Monthly Archives: July 2018

Atlas of the Dalradian from Scotland and Ireland

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The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source as indicated below (it is the polite, and professional thing to do).  I retain copyright of all images presented herein

Brian Ricketts –  www.geological-digressions.com

 

The Dalaradian, a 15+km-thick sequence of metasediments and metavolcanics occupies a broad swath through central Scotland. This is the Grampian Terrane, a patch of rock that originally accumulated on the Late Precambrian – early Paleozoic margin of the ancient continent Laurentia, washed by the equally ancient Iapetus Ocean. The Grampian Terrane is now sandwiched between two crustal-scale sutures: Great Glen Fault in the north, and the Highland Boundary Fault. Dalradian rocks overlie Laurentian basement.

Dalradian polyphase deformation, metamorphism, and syn- and post-kinematic intrusions have been the subject of intense study, conjecture and debate since the 1800s. In 1893 George Barrow published his discovery of a coherent metamorphic zonation in the Dalradian sequence, that ever since has promoted theoretical concepts of crustal processes such as pressure-temperature effects during burial, elucidation of structural complexities, and exhumation of deep crustal realms.  Barrovian metamorphism and deformation took place during the late Cambrian – Ordovician Grampian Orogeny, an early phase of Caledonian mountain building during closure of the Iapetus, and collision between Laurentia and an oceanic island arc.

Below are some images taken during a recent visit to Macduff and Portsoy (coastal Moray Firth, Banffshire, Scotland), and Connemara, County Galway, Ireland.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other Atlas categories.

Some useful references:

B.E. Leake, 1986. The geology of SW Connemara, Ireland: A fold and thrust Dalradian and metagabbroic-gneiss complex. J Geological Society of London, v.143, p. 221-236.

D.M. Chew and R.M. Strachan, 2015. The Laurentian Caledonides of Scotland and Ireland. Geological Society of London Special Publication 390, p. 45-91

Banffshire coast – an excursion – introduction to geology. British Geological Survey, 2015

 

Macduff: On the coast near the old swimming pool, thick bedded psammites, possibly crossbedded, with thinner intervals of pelite-shale, with a suggestion of ripple, despite the garnet-zone metamorphism. There is some penetrative fabric here, but not as intense as that seen along the Portsoy coast. An excellent Cullen Skink can be sampled in any of the local cafes in the nearby town of Cullen.

 

Massive bedded psammites and thin interlayered pelitic rocks. Bedding is well preserved, despite the relatively high grade metamorphism. There are hints of crossbedding. Cullen.

 

Thin bedded and laminated pelites with interleaved lenses of coarser-grained lithologies, possibly rippled.  Macduff.

 

Massive bedded psammites with internal discordant contacts resembling crossbeds. Macduff.

 

Portsoy (about 5km east of Cullen). The old harbour here was constructed in the 17th century and has stood the test of time and North Sea storms. Stone blocks (mostly psammite) in the original harbour walls were oriented vertically.  Dalradian outcrop (from the disused swimming pool west of the village, to East Point) are part of the Portsoy Shear Zone. Kyanite-zone metamorphism produced well developed micaceous foliation; folding and cleavage records 3 or 4 stages of deformation.  Post-tectonic pegmatites intrude the sequence. The first set of images are west Portsoy near the disused swimming pool. The second set is along a coastal transect towards East Point.

Blocks of psammite were aligned vertically in the old Portsoy harbour walls because it was thought at the time of construction, that this configuration would be more stable. It seems to have worked.

 

Elsewhere, house and wall construction used the more familiar horizontal block-stone style of construction.

 

Portsoy west (near the old swimming pool)

Views west of Portsoy. Large mullions in quartzite are clearly visible in the left image. Here, fold axes are steeply plunging (approximately north).

 

Another view of the mullions – located between the disused swimming pool and Portsoy village. (focus mot brilliant in this image – it was a very windy day)

 

Shearing in interlayered psammites-pelites has disrupted  the earlier foliation and folding.

 

A patch of reasonably coherent foliations and small, recumbent, isoclinal folds and boudinage.

