lawver

Earth’s mantle has cooled by 6–11 °C every 100 million years since the Archaean, 2.5 billion years ago. In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches. Here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time. We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1.7 km thicker on average than crust produced along the present-day mid-ocean ridge system. If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15–20 °C per 100 million years over this time period. The difference between this and the long-term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss. We also find that the increase of ocean crustal thickness with plate age is stronger in the Indian and Atlantic oceans compared with the Pacific Ocean. This observation supports the idea that upper mantle temperature in the Jurassic was higher in the wake of the fragmented supercontinent Pangaea due to the effect of continental insulation.

Continental insulation refers to the idea that an unbroken continental crust such as that provided by a supercontinent may act as a blanket resulting in a slow build up of heat over tens to hundreds of millions of years in the underlying mantle. Eventual continental breakup will lead to enhanced magmatism and thicker ocean crust along these previously insulated regions.

The Pangaean paleogeography of the Triassic (252 million to 201 million years ago) is depicted in the map below. The distribution of continents is lopsided covering the sites of the future Atlantic and Indian Oceans.

 Source: Paleobiology Navigator

Wednesday, March 29, 2017

Exploring India's Paleogeography And Fossils Using The Paleobiology Database Navigator

I was directed to the Paleobiology Navigator by a tweet from @avinashtn .

Great fun! The Paleobiology Database is being maintained by an international non-governmental group of paleontologists. Contributing members add to it fossil occurrences from scientific publications.  The Paleobiology Database Navigator is a web mapping application managed by the University of Wisconsin-Madison that allows you to explore the geographic context of these fossil locations. You can filter the data based on age, taxonomy and geography. You can also generate diversity trends for the selected set.

I played around a bit with India specific fossil locations.

Paleozoic versus Mesozoic Basins

The figure below shows the distribution of fossil localities for the Paleozoic Era. India is shown as it is today and in its Paleozoic geography.

Source: Paleobiology Navigator

You can clearly see that fossils in Peninsular India are predominantly located in one narrow band in the center and east of the country. These are the Permian Gondwana basins. They are, starting from the westernmost and going eastwards, Satpura Basin, Son Valley Basin, Damodar Valley Basin and the Ranjganj Basin.  These are continental interior basins comprising river, lake and swamp environments. Most of India's coal deposits come from these basins. These basins are rich in plant fossils, and reptile and amphibians remains.

Now take a look at India's geographic position (arrow) during the Permian (298-252 million years ago). Peninsular India occupies an interior location within Gondwanaland, far away from any ocean. Tectonic stability through most of the Paleozoic meant lack of crustal movements. During this time, peninsular India was an erosional landscape until the Permian basin formation in the east.

The one Paleozoic fossil location in Rajasthan shown here represents early Permian marine sediments formed by the flooding of the western region by an arm of the Tethys sea.

And this database has still not added one important fossil location. This is the early Cambrian age locality near Jodhpur where sediments of the Nagaur Group are exposed. They contain trilobite trace fossils.  No basin development and sedimentation took place in Peninsular India from Mid-Cambrian to Permian times (530 million years to 298 million years). 

In contrast, look at the northern edge of India, where the Himalaya stand today. That margin was submerged under the Tethyan ocean. A thick pile of marine sediment accumulated right through the Paleozoic, forming the fossil rich Tethyan Sedimenary Sequence of the Himalaya.

Continental configurations changed in the Mesozoic (252 million to 66 million years ago). The figure below shows Mesozoic fossil locations and the Cretaceous paleogeography of India.

Source: Paleobiology Navigator

There is now a wide swath of fossil localities across Peninsular India. The dotted lines trace important linear depressions where sediments were deposited. The east west oriented Narmada rift zone (NRZ; Jurassic and Cretaceous) and the NW-SE oriented Pranhita Godavari zone (PGR; Triassic to Cretaceous) are important fossil repositories.  The eastern India basins continued accumulating sediment. To the west are the basins which formed in Gujarat and Rajasthan (Jurassic and Cretaceous). The Kutch rift (KR) is outline by dotted lines. And to the south east in Tamil Nadu, marine flooding of the eastern continental margin in the Cretaceous resulted in the deposition of richly fossiliferous sedimentary sequences.

