Wednesday, August 30, 2017

Mapping: In Praise Of The Triangle

Jerry Brotton in his book A History Of The World In 12 Maps writes about France's National Map Project. Begun around the 1670's upon the establishment of the Academie de Sciences and the Paris Observatory and headed by the astronomer Cassini I, it first attempted to create an accurate geodetic survey of France using the latest surveying instruments. At the heart of the survey was the calculation of distances and directions using the method of triangulation. Latitude was calculated using a quadrant that measured the altitude of celestial bodies. Then, using a measuring stick, a baseline of a known length was established. A third point on the landscape was sighted. The angles between the three control points were measured. Using trigonometric tables the lengths of the remaining two sides of the triangle could be calculated.

I liked this passage:

In 1744 the survey was finally completed. Its geometers had completed an extraordinary 800 principal triangles and nineteen base lines. Cassini III had always envisaged printing regional maps as they were produced, and by 1744 the map was published in eighteen sheets. Its new map of France, on an approximately small scale of 1: 1,800,000, shows the country represented as a network of triangles, with virtually no expression of the land's physical contours,and with large areas such as the Pyrenees, the Jura and the Alps left blank. It was a geometrical skeleton,a series of points,lines and triangles following coasts, valleys and plains in connecting key locations from which observations were carried out. Over it all lay the triangle, the new immutable symbol of rational, verifiable scientific method. On Cassini III's map the triangle almost takes on its own physical reality, a sign of the triumph of the immutable laws of geometry and mathematics over the vast, messy chaos of the terrestrial world. The Babylonians and the Greeks had revered the circle, the Chinese celebrated the square, the French now showed that it was the application of the triangle that would ultimately conquer the earth.

Cassini III was the grandson of Giovanni Domenico Cassini (Cassini I). The directorship of the Paris Observatory remained in the Cassini family over four generations.

This surveying method was quickly adopted and adapted by others. The Ordnance Survey began mapping the British Isles using this method in the late 1700's.  William Lambton took the Ordnance Survey's acquired expertise and began the Great Trigonometrical Survey of India in the year 1800, a feat that took nearly 50 years to complete. John Keay's book The Great Arc details that mammoth effort.

Thursday, August 24, 2017

Field Photos: Glacial Deposits Of The Darma Valley, Kumaon Himalaya

During my recent trek to the Panchachuli Glacier in the Kumaon Himalaya, I obsessed about observing changes in metamorphic grade of the Greater Himalayan Sequence on the trek route and also about finding the South Tibetan Detachment fault system. I wrote about this in an earlier post.

But there were other interesting geological observations too. The Panchachuli Glacier has left a thick record of glacial deposits. The river Dhauliganga originates from this glacier. Along this river valley, glacial deposits can be observed to a distance of at least 5 kilometers downstream of the present location of the snout of the glacier, indicating that the glacier was much more extensive in the past. Tributary glaciers flowing out of the ranges east of the Dhauliganga have also left an extensive record in the form of thick fluvio-glacial deposits. These can be observed as far south as the village of Baaling.

We heard anecdotes in village Dugtu about how this glacier was much bigger in living memory and how it has been receding rapidly in the past few decades. On one level such stories are believable because studies of Himalayan glaciers have shown that many of them have been shrinking over the past few decades (ref). This is partly due to anthropogenic global warming, but glacial response to warming may be varied due to local variations in topography, precipitation and wind conditions. Some glaciers don't show retreat while some are actually seen to be expanding. Overall though, there a substantial ice loss observed across the Himalaya. Exactly how much of that is due to recent global warming and how much, as some scientists caution, due to natural factors is still being studied. Sustained warming though will cause these glacier to shrink further over the next century.

There is also a longer geological story of glacial advance and retreat written in these deposits.

I've embedded below an annotated interactive map of the glacial deposits of the Dhauliganga river valley in the Panchachuli Glacier area. This will enable readers to zoom in and recognize the various glacial landforms present in the valley. You can also access it via this Permanent Link.

