Lewisian Gneiss Complex controversies - setting the scene

On a holiday in 2008 to the NW Highlands Geopark I had the opportunity to view the Lewisian Gneiss Complex that represented part of the Earths lower crust and the oldest dated rock in the Lewisian Gneiss complex is just over 3 Billion years old, a vast depth of time even by the standards of geologic time.

Lewisian Gneiss

The literature I had read beforehand was aimed at the general reader and hinted at the nature of the geology, but didn't quite prepare me for the actual field exposures. Most of the rocks I had seen up until then were of igneous, volcanic and sedimentary origin, deposited on the surface or at a high level in the continental crust, at a low grade of metamorphism and relatively undeformed. Some of the rocks of the Lewisian Gneiss Complex have undergone episodes of high grade metamorphism and intense deformation, to produce a distinctive and perplexing rock. To make sense of it requires an appreciation of the heat and forces that the rock has been subjected to.

Granite sheets intruding mafic gneiss and subjected to deformation in a tectonothermal event.

There is unanimous consensus amongst geologists that pressure and temperature increases with depth in the Earth and that mechanical properties of rock progressively changes with depth in the crust, from brittle to ductile and finally molten.  Rocks buried deep in the earth, behave more like plasticine than those at the surface.  Fig 1 is an illustration of a simplistic high grade deformation of a rock body intruded by plutonic rocks and then subjected to progressive higher grades of deformation in a single episode. A ~ original relationships. B, C and D ~ increasing deformation or strain. To get to D requires a surprisingly large dimension image files in photoshop to simulate the strain to generate Gneiss banding.

Figure 1 Simulation of the intense strain to deform rock into Gneiss  

Gneiss is the name given to a metamorphic rock that has undergone intense deformation at high temperatures and pressures.  Fig 2 illustrates how previous deformation events can be overprinted by an event the causes intense deformation.

Fig 2 -  Intense deformation overprinting previous deformation events

Rocks that are several billion years old have unsurprisingly undergone several deformation events at high temperature and pressure (tectonothermal events). The banding in gneiss allows relict structures to be preserved in areas of relatively low strain and provide an insight into the geological history. Fig 3 A is a block of banded gneiss. B is a deformation event inducing structural folds with vertical axes. C is a deformation event causing a fold with a horizontal axis. D and E represent progressive deformation. F is a detail of the recumbent isoclinal folds

Fig 3 - Deformation of banded gneiss and relict structure

Recumbent folds in Lewisian Gneiss

Lewisian Gneiss terrain and cross cutting pegmatite dykes. Note figure for scale.

Illustrated cross cutting pegmatite dykes

Cross cutting relationships detail - Gneiss cut by pegmatite dyke and then both are cut by a 2nd (pink) pegmatite dyke

Highly inclined in-weathered Scourie Dyke cutting through Lewisian Gneiss

Illustrative interpretation of in-weathered Scourie Dyke cutting Lewisian Gneiss

In the late 19th Century, officers of the Great British Geological Survey started work mapping, recording, sampling and examining the petrology of the constituent rocks of the mainland Lewisian Gneiss Complex, to elucidate its history and nature. In 1907 a memoir titled "The Geological Structure of the North West Highlands of Scotland" was published, that covered the observations and findings of some of Great Britain's most experienced and respected geologists.

Gneiss (Grey) intruded by Scourie Dyke (Black) then intruded by Granite (Pink) and deformed by tectonothermal event(s)

The Geological Survey broke down the Lewisian Gneiss Complex into two divisions 1) Gneisses, the majority of which had affinities to plutonic igneous rocks with a wide range of petrological characteristics and a minority of presumed sedimentary rocks. 2) A great series of intrusive rocks that like the gneiss had a wide petrographical range. Intrusive rocks were used as markers to constrain deformation and mineral changes within the intrusive rocks and provided evidence and clues on the phenomenon of metamorphism.

Realtively low strain outcrop of Mafic gneiss (Dark) veined by Tonalite or Tondhjemite (white). Possibly a migmatite.

The effects of earth movements and metamorphism were also recognised in the structure with planes of shear, folding and thrusts, by noting the changes in gneiss banding orientation, deformation of intrusive rocks, relict structures and changes in mineral assemblages. The deformation intensity was to some extent heterogenous and also varied in style through the districts.

Weathered exposure of gneiss with a refolded fold.

Deformation and metamorphism were heterogenous in nature at all scales, it was observed that a rocks mineral assemblages could change across a hand specimen or even in a sample prepared for microscopic examination. The phenomena of changes in mineral assemblage of rock also seemed independent of deformation and gneisses with pyroxene passed imperceptibly into hornblende by the hornblende replacing pyroxene.