All these basins ultimately owe their origin to the forces exerted on the crust as India pulled away (arrow) from Gondwanaland.  Seaways formed along these rifts and crustal depressions. The Mesozoic, especially the Jurassic and Cretaceous, was a time of global high sea levels. The western margin saw marine incursions from the nascent Indian Ocean, while the eastern margin was submerged by the waters of the newly formed Bay of Bengal.  River and lake systems also developed in more continental interior locations. The northern margin (Himalaya) was mostly a marine environment through the Mesozoic.

Marine versus Continental Interior Basins in Mesozoic Central India

The distribution of terrestrial organisms versus marine organisms can tell us about the extent of marine flooding into Peninsular Central India in the Mesozoic.

I created these maps by using localities of dinosaur fossils (above) to map the distribution of terrestrial sedimentary environments. I used localities of invertebrate marine organisms, namely,  brachiopods, echinoderms and ammonoids  to delimit the extent of marine environments along the Central Indian basins (below).

 Source: Paleobiology Navigator

You can see that terrestrial environments were present right across the Narmada rift zone, the Pranhita Godavari rift basin and in the western Indian basins also. In the western basins, some of the dinosaur fossils have been found in marginal marine settings comprising coastal and estuarine environments.

Deeper water marine environments as evidenced by brachiopod, echinoderm and ammonoid localities are however restricted to Gujarat, Rajasthan and western Madhya Pradesh. The Cretaceous Bagh Beds in Madhya Pradesh is the eastern most limit of Mesozoic marine flooding into Central India. Seaways did not extend into eastern parts of the Narmada rift basins.

Global and Indian Dinosaur Diversity Patterns

I used the Stats tool to create graphs of dinosaur diversity. The number of Genus per Stage is being used as a measure of diversity. Geologic time is subdivided in to bins. An Age is a bin spanning a few million years. Stage represents rock layers deposited in an Age. So, a diversity measure has been created by counting the number of dinosaur genus reported from successive bundles of rock layers, each representing a few million years of time.

Source: Paleobiology Navigator

The global diversity pattern shows episodes of diversification and decline in the Triassic, Jurassic and the Cretaceous. There appears to be a trend of increasing diversity through time with peak diversity in the Mid-Late Cretaceous. The Late Cretaceous extinction of dinosaurs forms the right side boundary.

The diversity measures in India show some differences with global trends. The number of Genus sampled are less. This is due to regional versus global sample. A smaller locale will generally have less of the total observed variation. The trends in diversity with time also is different from the global trajectories. There are a couple of reasons for this. First, this is a preservation artifact. Mesozoic terrestrial basins in India were receiving sediment only episodically. Depositional phases were interrupted by erosional hiatuses. Rock sections thus have been removed as well.   There was little to no sedimentation from Mid-Jurassic to Mid-Cretaceous in the Narmada rift basins. Hence, no fossils either. The lost diversity from this interval is irretrievable.

The second reason gives more hope. A couple of years ago, Dr. Dhananjay Mohabey of the Geological Survey of India gave a talk in Pune on Late Cretaceous dinosaurs of India. He mentioned that there are roomful of dinosaur fossils in government archives that are yet to be studied and catalogued. There is scope then to enhance our understanding of at least late Cretaceous dinosaur diversity of India.

I have barely scratched the surface. There are many more stories and patterns and trends in the Indian fossil record waiting to be teased out from this database. Dive in!

Saturday, March 18, 2017

Comments On The 1.6 Billion Year Old Red Algae From Central India

The Proterozoic Vindhyan sedimentary basin in Central India contains sediments ranging in age from 1.7 billion years to about 600 million years ago. Bengtson and colleagues report three dimensional preservation of cellular structures which they interpret as multicellular red algae. These fossils have been found in the Tirohan Dolomite dated to about 1. 6 billion years. Before this discovery, the earliest fossils of multicellular eukaryotes was the rhodophyte Bangiomorpha, dated to about 1.2 billion years.

The Tirohan Dolomite is exposed in the Chitrakoot region of Madhya Pradesh. The fossils occur in patches of carbonate sediment which was replaced by the calcium phosphate mineral apatite just after their deposition in a shallow marine setting. Phosphotization is often a very delicate process enablng the preservation of fragile cell structures.