The annotations depict:

a) The dark blue lines are the snout of the glacier.
b) The light blue lines are the recent terminal moriane fields.
c) The pink lines are older lateral moraines.
d) The yellow lines are outlines of older fluvio-glacial deposits
e) Numbers 1 -12 mark the locations of glacial deposits.

I have mapped only a few representative examples of each of the feature types. Readers can use these to explore similar features scattered throughout the valley. 

Location 1: This is the snout of the glacier. It is a mass of ice and frozen mud. The river Dhauliganga emerges out of an ice cave.

Location 2: Taken from near the snout of the glacier looking downstream. Ridges of the terminal moraine can be seen in the foreground. The arrows in the background outline a ridge of an older lateral moraine. Notice how the ridge decreases in elevation downstream suggesting that the terminus of this older glacial phase in somewhere nearby downstream.

Location 3: The older lateral moraine can be clearly seen as a sharp ridge line (arrow) separated from the valley wall by a depression. 

This moraine top is a few hundred meters above the valley floor implying that the glacier was thicker in the past. When was this lateral moraine deposited? It may be at least a few hundred years old. In the Garhwal Himalaya, similar older lateral moraines close to the glacier has been dated to be several hundred years old. They have been interpreted to be a result of glacial growth and deposition during the Little Ice Age, a period of earth cooling and climate instability that lasted from around the 1300's to the mid 1800's (for more on this climatic episode, I recommend Brian Fagan's book The Little Ice Age: How Climate Made History 1300-1850).

Location 4: A view of the glacier and an older lateral moraine (arrow) on the other side of the valley.

Location 5: Further downstream are thick glacial deposits. The river has incised or cut through these sediments. As a result the deposits form flattish plateaus or terraces that hug the mountain slopes. Village Dugtu, where we stayed, has been built on top of one such glacial terrace. The arrow in the top picture points to an exposure of these glacial deposits. A close up of this deposit is seen in the bottom picture. Notice the extremely ill sorted texture. Such ill sorted sediment deposited by glaciers is called Till. Large boulders are mixed in with  gravel, pebbles and much finer sized rock flour (the light to brown colored matrix).

Location 6: Another exposure of a glacial deposit near Dugtu. Again, notice the ill sorted deposit. However, at the top is a well sorted pebbly layer. This suggests deposition in more vigorous flowing water. Glacial retreat from time to time would have resulted in the establishment of a fluvial regime and deposition in these streams. These deposits may be a few hundred to several thousand years old.

Location 7: The glacial terrace on which village Dugtu is built is seen in the lower right corner. Farther away is village Philam built on the thick fluvio-glacial deposits of a tributary glacier originating in the range east of Dugtu. At village Dugtu, the east flowing river Dhauliganga makes a sharp southerly turn. The river has cut through these deposits and the slopes of the valley are thickly forested suggesting the great antiquity of these deposits.

Location 8: A nice view of glacial deposits south of village Baun along a smaller tributary of the  Dhauliganga. Notice the waterfall!

Location 9: A walk right through these thick fluvio-glacial deposits along a forested section of the valley slope. Again, notice the ill sorted nature of the deposits. Glacier are viscous and cannot sort sedimentary particles like water or air can. The result is a jumble of boulder, gravel and rock flour.

Location 10: Another cliff made up of fluvio-glacial deposits. I'm calling the deposits east of Dugtu as fluvio-glacial, since I observed intervals which show layering. This suggest deposition in water, either in streams or in melt water lakes and ponds that form in front of glaciers.

Location 11: A thick sequence of fluvio-glacial deposits along the Dhauliganga river. If you zoom and pan the satellite image you can recognize these terraces  southwards almost up to the village of Baaling.

