Monthly Archives: August 2018

Tidal waves; prisoners of celestial forces

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We are told that a tide waits for no one, the impatient cousin of time. In its early 13th century idiom (the oldest known quotation is 1225 AD) the word ‘tide’ was associated with time, as in a season, or an instant. Somehow this has morphed in modern English to mean the daily rise and fall of sea level, although the association with time and periodicity remains; the regular advance and retreat of the water’s edge. In modern usage, the word ‘tide’ also connotes an association with the Moon and Sun. Tides, as we now understand them, have helped shape our world for the last 4.6 billion years: kept the oceans honest, tidied up our coastlines, and defined the character of harbours and estuaries. Ships enter and leave on the tide, recycled from one coastal haven to the next.

Ocean tides are the natural response to the forces of gravity acting on earth, its moon, and the sun. Tidal forces act on other planets and their moons. The sulphurous moon Io develops a distinct bulge during its close approach to Jupiter. It is hypothesized that moons Enceladus (Saturn) and Europa (Jupiter), maintain liquid water oceans beneath their frozen surfaces because of the heat generated by the forces of gravity.

Earth tides cycle through highs and lows depending on the relative positions of the moon and sun. Tidal highs and lows also vary from place to place, for example a high tide on the west coast of New Zealand may occur at the same time as a low tide on its east coast. To explain this phenomenon, we first assume a simple model where earth is covered completely by ocean – once we have established an explanation using this simplification, we can add the continents to create a more complicated, real world explanation of tides.

The Earth and Moon are in a state of balance; the Moon exerts a gravitational pull on Earth (and vice versa), and because the Moon orbits Earth (the orbit is slightly elliptical), there is an opposing force – the centrifugal force. As a crude analogy, imagine riding a fast-moving ferris wheel; you are moving in a circle. If you release the safety harness, you will fly off at an angle, propelled from the safety of your seat by a strong centrifugal force. In our celestial system, these two opposing forces keep the Moon from crashing into Earth (and likewise, Earth into the Sun).  Centrifugal forces are the same everywhere on Earth, but the Moon’s gravitational pull changes with distance; it is strongest on the side closest to the Moon, and weakest on the opposite side. Thus, at different points on the Earth surface, there is a slight difference between the two forces. The difference is not enough to upset the overall balance between Earth and Moon, but it is strong enough to create a bulge in the ocean mass; one on the side facing the Moon, the other on the opposite side of Earth. The bulges correspond to high tides.  However, Earth rotates on its axis, which means that different parts of Earth experience the bulge at different times – note the bulge itself is always aligned with the moon. In this simple model, the bulges on opposite sides of the earth mean that there are two tides every 24 hours, 12 hours apart.  These are semidiurnal tides.

The Sun exerts a similar effect on Earth, but its influence on tides is about half that of the Moon. Nevertheless, the Sun’s gravitational force will reinforce that of the Moon during full and new phases of the Moon, resulting in spring tides; the opposite effect, neap tides occur when the two gravitational forces oppose each other. Other tweaks to this relatively simple celestial model are the elliptical Earth-Moon-Sun orbits (which results in some changes to the gravitational effects), and the tilt of Earth’s own axis of rotation – hence the monthly lunar cycles, and seasonal solar-Earth cycles. Centuries of sky gazing have taught us that all these cycles are predictable which means we can foretell tides well into the future.

To better understand tides in the real world we now need to complicate our model by adding continents.  These massive landmasses have created a degree of ocean isolation (Pacific, Atlantic, Indian) such that tidal cycles can be considered separately for each ocean. The tidal bulge, or tidal wave, is slowed as it enters shallow coastal waters and is also is deflected, such that it moves, wave-like, around each ocean margin. Tidal waves in the northern Hemisphere move anticlockwise, while those in the south move clockwise.  Thus, high and low tides will also migrate along ocean coasts. These patterns apply to the oceans as a whole, but on a more local scale, the tidal wave can be deflected, amplified, or weakened, depending on the shape of the coastline and variations in water depth. A classic example of tidal amplification is Fundy Bay, a narrow stretch of water between New Brunswick and Nova Scotia (eastern Canada).  Here tidal ranges of 17m are common, in marked contrast to those on the opposite coast (e.g. 2m tides in Halifax). Tides here flood very quickly, initially as a tidal bore that in places may be a 2-4m high wall of water (check out this link to a short video, taken near Moncton, New Brunswick).