Here is a picture of the cellular structures of red algae imaged by SEM (scanning electron microscope)

Source: Bengtson 2017

And another rendering of the three dimensional structure of the red algae imaged using Synchrotron-Radiation X-ray Tomographic Microscopy (SRXTM). The green objects inside the cell are interpreted to be organelles, components of eukaryotic cells which aid in different physiological functions. Prokaryotes (Bacteria) lack such organelles.

Source: Bengtson 2017

I don't want to dwell on this study too much. The paper is open access for those who want to explore further.

There are two side stories that I want to comment upon.

First. The Tirohan Dolomite and its fossil assemblage has a controversial past.

They were discovered about twenty years ago by Dr Rafat Azmi, a paleontologist working with the Wadia Institute of Himalayan Geology. He reported from the Rohtasgarh area in 1998 a rich trove of filamentous and spherical forms, and odd shaped mineral fragments. He interpreted the mineral fragments as "small shelly fossils" representing fragments of animal shells and the spherical forms as possible animal embryos. Later in 2006 he reported tubular forms which he interpreted as Cambrian animal taxa. The problem was that animals are thought to have evolved by the latest Neoproterozoic- early Cambrian (600 mya -540 mya), while the understanding then was that the Tirohan Dolomite is likely 1 billion to 1.5 billion years old. Azmi's interpretation carried two enormous implications; either a) the Tirohan Dolomite was much younger in age. This would have required a major revision of the ages of Vindhyan sediments or b) that the rocks were old (~1.5 billion years), but that animals evolved much earlier than the current fossil record indicated.

These very significant implications caught the attention of geologists and media alike. The Geological Society of India sent a team to investigate Dr. Azmi's claims. They reported that they were unable to find the fossils Dr. Azmi had claimed to have found.

 Memories of an earlier scandal in Indian palaeontology were still fresh. In the late 1980's Vishwajit Gupta of Punjab University was found guilty of fraud and plagiarism. He had been misreporting fossil discoveries from the Himalayas by using museum specimens from all over the world. He had  constructed an entirely fake narrative of Himalayan fossils and stratigraphy. Scientific journals were forced to retract his papers. The Paleontological Society of India produced a book authored by S. K Shah titled "The Himalayan Fossil Fraud".  Punjab University, disgracefully, allowed Dr. Gupta to remain in service till he retired in 2004.

Under this shadow, Azmi's fossils came under similar suspicion. Fortunately, Bengtson and colleagues in a study some years later confirmed that these fossils do exist in the Tirohan Dolomite. However, they sampled the Tirohan Dolomite at Chitrakoot and not its stratigraphic equivalent (Rohtas limestone) at Rohtasgarh where Dr. Azmi's initial claims came from. They established using absolute radiometric dating that the Tirohan Dolomite is 1.6 billion years old. And they showed that the forms, similar to those Dr. Azmi found, are not multicellular animals. The spherical forms were all likely gas bubbles. Some of the larger tubular forms were revealed in the present study as red algae. Animal evolution didn't take place that early after all. The claim of the "small shelly fossils" has not been resolved fully. Bengtson and colleagues work doesn't address them. Some other researchers though have interpreted them as non-biogenic mineral growths. The stratigraphy and broader fossil content of the Rohtas limestone from where Azmi collected his fossils firmly indicates that it is not Cambrian but Proterozoic in age. .  In this present paper, these scientists have named one of the red algal forms Rafatazmia chitrakootensis in honor of Dr. Razat Azmi.

The second comment I have is on multicellularity. These red algae are the oldest multicelluar eukaryotes found anywhere. Plants, Fungi, Protists (amoebas) are eukaryotes.  They share a common eukaryote ancestor which was unicellular. That means there was just one origin of the eukaryotic cell type. However, multicellularity has evolved many times independently in different branches of the eukaryote family.

Multicellularity comes in different flavors. In simple forms of multicellularity, organisms are made up of sheets and aggregates of cells sticking to one another. There is differentiation of somatic and reproductive cells. Communication between cells is limited. One important aspect is that all the cells are in direct contact with the environment, since in these organisms, nutrient transfer takes place by diffusion from the environment to the cell. More complex types of multicellularity require the evolution of not just cell to cell adhesion, but elaborate cell to cell communication systems and a division of labor i.e. cells specialized for different functions. Also, these organisms have a three dimensional arrangement of cells wherein only few cell types are in direct contact with the environment. Diffusion is not efficient enough to supply internal cells with all the necessary life support. Molecular conduits and tissues that facilitate bulk transport and circulation of nutrients need to evolve to build this type of multicellularity.