I did not observe such deposits south of Baaling. However, there are smaller glaciers, such as the Naagling glacier, originating in the ranges on either side of the Dhauliganga. There would be smaller deposits scattered in these tributary valleys.

I have been vague about how old these deposits could be. If we assume that the Panchachuli glacier would have attained its maximum extent in the Pleistocene during the Last Glacial Maximum about 20,000 years ago, then the deposits furthest away from the present location of the glacier would be the oldest. As the glacier recedes one should find younger and younger deposits closer to the active glacier.

A study by Dirk Scherler and colleagues in the Garhwal Himalaya found such a pattern. They studied deposits of the prominent Jaundhar Glacier and the Bandarpunch Glacier in the Tons Valley. I've posted below a map showing the interpreted ages of deposition of glacial sediments.

 Source: Scherler et. al. 2010

Notice how the oldest deposits are further away from the present location of the glaciers (eastern most extremity of the map). These oldest deposits point to the maximum extent of the glacier that was reached in the Pleistocene during the Last Glacial Maximum.  However, the decreasing ages of the deposits upstream aren't the result of a uniform recession of the glacier. Instead, they point to several glacial episodes during which the glacier advanced, then receded, and then advanced again during the Holocene. Their data shows five such episodes of glacial growth dated to approximately 16 ka (ka = thousand years ago), 11-12 ka, 8-9 ka, 5 ka and less than 1 ka.

It turns out that the climate history of the Holocene is not one of uniform warming since the end of the last glacial period. The earth has gone through several minor cooling phases during the Holocene. The well known Younger Dryas Event around 12.9 -11.7 ka is one example.  Some studies suggest cooling episodes around 8.2 ka  and around 4.2 ka . And there is the Little Ice Age during the last millennium.

Another climate dynamic is fluctuating monsoon strength through the Holocene. The authors don't favor the explanation that these periods of glacial growth were triggered by global cooling events.  They argue that glacial growth corresponds to small phases of increased monsoon strength interrupting a longer trend of decreasing monsoon strength. More moisture means more snow and glacial growth. Since the long term trend in this part of the world is one of decreasing monsoon strength, every successive phase of glacial growth was smaller than the previous, resulting in younger and younger deposits upstream. The Little Ice Age deposits (which were likely driven by global cooling and not necessarily increased precipitation) mark the last major phase of glacial growth.

How are these deposits dated? Scherler and colleagues use a technique known as cosmogenic nuclide dating. This technique is one way to date the timing of surface exposure. Glaciers carry rock debris. These form a layer below the moving ice. When the glacier recedes the rock debris is deposited as a moraine or as an erratic boulder. It is exposed to the atmosphere and starts getting bombarded by cosmic rays. Energetic cosmic ray neutrons falling on atoms of minerals like quartz results in spallation reactions. This means that the collision of neutrons is energetic enough to fragment the nucleus. Oxygen bound up with silicon in the mineral quartz gets converted to an isotope of Beryllium (10Be). The amount of nuclides generated this way is proportional to the length of exposure. By measuring the amount of 10Be and comparing it with other isotopes, an 'exposure age' is estimated. This is essentially the age of glacial recession and the deposition of glacial sediment.

Samples have to been selected carefully for this method to give a true estimate of surface exposure and deposition. Care must be taken to avoid sampling rocks that have been repeatedly buried and exposed. Rocks which show signs of being subjected to prolonged glacial erosion are selected since  erosion will remove outer shells of material that may have accumulated nuclides during an earlier period of exposure.  Debris with a polished surface or with striations and grooves generally suggest subglacial transport and prolonged glacial erosion and are preferred samples.

 The figure below taken from the same study shows the reconstructed glacial extents using exposure dates of the moraine sequences in the upper Tons Valley.