 

Folding in thin bedded psammites-pelites and limestone (fold axes here are almost horizontal). Folds In the right image have been sheared.

 

Foliated and folded, thin bedded psammites.  This exposure is very close to the two images immediately above; here the fold axes have been rotated.

 

Detail of shearing and stretching of small-scale folds.

 

Tension fractures in bedded psammite.

 

Portsoy east

Foliated psammites, steep-dipping cleavage, and some shearing.

 

Small-scale folds in thin bedded psammite-pelite sequence

 

 

Left: folded psammites and boudinage of thicker limestone (light grey). Right: Small-scale folds in thin bedded psammite-pelite.

 

Small-scale folding and boudinage in laminated pelite-limestone.

 

Post-shear pegmatite dyke in sheared psammites (near East Point)

 

Clifden coast, Connemara

The Sky Coastal route west of Clifden. The Dalradian here is part of the Connemara Metamorphic Complex and is a continuation of that seen in Scotland. It contains quartzites, schists, marbles and amphibolites.  The outcrops shown here are primarily strongly foliated schists with a later phase of tight, isoclinal folding.

Left: the open Atlantic coast; Right, one of several estuaries, home to small fishing villages,

 

Strongly foliated and folded amphibolites exposed in the shore platform opposite Omey Island

 

      

Small-scale, recumbent, isoclinal folds

 

Small-scale folding, near Cleggan Harbour, NW of Clifden. Note the small thrusts near the center of the left image.

 

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The Pink and White Terraces, Magic Lanterns, and 19th Century Narratives

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Preamble

A collection of late 19th century lantern slides made by Arthur Whinfield, a native of Worcester, England, has recently been restored and digitized.  I was alerted to the Whinfield collection by an old friend Justin Hughes, an archaeologist working in Worcester, UK, who indicated that the collection contained several slides of the famed Pink and White Terraces near Rotorua, New Zealand. These iconic geothermal wonders were destroyed by the eruption of Tarawera, on June 10, 1886.

Lake Rotomahana, in the shadow of Tarawera volcano, looks peaceful enough. Its waters are ruffled only by wind and the wakes of small boats. No hint of impending doom. No hint of the destructive explosions 132 years ago, eruptions that completely changed the local landscape, destroyed a village and its inhabitants, and obliterated a geological icon – the Pink and White Terraces.

New Zealanders look upon the Pink and White Terraces with a kind of fondness, even though no one alive has seen them. Mineral-rich waters spurting from geothermal springs and erupted from geysers above the former Lake Rotomahana, deposited silica in a cascade of rimmed terraces and pools; ever shrouded in steam. As their name indicates, there were two sets of terraces. The larger White Terraces descended 25m, stair-like, into the lake.  Their pink counterparts were terraced through 22m. Mineral content was more pronounced in the pink variety, with precipitation of arsenic and antimony minerals, and gold.

The terraces disappeared on June 10, 1886. Eruption of Tarawera was focused along a 17 km rift that extended from the volcano summit, through the terraces, and into Lake Rotomahana. Whether the terraces were obliterated, buried, or partly submerged in modern Lake Rotomahana is still debated.

By all accounts the terraces were spectacular; witness the written testimonies of geologists (like Ferdinand von Hochstetter in 1859) and Victorian gentlefolk, renditions in oil and water-colour (like the Blomfield painting above), and photographs – grainy, black and white, tinted amber with age. This was the late 1800s, and photography was in its infancy, the act of recording an image a laborious process.

Enter Arthur Whinfield. Whinfield was a peripatetic photographer who in the 1880s captured the magic of cities and landscapes in the Americas, Asia, Africa, Europe, Australia, and New Zealand. Part of his legacy resides in a collection of lantern slides (more than 2100 of them) that were donated to the Worcester Diocesan Church House Trust by his wife in 1918. A century later, in partnership with the Worcestershire Archive and Archaeology Service, the slides have been restored, digitized, and made available for public display. Included in the collection are 11 glass slides of the Pink and White Terraces, and the aftermath of the Tarawera eruption.