The figure below shows the many origins of the complex type of multicellularity (in red) in different eukaryotes branches.

Source: Andrew H Knoll 2011

Based on cell type, life is divided into two domains. The Prokaryotes (Bacteria and Archaea) have smaller simpler cells. Eukaryotes are generally larger and are made up of more complex cells. This cell type evolved by a symbiotic merger between two types of prokaryote cells. Prokaryote fossils have been found in rocks older than 3 billion years. The eukaryote fossil record begins in rocks younger than 2 billion years. The timing of the origin of eukaryotes is unclear. Estimates range from  2.5 billion to 1.5 billion years ago. These red algae fossils show that eukaryotes had already diverged into different branches by 1.6 billion years ago, which means that the unicellular ancestor of eukaryotes evolved before that. It also means that red algae took the road to multicellularity much earlier than animals.

Does complexity evolve necessarily whenever genetic potential is available or does it depend on ecologic opportunity? If the cellular machinery and the underlying genetic regulatory systems required for multicellularity evolved in the ancestors of red algae by 1.6 billion years ago, why did multicellular animals not evolve earlier as well? It could well be that there were ecologic conditions limiting the evolution of physiologically demanding creatures like animals. The end of Neoproterozoic ice-ages by about 650 million years ago and the break up of supercontinent Rodinia impacted sea water chemistry. Sea water oxygen increased to threshold levels permitting a more active life style. Increased weathering of continents brought into the oceans metals like zinc which are crucial for physiological functions. Creation of larger continental shelves and shallow water zones due to continental breakup provided varied ecologic spaces for diversification. Animal evolution was triggered in this ecological context.

Wednesday, March 8, 2017

Papers: Tectonics And Physical Volcanology Of Deccan Traps

There are plenty of research papers on the geochemistry of the Deccan Basalts. But nature lovers and trekkers like me come face to face not with chemistry but with the physical forms of lava and the structural elements of the volcanic pile.

I found this list of papers most useful. They have helped me sort out my confusions regarding lava morphology and taught me something about the structural fabric of the western margin of the Deccan Volcanic Province.

1) Near N–S paleo‑extension in the western Deccan region, India: Does it link strike‑slip tectonics with India–Seychelles rifting? - Achyuta Ayan Misra Gourab Bhattacharya, Soumyajit Mukherjee, Narayan Bose

This is a structural analysis of the fracture systems that cut across the western margin of the Deccan province.  The area of study is the coastal plains, about 100 km north and south of Mumbai. The Indian western margin is a rifted margin i.e. it formed by the breakup of India with Madagascar (88 million years ago) and then Seychelles (64 million years ago). This type of margin is formed by tensional forces splitting apart continents and so you would expect normal faults, wherein blocks of crust have moved down along inclined fault planes.  Except here, the researchers find evidence of strike slip movement along sub-vertical fault planes. This means crustal blocks slid past each other. This implies oblique rifting with components of both extension and transverse movement between India and Seychelles. There are some really revealing field photos of this transverse (strike slip) movements.

2) Geology of the Elephanta Island fault zone, western Indian rifted margin, and its significance for understanding the Panvel flexure- Hrishikesh Samant, Ashwin Pundalik, Joseph D’souza, Hetu Sheth, Keegan Carmo, LoboKyle D’souza, Vanit Patel

Wait a minute. There are normal faults with downthrown blocks in this region too. And from the famous Elephanta Island. The fault planes dip eastwards producing easterly downthrows. That means the easterly crustal block has moved down. Again, some good field photos of fault planes and slickensides ( fault surfaces which get a polished striated appearance due to the frictional movement of rocks). These faults with easterly downthrows are found all along the west coast.  There is one near the proposed site of the nuclear power plant at Jaitapur in southern Maharashtra, which shows signs of intermittent movement over the past fifty thousand years. So, there is a very practical reason for understanding these faults.

3) Deccan Plateau Uplift: insights from parts of Western Uplands, Maharashtra, India- Vivek. S Kale, Gauri Dole, Devdutta Upasani and Shilpa Patil Pillai

This is a study of part of the Deccan plateau. I visited this region a few weeks back.  Very useful information of the various fracture systems that cut across the stacks of lava and their significance in terms of recent (Quaternary) crustal movements and controls on the drainage systems. Well thought out block diagrams illustrate the authors ideas very clearly.