Source: Scherler et. al. 2010

Such dating of glacial deposits at other locations in the Garhwal Himalaya (ref) tell a similar story of glacial growth and decay over the Holocene. And what about the Pleistocene? Is there evidence of older glacial cycles in the Himalaya? There are many studies that have identified glacial phases during the Pleistocene as well. For example, in northwest Garhwal, the Bhagirathi Glacial Stage has been dated to 63 ka (ref). And in the Ladakh Himalaya the oldest glacial stage has been dated to 430 ka (ref). Pleistocene ice ages have impacted glacial dynamics in the Himalaya too although more work needs to be done to understand the specific mechanisms of glaciation.

Location 12: Its back to the Dhauliganga valley floor. This moraine ridge (arrows) may be the remnant of an older terminal moraine. It is located about 2 kilometers downstream of the glacier.

The Panchachuli and other glaciers in the Kumaon region to the east of the Garhwal will also have their own history of past glory and recession. How much of the retreat of the Panchachuli and other Kumaon glaciers due to recent global warming?  And what is its fate? Hopefully, someone will study them with more precision in the future.

Tuesday, August 15, 2017

Links: Human Evolution

Sharing links to interesting articles I have read in the past few months. Better understanding of human evolution is being driven by a) New fossil finds giving valuable insights into morphologic variation and geography, b) DNA analysis of both modern and extinct populations giving us an understanding of genealogical relationships and migration histories and c) better absolute dating of fossils that constraint evolutionary scenarios.

1) What Are Our Best Clues To The Evolution Of Fire-Making? Anthropologist Barbara J King examines the physical evidence of fire making by ancient hominins and presents speculations on how natural fires may have played a role in hominin cultural evolution.

2) A world map of Neanderthal and Denisovan ancestry in modern humans- Phys.Org. " There are certain classes of genes that modern humans inherited from the archaic humans with whom they interbred, which may have helped the modern humans to adapt to the new environments in which they arrived," says senior author David Reich, a geneticist at Harvard Medical School and the Broad Institute. "On the flip side, there was negative selection to systematically remove ancestry that may have been problematic from modern humans. We can document this removal over the 40,000 years since these admixtures occurred."

3) Three new discoveries in a month rock our African origins- Prof. John Hawks on new fossil dating of hominin fossils from Morocco and evidence from archaic DNA from S. Africa that complicates the African story of the origins of Homo sapiens. The scenario suggested here is that Homo sapiens did not evolve due to changes in a population which was genetically isolated from other Pleistocene African hominin groups. Rather there was a pan-African gene flow. This is multi-regionalism within Africa.

4) Out of North Africa- Dienekes argues the exact opposite.. that the Morocco fossils imply that Homo sapiens evolved in north Africa from a reproductively isolated population and that multi-regionalism is wrong.

5) Features of the Grecian ape raise questions about early hominins- Did the hominin clade evolve in Europe and not Africa? Prof. John Hawk's critique of a recent paper suggesting that view. He cautions that convergent evolution is common among different hominin lineages. A single feature, such as the mandible used in this paper, cannot indicate relationships.

6) Early modern humans in Sumatra before the Toba eruption- Steve Drury in Earth Pages summarizes new evidence that indicates early ( more than 70,000 years ago) migration of Homo sapiens into SE Asia. .." Together with the dating of the earliest Australians the Sumatran evidence is at odds with the view, widely held by palaeoanthropologists, that the ‘Out of Africa’ exodus began by crossing the Straits of Bab el Mandab between 74 and 58 ka when global sea-level fell markedly during marine oxygen-isotope Stage 4 (MIS4). A problem with that hypothesis has been that climatic and ecological conditions in southern Asia during MIS4 were unfavourable. But is seems that modern humans were already there and capable of adapting to both the climate shift and to the devastation undoubtedly caused by Toba."