Whinfield took many of the photographs he used in his slides, but also borrowed from other photographers, and this seems to have been the case for several Pink and White Terraces images.  Te Papa Tongarewa  (the New Zealand National Museum) has an extensive, publicly accessible collection of photographs, paintings and prints related to the Terraces. I was able to identify individual photographers in some of the slides from the Te Papa collection.

Click on each image below for a larger format, then use the back-click arrow to return to the article.

The classic image of the Pink Terrace with Maori guides (or is this a family scene?) and a canoe in the foreground is shown below. This iconic photograph was taken by Burton Brothers Studio in 1885, and later used by Muir and Moodie Studio in a popular postcard (early 1900s; one penny postage required). Whinfield’s slide (left) is a copy of this scene (acknowledging the photographer on the lower left corner).

Whinfield slide left; Muir & Moodie postcard right

 

Moodie and Muir also produced a postcard from the White Terraces image below (left); the colour would have been added by hand to the printed photo. The Whinfield slide (right) is an uncoloured version of this image (compare the shape of the steam clouds at the top).  The terrace flights are nicely portrayed in this slide.

Muir & Moodie postcard left; Whinfield slide right

 

One of the more panoramic views of the White Terraces shown by Whinfield (left) is similar to a photograph taken by Burton Brothers in January-May 1886 (in the Te Papa collection) but is viewed from a lower elevation and records a different steam profile; the original photographer may have taken more than one shot from this location (I have not been able to determine who the photographer was for Whinfield’s slide). Here the terraces clearly dip their toes in Lake Rotomahana.  The terraces were a popular tourist attraction, in part because bathing was possible in the lower pools.

Whinfield slide left; Burton Brothers photo right

 

The original black and white photograph for the three slides below (Pink Terraces), was taken by Charles Spencer.  The three Whinfield slides are identical, with the right image slightly enlarged and a mirror of the other two. The view provides some detail of several small pools that appear to have been filled completely by silica.

 

The two slides below show detail of White Terrace pools and the intricate patterns wrought by precipitation of silica.  Dark stains daubed on the pool walls may have been algae. I have not been able to determine the attribution of either slide. The slide on the right is labelled ‘White Terrace Crater’ and may have been taken close to one of the active geysers near the top of the terraces.

 

Whinfield’s slide (below, left) showing the aftermath of the eruption at McRae’s Hotel is slightly different to one taken by photographer George Valentine (1886, McRae’s Hotel and Sophia’s whare) – the man on the right in Valentine’s image has his arms folded; in Whinfield’s slide they are not. The viewing angle is also slightly different – the ladder (foreground) is more oblique in the Whinfield slide.  The hotel probably collapsed under the weight of volcanic ash. Other photographs (not in the Whinfield collection) show the back of the hotel to be demolished completely. The trees were also stripped of foliage.

Whinfield slide right; George Valentine photo right

 

The title of the slide below Rotomahana Looking to Site of Pink Terrace, indicates a view towards the former terrace, or perhaps close to it, in the aftermath of the eruption. If the location is correct, the image is important because it shows that destruction of the Pink Terraces was complete. The Mounds of volcanic ash cover almost everything. Characteristic erosional rills suggest rain soon after the eruption, where surface water run-off redistributed the ash (probably towards the lake).  I could not determine who the photographer was.

Whinfield’s slides, recently brought to life, are delighting and informing audiences today, just as they must have done when he presented them to an eager 19th century public. These days we never think twice about the projection media at our fingertips. It seems almost to be part of our subconscious, but to Whinfield’s audiences there must have been a sense of excitement, awe, and puzzlement, not just in the images they were seeing, but the fact they were seeing them at all. The havoc wreaked by distant volcanic eruptions, was delivered to their living rooms by a rapidly developing technology.

The Whinfield Terrace collection may not contain photographs of his own taking, but this is not important. An iconic landscape was taking shape in people’s minds, a narrative in images. Folk who may never have left their own village became informed; witnessing the real world shaped by unimaginably ferocious forces – a kind of 19th century Scicomm.

 

I would like to hear from anyone who has additional information on the images in Whinfield’s slides, particularly information relating to the original photographers.