4) Pahoehoe–a'a transitions in the lava flow fields of the western Deccan Traps, India-implications for emplacement dynamics, flood basalt architecture and volcanic stratigraphy-  Raymond A. Duraiswami, Purva Gadpallu, Tahira N. Shaikh, Neha Cardin

Good explanations of the morphology of basalt lava flows.  I really liked the sketches showing the internal structure of lava flows and the emplacement of pahoehoe lava fields with its transformation into transitional and a'a type lavas. Very useful guide for my next outing into the Deccan basalts!

Saturday, February 25, 2017

Field Photos: Western Uplands And Giant Plagioclase Basalts

Last Sunday I visited Chavand fort near the town of Junnar, about 110 km north of Pune. This is a rugged terrain marked by several NW-SE oriented ridges separated by broad U shaped valleys. In the map below the black cross marks the location of Chavand. WGE refers to the Western Ghat Escarpment and KCB refers to the Konkan Coastal Belt. Trekkers familiar with this region will recognize the hill ranges, especially the Bhimashankar range and the Harishchandragad range. And near the town of Junnar is Shivneri fort, birthplace of the Maratha king Shivaji.

Source: Kale et. al, 2016

When Deccan volcanism ended, this region would have been a vast flat- to- gently undulating lava surface. At that time, some 60 million years ago, you could have walked from where Bhimashankar temple now stands to the present location of Harishchandragad along broadly the same elevation without the need to climb down several hundred feet or so into a valley and then climb up again. Over time however, south easterly flowing rivers and tributaries have gouged out grooves within this large plateau, dissecting it into a valley and ridge terrain.

The geomorphology of this region therefore reflects the creation of relief due to removal of material by erosion. This contrasts with other areas like the famous Basin and Range Province in western United States, where parallel faults have moved blocks of crust hundreds of feet to form a system of flat bottomed valleys (grabens) and flat topped ridges (horsts).

The Western Uplands end abruptly along the Western Ghat Escarpment, a sinuous west facing cliff overlooking the Konkan coastal plain. The escarpment is the edge of the Deccan Plateau.

There is another factor that has shaped this landscape. Take a look below at a satellite imagery of this area. The arrows mark fracture systems that have broken this plateau.

Erosion along these zones of weakened rock results in slabs of basalts peeling on rock faces. Over time, the result is a landscape that fragments into mesas, buttes and pinnacles. Chavand is one such mesa. Notice its straight edged polygonal shape suggestive of erosion along fracture planes.

Many of these fracture systems originated in the tensional forces that the western margin of India experienced during rifting and associated Deccan volcanism. After its separation from Madagascar around 88 million years ago, the Indian continent's rifted and the fractured western margin migrated over the Reunion hotspot, an unusually hot area of the mantle. The result, beginning around 68 million years ago was Deccan volcanism. Some of these linear structures, common in the area around Sangamner, are dikes. They are the pipes which brought up magma from deep subsurface chambers to the surface. Continued rifting of the western margin resulted ultimately in crustal blocks subsiding along a series of N-S oriented parallel faults. The Western Ghat Escarpment likely originated as a west facing fault scarp, but it would have been located as much as a 100 km to the west of its present location. Erosion over million of  years has resulted in an eastward retreat of this feature to where it stands now.

Picture below shows how hill ranges have been broken by a fracture system, resulting in isolated pinnacles.

And here is a picture of Chavand.

Recently my friend Vivek Kale and colleagues complied some very interesting geomorphologic, structural and sedimentology data to suggest that these western uplands have experience some tectonic movements during Quaternary times (past 2.58 million years). They emphasize that the Deccan plateau and Western Upland should not be regarded as a monolithic stable crust block. They point to three major fracture systems (F1, F5, F7 in the map below) which have segmented this part of the western upland. The central segment, i.e. the area north of Chavand, roughly between fracture systems F1 and F5 has moved up relative to the blocks to the north and south. The presence of sediments deposited in the Pravara river system and along F1 and some streams to the south  is evidence that the these blocks subsided somewhat, resulting in stretches of streams becoming sediment traps.