Highly inclined pegmatite sheet cutting moderately inclined gneiss 

Despite the great heterogeneity of rock petrology and deformation, the Lewisian Gneiss Complex was divided into 3 districts : North, Central and South, on the basis of each district having its own fairly distinct mineral assemblages and representative styles of structural deformation.

Granite sheets intruding mafic gneiss and isoclinally folded 

The fieldwork and petrological analysis by officers of the Geological Survey produced a solid foundation for geologists to continue further investigations into the geological processes that had acted upon the Lewisian Gneiss Complex. The next post will review some of the controversies the geological processes that have formed the Lewisian Gneiss Complex and its history.

Geology Controversies

I find it interesting to read about geology controversies and the rationale underpinning the opposing hypotheses, that provides insight into the evolution of the 'discussions' to a general consensus. The greater the controversy, the more likelihood it has of making it to a wider audience and that is when someone with sufficient perspective can outline the respective arguments in terminology a non specialist can understand. Geology controversies tend to be long running affairs as entrenched parties present more evidence to substantiate their hypotheses and refute opposing hypotheses. When the evidence comprises actual bedrock outcrops, there is also the opportunity to walk the same sections and understand better their significance.

Some older major controversies have been well documented with books written to provide an historical overview of the controversy. Two books that I found particular interesting as they cover rocks in areas that I have visited, deal comprehensively two notable 19th Century controversies in the UK :

The Cambrian Silurian controversy  ~ Ordering geological stratigraphy which ultimately resulted in the establishment of the Ordovician.



Highlands Controversy ~ A bitter dispute on geological stratigraphy and a triumph of geological fieldwork to elucidate the tectonic structure.



It would be reasonable to suggest that geologists have a greater appetite for robust discussion than other sciences, which does add a certain frission to reading a geology paper on a controversial topic. One current long running 'heated' controversy is Archean Plate tectonics and as an example of robust discussion I have cherry picked some statements from the Introduction to a paper by Warren B. Hamilton in a memoir by the Geological Society of America.

"This essay is a study in alternatives. Most current interpretations of geodynamics and of evolution of Earth are forced to fit popular but dubious assumptions that Earth fractionated slowly and is still largely unfractionated, and that rocks of all ages must be explained with plate-tectonic processes combined with plumes rising from basal mantle. The data accord better with opposite interpretations. Earth largely fractionated very early in its history, plate tectonics did not begin operating until late Proterozoic time, and deep-mantle plumes do not operate now and did not affect the Archean Earth."

"Our planet had a hot, violent beginning, and has evolved only slowly toward its present dynamic patterns. The geologic record of the young Earth differs profoundly from that of the modern one in crustal architecture and in rock types, assemblages, and structural and magmatic histories."

"I have seen hundreds of exposures of early Paleozoic to middle Tertiary subduction mélanges around the world, but not a suggestion of one in the Archean."

"I see no plate-tectonic interactions in the Archean record. Most plate-tectonic interpretations for the Archean are based on weak compositional analogies with modern rocks of known settings, or with imagined products of hypothetical plate-related settings that have no modern analogues. Although this is a valid approach in the search for explanations, its implicit predictions are not then tested against geologic data." 

"The cartoons drawn to illustrate such schemes (e.g., Kerrich and Polat, 2006) are unrelated to anything seen on the ground." 

"Modern-mode plate tectonics, with high-pressure, low-temperature metamorphism in its sutures, began only much later, in Neoproterozoic or very early Paleozoic time (Stern, 2005, 2007; Tsujimori et al., 2006)." 

Hamilton, W.B., 2007, Earth’s first two billion years—The era of internally mobile crust, in Hatcher, R.D., Jr., Carlson, M.P., McBride, J.H., and Martínez Catalán, J.R., eds., 4-D Framework of Continental Crust: Geological Society of America Memoir 200, p. 233–296  

In the next post I will look at an area of the UK that with an exposure of Archean bedrock and the current controversies associated with it.

Mise-en-scène

With vast sums of money in play in the film and television industries, there is an incentive to examine the variables that contribute to successful and not so successful productions. It is not an exact science as there are some productions that are phenomenally financially successful whilst failing to satisfy many variables considered important and conversely there are productions that are miserable financial failures, despite satisfying many variables considered important. One of the variables is the Mise-en-scène, a term that appears to have many definitions for what is in a scene that contributes to the film/production narrative.  The average feature film has 144,000 frames, the success or failure of a feature film rarely hinges upon a single frame, whereas landscape photography is all about a single frame.

In my humble opinion mise-en-scène has some relevance to a landscape photographer in determining a narrative for an image. For viewers of landscape photography, especially critical ones, there's the objective application of the mise-en-scène to an image. Subjectively you may not like an image, but objectively you can appreciate an image by considering the mise-en-scène.