Thursday, August 10, 2017

China Hydraulic Engineering - Yellow River And The Grand Canal

I am reading Philip Ball's excellent book The Water Kingdom: A Secret History Of China. It describes the epic problems of river flow management that China has grappled with over millennia. Enormous floods have always ravaged China. At the site of a large dam at Sanmenxia on the Yellow River is an inscription in honor of the Great Yu ( ~ 2200- 2100 B.C), who as the story goes,  conquered a flood. The inscription says " When the Yellow River is at Peace, the Nation is at Peace". Flood control required enormous civic resources and cooperation and the ability to tame river waters gave political and moral legitimacy to the ruling class.The taming of nature using cooperative people power and as a sign of a strong united society has deep roots in Chinese political thinking.

One particular vexing problem was the very high rates of silt load carried by the Yellow River. A vast area of the Yellow River watershed drains the Loess deposits of north central China. This is a plateau made up of loose friable sand and silt blown in from the Gobi desert of Mongolia. Erosion of Loess fills the river with sediment. There is 300 grams of sediment for every kilogram of Yellow River water giving the river a reddish golden color. Erosion has carved the Loess Plateau in to a landscape of ravines and gorges. The American journalist Edgar Snow in the 1930's described it thus:

" an infinite variety of queer, embattled shapes - hills, like great castles, like ranges torn by some giant hand, leaving  behind the imprint of angry fingers. Fantastic, incredible and sometimes frightening shapes, a world configurated by a mad God - and sometimes a world of strange surrealist beauty. "

High rates of sedimentation meant that the Yellow River bed could aggrade or rise, increasing the risk of the river breaking its banks and flooding the countryside. Dyke building to constraint the river channel began as early as the seventh century B.C. by the state of Qi.

Constant dredging of the Yellow River and the associated tributaries and canal systems ( the Grand Canal) was also required to maintain a channel deep enough for navigation to move armies and grains from south to north.  River channel and canal maintenance acquired a new urgency when Zhu Di known as the Yongle Emperor of the Ming dynasty moved the capital north from Nanjing in the eastern province of Jiansu to Beijing in the early 1400's. The rational was probably to keep the political center closer to the armies amassed on the northern frontier where the Ming faced a threat from the Mongol and Manchurian steppe people. Later in the 1600's, the Manchurians overthrew the Ming and established the Qing Dynasty. Although over time, the Qing assimilated into the larger Han cultural milieu, they felt more at home in the north of the country. That meant the Yellow River and the Grand Canal system had to kept in top navigable order.

Desperation to unclog  the river channel spurred technological innovation. 

An extract:

Removing silt from the Yellow River demanded some impressive technology, not to mention serious organization. The Song government set up a Yellow River Dredging Commission in 1073 which began to deploy boats equipped with dredging tools. The vividly named  "iron dragon-claw silt dispersing machine" was a great rake pulled along the riverbed to agitate the silt and return it to the flow. This principle was extended with the 'river-deepening harrow', a 2.5 metre-long rotating beam fitted with iron spikes, like a thresher for riverine mud. The Ming imperial censor Chen Bangke introduced new techniques in the late sixteenth century, such as wooden machines set rolling and vibrating by the current to constantly stir up the sediment. In the dry season Chen proposed simply digging out the silt manually.

The Ming official Pan Jixun in 1565 or so came up with a solution that has made him one of China's water heroes. He pointed out that if one confines the water flow to a narrow channel, it will have enough strength to scour the sediment off the river bed. There would be minimal need for laborious manual dredging. He may have borrowed the  idea from a Confucian text of the Han era (~200 B.C - 220 A.D) called Zhou li (Rites of Zhou) which stated "A good canal is scoured by its own water"

Chinese philosophical tradition impacted river management strategies. Daoists argued that the river be given room to spread and build wide floodplains in concert with the principle of wu wei which could be read to mean "do nothing" or "having a yielding attitude".  Confucians on the other hand wanted the river to be managed by human  engineering and recommended the construction of high dykes to keep the river channel narrow and constrained.

Joseph Needham, the noted historian of China writes that 'during twenty centuries the two schools contented'.. 'and neither proved wholly successful'.

Highly Recommended.