 

Credits: The Whinfield collection is owned by the Worcester Diocesan Church House Trust. I am grateful to the Trust for permission to use the digital images of the slides. Thanks also to Justin Hughes of the Worcestershire Archive and Archaeology Service for bringing the slide collection to my attention, for arranging to forward the images, and for his patience with my incessant questions.

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Crème brûlée, jelly sandwich, and banana split; the manger a trois of layered earth models

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Some things in science are just too difficult to comprehend: the temperature at the center of the sun (15,000,000oC), the age of the earth (4.6 billion years), the size of a nano-particle (1-100 nanometres, or billionths of a metre). We can include in this list of imponderables, the skinny outer layers of the earth: the one we are in daily contact with (the crust), and other layers beneath it. Our familiarity with the crust is usually in terms of the dirt, rock, and water we work with. But what is it like 30km down? And, beneath the crust, the upper mantle is beyond reach of our senses. What does this layer look like? How does it respond to being pushed around?

Some scientists (geologists, geophysicists) spend a great deal of time pondering questions like these. The crust and upper mantle layers are collectively referred to as lithosphere. Beneath the continents it averages 150 km thick; beneath the oceans, it is as thin as 10 km beneath the mid-ocean ridges. Given we spend our entire lives on the uppermost veneer, a reasonable person might ask ‘why is it important?’.

A few common answers include: Most earthquakes are generated in the lithosphere; Magmas erupted at volcanoes melt at these depths. But the overarching reason is that all tectonic plates are born and destroyed as lithosphere. Plate tectonics governs pretty well everything that happens on earth over geologically short and long time-scales. So, what appears arcane at first sight, does have practical applications.

Enter the dessert trolley. There are three choices: a crème brûlée, a jelly sandwich, and a banana split. Proposed as models of the layered earth, they serve a dual purpose: they provide visual descriptions of how the lithosphere might be structured and, after evaluating the merits of each, they can be consumed.

The crème brûlée is a two-layered model.  A viscous fluid base (custard) is capped by a thin crust of caramelized sugar. The crust behaves in two ways. Poke it gently in the centre, and it will bend slightly – release the pressure and it will return to its original shape.  This represents elastic behaviour (think also of wire springs, or rubber bands). Press it too hard and it will break into several ragged pieces; in this instance, you have exceeded the elastic limit, or strength, and induced brittle failure. Earthquakes represent brittle failure where earth’s crust fractures, is displaced, and in the process causes mayhem. The crème brûlée model is probably the simplest of the dessert trio in terms of its relevance to the lithosphere.

The jelly sandwich is potentially the more variable of the three analogues. It is a three-layered model where two pieces of bread are separated by a layer of jelly.  Here, the upper bread layer represents a strong upper crust, and the jelly a weak lower crust. The bottom bread layer is compared with a strong upper mantle – in contrast to the weak custard (mantle) layer in the crème brûlée. The upper and lower bread layers are both quite bendy (unless you have toasted the bread). If you use plain white bread, then bending will be uniform. But if you prefer whole-grain slices there will be lots of lumps and greater heterogeneity, and hence a less predictable response to the application of pressure, or stress. The jelly is much less fluid than custard. It can behave elastically – witness the wobbling, that represents deformation from which it recovers, but at a certain point it too will fail.  Bread is less rigid than a crème brûlée crust; any kind of twisting or bending will probably result in some permanent deformation (i.e. it doesn’t bounce back to its original shape). Unlike the crème brûlée crust, bread is less prone to brittle failure.

The banana split adds another level of complication to models of the lithosphere. The rationale for this model is that the lithosphere contains zones of weakness, particularly near the boundaries of tectonic plates – imagine these plates colliding or sliding past one another, where the forces are large enough to create mountain belts and consume oceans. Here, scoops of ice-cream represent blocks of crust and mantle that are separated by large, very deep faults. This is a very temperature-dependant model. As the ice-cream melts there is a zone of weakness between it and the adjacent scoop (block). The presence of fluid, particularly water, exacerbates this weakness. In this dessert, we need to translate the fluid boundary between scoops of ice-cream, to structures 10s of kilometres deep. Modern examples include the Alpine fault in New Zealand, and San Andreas Fault in California. Some of these large structures can last for very long periods of geological time (100s of millions of years), and potentially influence events in the crust-upper mantle long after they first formed.