The wearing down of mountains ~ Norway 2012

The image above was one that resulted from a desire to make a landscape photograph with a narrative of active geomorphological processes. The last glaciers melted somewhere around 12,000 years ago in the UK, so a trip was made to Norway and Google maps identified a suitable area. The foreground is an outwash plain with a braided meltwater stream undergoing channel avulsion. The backdrop is a glaciated mountain environment, with some relict glacial landforms and mantled in part by flora in autumnal colour. The lighting could be better, but I'll take dappled sunlight over clear blue sky/leaden blanket cloud. The image will not be to most viewers liking, that's the nature of the world, but appraising the mise-en-scène and hopefully an objective appreciation of the image narrative becomes apparent.

The narrative of an image

A recent review of my image archive and a question I repeatedly asked myself :

What is the narrative of the image?

Many of my early images were trying to conform with paradigms of composition rules for landscape photography : lead in lines, zigzags, rule of thirds and with the benefit of hindsight way, way too contrived. Some may think this a virtue, it really isn't, in too many instances it just disguised my ignorance of the landscape. An image narrative of ticking composition boxes is no substitute for interest and it is dispiriting to make images that hold very little interest.

2008 ~ Composition is no substitute for narrative

The images that held appeal were those that conveyed interest in the landscape and generally where I actually knew something about the subject matter. Which exposed an inconvenient truth: There were many aspects of the landscape that I was blissfully ignorant of. I needed to understand the landscape and know something about the landscape I was attempting to photograph.

2008 ~ A narrative of geology, ice phenomena and geomorphology.

The allure of waterfalls

The 18th/19th Century witnessed Great Britain's leisured classes undertaking tours specifically to view and experience landscape scenery from the perspective of Romanticism and then specifically to locations that were deemed to satisfy the aesthetic ideals of the Picturesque. It was a time that heralded the start of the British tourism industry, with guides, guidebooks, transport, accommodation and infrastructure, to satisfy the leisured classes. It is a happy coincidence that waterfalls are generally best experienced with high water flows and an ideal subject for itineraries in an oceanic climate, when weather negates the views from elevated locations.  Two centuries on and waterfalls, provide inspiration for landscape photographers, on days when weather negates elevated views.

From a purely aesthetic perspective there is something visually compelling in the juxtaposition and interplay of colours from white water, dark rock and verdant greenery. The camera may not capture the sound, but it can capture movement and long exposures create an ethereal effect on foaming white water.

Coincidentally in the 18th Century, waterfalls were also the focus of inquiry by early geologists, who investigated their relationship with the underlying geology to elucidate their nature. The geologists determined that waterfalls exploited weaknesses in the bedrock and their nature influenced by : joints, rock competencies, fractures, cleavage, bedding planes, faults, structures etc The character of a waterfall was determined by Geological control. There was also a dawning realisation that the waterfalls in the context of the wider landscape, represented a vast amount of time, to allow for fluvial erosion of bedrock. The vast time for waterfall formation were used by geologists championing Uniformitarianism and challenging the prevailing paradigm of Catastrophism.

An enlightening exercise

After years of prevarication and half hearted attempts, I recently undertook a comprehensive image library review, primarily to delete images and free up disk space. As it turned out, it proved to be an exercise that was enlightening for a number of reasons. On a mundane level a not insignificant number of images were ruthlessly deleted, for varied reasons, many of which I would euphemistically categorise as embarrassingly poor demonstrating considerable scope for improvement.  What was particularly thought provoking was revisiting images with fresh eyes and a measure of objectivity.

Locations to be revisited

A number of areas/locations had fallen off my radar in recent years, I have no idea why, nonetheless I'll rectify that with some future excursions.

2009 ~ A revisit is long overdue to this area of the Cambrian mountains

Weather, Air clarity and Lighting

It was disheartening to view images that suffered from poor air visibilty, numerous excursions suffered from haze caused by sublimated frost, water vapour, particulate matter or wildfires.

2014 ~ Haze in the Cairngorms

2011 ~ Haze courtesy of wild fire smoke, clear skies courtesy of a weather front that failed to materialise.

Then there were days when the cloud didn't break up.

2011 ~ Geologically compelling scenery that would benefit from more than flat lighting

Or hill fog put in an unwelcome appearance

2012 - Hill fog bubbling up at sunset on Aran Fawddwy

Then there were forecasts where the cloud base was too low.

2013 ~ Low cloud base in the Cuillinsv

I make a point of taking and saving a screenshot of the weather forecast before a trip, to see the difference between theory and practice. It certainly pays to be philosophical and accepting error margins with weather forecasts, as there are also days when the cloud base lifts and a break appears.