Source: Adapted in Kale et. al 2016 from Dole 2000, Dole et. al. 2002, Bondre 2006

These sediments represent deposition over the past 100,000 years or so. At some localities along the Mahalungi river, they have been deformed. Soft sediment deformation structures such as slumping, load structures and sand dykes have been recorded by Dole and colleagues. Such structures are evidence of ground shaking and sediment liquefaction and remobilization during earthquakes.  Also observed at one locality is reverse faulting. The faulting has been inferred to be of Holocene age, as recent as the past 10,000 years.

These structural movements have also disrupted and modified the antecedent drainage of this region. The map above shows several easterly flowing streams (R. Mahaludi, R. Adula, R. Mula, R. Madvi, R. Pushavati) in this region abruptly turning southeast as they intersect NW-SE trending fractures. The yellow overlay on the map indicates sedimentary deposits. The major fracture systems F1, F5, F7 likely reflect faults in the Precambrian continental crust underlying the Deccan volcanics. They have been rejuvenated in Quaternary times and have cut across and caused dislocations of the volcanic pile.

There are other interesting drainage features in this region. Many streams have stretches with potholes (Nighoj on R. Kukdi), cascades (stretches of R. Pushpavati, R. Mula, R. Ghod, R. Bhama) and entrenched meandering channels (R. Pravara, R. Mula, R. Ghod, R. Vel) all suggesting episodes of increased vigor of stream down cutting. Whether this is a climatic signal (e.g. increase in rainfall will increase water flow and stream erosive power) or is tectonically triggered (e.g. slight uplift and  tilting of land will increase stream gradient, resulting in more vigorous stream flow and down cutting), as Kale and colleagues have recently argued, is an active area of research.

Finally, the giant plagioclase basalts. Plagioclase, which belongs to the feldspar family of minerals, is a major component of basalts. The entire Deccan volcanic lava sequence is subdivided into three subgroups based on geochemical differences. The giant plagioclase basalt lava flows occur predominantly in the lower part of the volcanic sequence. The table on the left (Kale et. al. 2016) shows the geochemical stratigraphy of the Deccan volcanic sequence with the location of the giant plagioclase basalts (GPB). The GPB flows cap individual formations within the Kalsubai Subgroup. This has been  interpreted by many geologists to mean that they mark the final eruptions of a magmatic cycle. Because of their distinctive appearance these GPB flows have proved to be useful as marker horizons in stratrigraphic mapping.

The plagioclase crystals are greater than 1 cm and often have grown to several centimeters long. They are surrounded by a fine grained to glassy matrix. The picture below is a close up of a giant plagioclase basalt, showing tabular plagioclase phenocrysts (arrows).

Geologists agree that these giant crystals grew slowly in magma chambers tens of kilometers deep in the subsurface. These crystals were then brought to the surface by ascending magma, which then cooled rapidly on the surface forming a fine grained to glassy matrix. This two stage crystallization history is the reason for the two distinct grain sizes in this rock.

The picture below shows Chavand fort hill face. The giant plagioclase basalt lava flow makes up the shrubby gentler slope.  These basalts are vesicular (containing pits and holes due to trapped gas bubbles in the lava) and softer and have weathered to form the gentler slopes. The upper harder and more compact basalt forms cliffs.  

There are differing views however on how long these crystals were growing in the subsurface and what that implies for the mode and duration of Deccan eruptions. I"ll leave the details for another post. Meanwhile, here is another picture of the giant plagioclase basalt showing lath shaped plagioclase grains (1) and a rosette of plagioclase phenocrysts (2).

...And a few more pictures of the terrain as seen from the top of Chavand.

A large water tank dug out from the hard basalt

Mesa top grasslands give way to the rugged ranges of the Western Uplands. This is a north facing view with the Harishchandra range at the far end.

A south facing view with the Bhimashankar range with its forested plateaus.

..did I mention it was a pretty steep climb?

Until next time...

Monday, February 20, 2017

Aristotle And Darwin: Why?

A thought provoking passage from The Lagoon: How Aristotle Invented Science by Armand Marie Leroi-

The history of Western thought is littered with teleologists. From fourth-century Attica to twenty-first century Kansas, the Argument from Design has never lost its appeal. Aristotle and Darwin, however, share the most unusual conviction that though the organic world is filled with design there is no designer. But if the designer is dead for whose benefit is design? It's the prosecutor's question: cui bono?