All models in science are simplifications of the things we try to explain. It may be the case that some consider the dessert trio to be trivial, even silly, providing little useful scientific information for the representation of the crust and mantle.  But the utility of models and analogues is not only in scientific explanation, but to present a complex world in visually interesting, and yes even amusing ways. Models and analogues need to stir the imagination of folk who are not directly involved in this kind of research but have a vested interest in it. In this regard, the dessert trio works, even if folk can relate to them only via our taste buds.

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A chance encounter with James Ussher, circa 1650

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A chance encounter of a different kind. The 9th century Book of Kells is exhibited in the Trinity College library (Dublin). The book contains 340 sumptuously illustrated folios written on vellum (calf skin), that make up the four Gospels (the entire book can be viewed online). It was written and illustrated around 800AD, in either Iona (Argyllshire, Scotland) or Kells (County Meath, Ireland), and possibly both. Monks from the St. Colum Cille Monastery in Iona, fled to Kells in 806 to evade further slaughter by marauding Vikings (who seemed to have a penchant for religious orders). Hence the uncertainty surrounding the location of the Book’s compilation.

Two or three folios are exhibited at any one time in the library.  There is something special about seeing this, regardless of one’s beliefs. That a document this ancient has survived pestilence, religious and political purges, plundering and the general ravages of time, is a testament to the enduring belief that human history is worth preserving.

A visit to the Book of Kells exhibit eventually leads to the Long Room, a cathedral-like hall with high barrel-vaulted ceiling dedicated to preserving old and ancient manuscripts, tomes, letters and assorted documents. It was first built in 1712) but enlarged in 1860 to accommodate a burgeoning collection.

One of the College’s 17th century benefactors was James Ussher, Archbishop of Armagh (although he never lived to see the actual Long Room). He was instrumental in purchasing books for the then nascent College library. Ussher became famous, or infamous, for his dating of Genesis creation at October 23rd, 4004 BC. He based this calculation (published in 1650) on biblical genealogies and historical events. Given what we now know about the age of the earth, Ussher’s calculation is generally viewed with a mix of ridicule and begrudging respect.

Acting on a hunch, I asked one of the library attendants if any of Ussher’s books were available for viewing. He suggested I check with the folk in charge of ‘reading room’ passes. This took a while but I was eventually issued a pass and ushered (sic) to the reading room. The first task was to search the catalogues.  One index of books had an entry annotated with scribbled comments on “creation dates”. Bingo. This particular volume appeared shortly thereafter in its own protective box. A rather plain unassuming book was placed carefully on a special reading rack. Instructions were given on how to turn the thin, brittle, dog-eared pages.

The journey through 400 year old documents like these is more one of personal discovery than some momentous event of universal import. The book was a collection of Ussher’s notes, annotated texts, and the odd piece of correspondence. Flicking through these pages gave, for a moment, a view into Ussher’s thinking. Here were his working notes, in a mix of Latin and English. Marginal scribblings on printed text and fragments of scripture, ecclesiastical compositions, calculations of cumulative dates gleaned from the scriptural register of births and deaths, crossings-out and corrections. Here was a person, a Bishop of repute, who tried genealogical combinations, recognized errors, approximated, and perhaps even fudged the odd date. Maybe there were even doubts about the progression of time.

The opportunity to look at Ussher’s notes came out of the blue. It was worth the multiple forms I was required to sign, acknowledging Trinity College’s copyright. I took photos of the text, but cannot display them. What a shame. A 400 year-old document originally intended to be read by all and sundry, now (figuratively) locked away by some arcane law (unless I payed a fee). I don’t decry all copyright conditions, just this one, that applies to a document written by a person who probably never intended such conditions of ownership to apply.

We now know that Ussher’s date for the beginning of earth history has a rather large margin of error. As absurd as his date may sound, it was derived from serious scholarship, in a manner we would now call ‘multidisciplinary’. This in itself is something to celebrate.

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