2013 ~ Cloud base lifts and a break appears in the Black Mountains

The attrition of wind vibration on image quality can be profound and unsurprisingly there are a number of 'soft' images deleted. Wind afflicted excursions are signified in my image library by a series of apparently identically framed images.

2015 ~ crop of wind vibration effect on image quality

Then there are wrong season and/or wrong place and/or wrong time, AKA hard earned field experience.

Experimentation

2009 ~ Polarising filter and peat stained water - not the best. 

In a moment of weakness I purchased a polarising filter and was underwhelmed with the lurid colours it produced across a range of subject matter. The small positive was the insight into marketing of photography products.

Image sets taken for panoramic stitching, in practise frustrating to execute well in dynamic lighting of dawn/dusk and/or atmospheric weather. I cannot recall who said it, but I wholeheartedly agree with the statement :

"A panoramic stitched image looks exactly like a stitched panoramic image, taken in weather and lighting amenable for panoramic stitching."

Sometimes it pays to set a manual white balance, most oftentimes it doesn't.

Philosophical

2009 ~ Abstract view of Dam overflow

From 2006 I followed my landscape photography journey and noted the change in subject matter preferences. Getting to grips with the challenge of woodland photography, the drift away from coast and seascapes, woodland plantations, ancient sites and the increase in visits to locations rich in geology and ecology.

2011 ~ Stone circle

It was a very worthwhile exercise and one I would recommend for the insight gained.

Snowdonia trip - December

The opportunity arose at the start of December for another week away and after very little thought I decided on Snowdonia National Park, a search for accommodation on the Friday, resulted in a cottage booked up for the next day. Nic, was left at home to fend for herself and look after the cat, whilst moi, would enjoy a week with no one to please, but myself. It's a tough life.

Migneint, Snowdonia National Park

I have a reasonable overview of the Snowdonia National Park and booked a cottage in Blaenau Ffestiniog a central location and with good road access to other parts of the Snowdonia NP. There are also interesting landscapes and geology in close proximity to Blaenau Ffestiniog, these being : Ceunants, Migneint and Moelwyns. Excursion itineraries were subject to the weather and weather resilience, with a list of locations to explore and subject matter, to cherry pick from : hills, moorland, ceunantydd, geology, mosses and lichens.

Ceunant

Ceunants are the Welsh name for gorges and ravines that have been fluvially eroded into the bedrock by water draining the adjacent higher ground and possibly sub glacial drainage. One ceunant is also a Geoconservation Review site for its fluvial geomorphology. Geologists have estimated their development has taken place over millions of years.

Ceunant

The steepness of slopes and inaccessibility of ceunants to grazing herbivores means that most have ancient semi natural woodland. The high annual rainfall maintains a high humidity and an ideal habitat for the bryophytes and lichens, of the Celtic Rain forest. These fragmented woodlands are part of a Special Area of Conservation (SAC) for old sessile oak woods.

Taylor's flapwort (Mylia taylorii) 

It would be fair to state that the Migneint is an extensive area of upland bog and moorland, courtesy of the climate and elevation. It is also where ice accumulated during Quaternary glaciations and evidence of ice flow can be gauged from glaciated bedrock and the quarrying/plucking of exposed bedrock outcrops.

There are also some compelling views of the Migneint and from it.

The Moelwynion are a range of hills, overlooking Blaenau Ffestiniog, little visited by walkers, who overlook them for the hill ranges of more commanding stature. Nonetheless the views are good.

Flush lines, blanket peat and glacially scoured depressions, provide a habitat for sphagnum mosses.

Whilst bryophyte and lichen communities take advantage of bedrock outcrops and boulders.

The Snowdonia National Park is predominantly underlain by Cambrian and Ordovician aged, sedimentary, volcanic and igneous rocks, that have been weakly metamorphosed and folded during the Caledonian orogeny. Some of the rocks tell the story of explosive volcanic eruptions generating pyroclastic flows with bedding surfaces, indicating pulsed flows, ash still hot enough to weld when it settled and volcanic pyroclastic breccia.

Pyroclastic flow with planar foliation and cross bedding 

Welded tuff with fiamme in-weathered

Volcanic pyroclastic breccia

I also chanced upon bedrock outcrops with igneous intrusions of flow banded rhyolite into tuff, cut by vein quartz (segregated quartz?) and intrusive rhyolite breccia with net veins of quartz.

Some outcrops of intrusive rhyolite presented an opportunity to photograph miniature landscape with an accompanying flora of saxicolous mosses and lichens

One dawn outing, suffered from a number of unforeseen events and own goals, on the journey and at the location, that culminated in a radical change of plan. The dawn colour was good, followed by a brief window of sunlight, before the sun rose into cloud.

Overall a good trip, a mix of weather, varied subject matter and plenty of photography opportunities.