Darwin answered that individuals benefit. Biologists have batted the question about ever since. The answers they've essayed are : memes, genes, individuals, groups, species, some combination or all of the above. Aristotle, however, generally appears to agree with Darwin: organs exist for the sake of the survival and reproduction of individual animals. This is why so much of his biology seems so familiar.

Yet there is a deep difference between Aristotle's teleology and Darwin's adapatationism, one which appears when we follow the chain of explanation that any theory of organic design invites. Why does the elephant have a trunk? To snorkel. Why must it snorkel? Because it's slow and lives in swamps. Why is it slow? Because it's big. Why is it big? To defend itself. Why must it defend itself? Because it wants to survive and reproduce. Why does it want to survive and reproduce? Because..

Because natural selection has designed the elephant to reproduce itself. Darwin gave teleology a mechanistic explanation. He halted the march of whys.

Aristotle was an eternalist. In his cosmos, organic beings were produced by a union of their parents. They in turn by a union of their parents.. continued into infinite regress. Organisms wanted to survive and reproduce so that their "kind" persist for ever. This static world had always existed. He saw divinity in immortality.

In Aristotle's world organic beings don't change and transform into anything else. He never did come around proposing a theory of transmutation or change or evolution. He had some of the raw material to advance such thoughts.

His extensive dissections and comparative anatomy had given him an understanding that life is arranged in a hierarchical manner. Dogs and foxes are more closely related to each other than either is to lions and leopards. Both, canids and felines though are part of a larger mammal family. Members within this family are more similar to each other than they are to members of reptiles. Aristotle recognized that there are groups within groups. He termed the basic taxonomic units as genos. These are part of the larger magiste gene. Ikthis (fish), entoma (insects),ornis (birds), zootoka tetrapoda (live bearing tetrapods- mammals), oiotoka tetrpoda (egg laying tetrapods -reptiles, amphibians) were some of his "greatest kinds" or  magiste gene.  Yet, he didn't ponder upon the relationship between life's groupings and never realized that this pattern is a tree like structure, resulting from common ancestry and subsequent lineage branching. The figure to the left is Darwin's sketch (Notebook B 1837) of the tree of life depicting common ancestry and the branching nature of life.

Aristotle was aware that there is variation within each "kind" or genos. Organisms within a genos varied in their eidos or form. He also had a theory of inheritance. Parents passed on their eidos to progeny, often not exactly. Progeny then, occasionally, could be somewhat different from parents. Moreover, his understanding of inheritance was surprisingly modern. A child might inherit her father's nose or her mother's nose but not something in between. So, he had a particulate view of inheritance. This also meant that traits remain stable and may pass on unaltered over many generations.  Father's and mother's contributions don't blend to form some average feature, but remain discrete. By this he did not mean that actual particles were being transmitted. Rather, traits were reproduced by the movement and heat of either the semen or 'menses' (menstrual fluid). Whichever was stronger determined whether the child resembled her father or mother.

Darwin never understood inheritance very well. He was sure that variation is the  fuel of evolution. But he was troubled that blending would wipe out variation in populations. He struggled with the problem of inheritance for decades.

Aristotle's limitation was that he didn't think anomalous features or differences offered any advantages to the individual. His view was that creatures are born within the limits of their physiology and there was no room for improvement. In that, he should have listened to Socrates! In a Greek society with very particular notions of beauty, Socrates, with his snub nose and flabby lips was an anomaly. He boasted though that his lips worked better than anyone else's. The mutant feature provided an advantage. Aristotle rejected such notions.

Perhaps this is why he couldn't take the next step.... That advantageous variation could be selected upon by nature. Possessors of that trait would on average leave behind more progeny. A certain new form would thus become more common over generations and while some other form disappeared. Some people argue that the lack of a fossil record may have been one reason why Aristotle never appreciated that organic beings have changed over time. However, as Leroi describes, Greek travelers and physiologoi (naturalists) from his time had written about fossil sea shells found high on mountains and fish imprints on stone. Theophrastus, his student and protege, describes dug up ivory. Dwarf elephants were among the many remains from the Pleistocene megafaunal fossil beds of Samos, Kos and Tilos islands.

To someone wedded to an unchanging cosmos, this may not have made any difference, Leroi argues.

It would be more than two millennia before Darwin and Wallace put all the pieces together.

Highly Recommended.