The simple Earth ocean model predicts two tides every 24 hours, or semidiurnal tides. Adding continents, coastlines and varying water depths further complicates this picture such that in places only one tide occurs each day; these are diurnal tides (e.g. Gulf of Mexico, the Kamchatka coast), or mixed semidiurnal tides where one high tide is significantly higher than the other (i.e. one high tide is weakened).

Along the New Zealand coast, both lunar and solar tidal waves move anticlockwise (the opposite direction to much of the southern hemisphere). Tidal ranges are highest on the west coast, where both tidal waves reinforce the tidal signal, and are lower on the east coast where the solar tidal wave is weakened. The National Institute of Water and Atmosphere (NIWA) has produced animations of the lunar and solar tidal waves. High tide in the far north of New Zealand is about 6 hours ahead (or behind) that in the far south. It takes 12-13 hours for the tidal wave to traverse around the entire New Zealand coast, which means that a high tide at any location, will have a corresponding low tide somewhere else. This difference is nicely illustrated in Auckland city which lies between two harbours; Auckland Harbour on its eastern coast (Hauraki Gulf – Pacific Ocean) is geographically separated from Manukau Harbour on the west side (connected to Tasman Sea), by an isthmus that in places is barely 1000m wide.  It takes about 3.5 hours for the tidal wave to travel north up the east coast, and south down the west coast to Manukau Harbour.

Tidal range, the elevation difference between high and low tides, is also affected by weather. High pressure weather systems tend to lower sea level, whereas low pressure systems result in higher than normal sea levels. Coastal regions can experience serious problems from flooding, when a spring tide corresponds with the passage of major storms, particularly cyclones and hurricanes. To make matters worse, storms like these usually generate high rainfall. Elevated tides plus storm surges will cause rivers to back-up, flooding low lying areas.

Tidal ebbs and flows have moved ocean water masses, flushed embayments, moved sediment, and helped shape coastlines ever since Earth acquired a Moon. Tides are periodic; they cycle endlessly through their highs and lows. They are also superimposed on the straight arrow of time (borrowing a phrase from Stephen J. Gould), such that we can now predict the times and ranges of tides well into the future.

Note: The name tidal wave is sometimes used to describe a tsunami. Tsunamis have no relationship with tides. Tidal wave should only be used to name long period, gravitationally induced waves.

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The Burrens: An understated beauty

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The Celts must have known a thing or two about rocks. They certainly recognized the bare hillslopes of western County Clare (Ireland) for what they were – Burren, an anglicized version of the Irish Boíreann (a stony or rocky place). Even the grasses and wild flowers in a myriad nooks and crannies, have trouble eking out a living. The lower hillslopes and valleys between these stony, limestone hills seem verdant (although the soils there must be alkaline), but the hills themselves…! Patchwork dry-stone walls ascend and encircle the hills, evidence of the optimism of farmers, the odd cow or horse searching for an elusive blade of grass. Whoever worked these stones are distant memories.

The Burren and the Cliffs of Moher are a UNESCO Geopark (established in 2011). From a geological perspective, the two sites are diametrically opposed – one a cluster of bare limestone hills and the other, vertiginous coastal cliffs that are in a constant state of renewal from pounding Atlantic waves.

The limestones formed about 350 million years ago, part of the geological period known as the Carboniferous. Ireland back then was closer to the tropics; seas were warm and teeming with life. This was a time of relative stability, following on the heels of momentous events caused by continental collisions and mountain building – the Caledonian event. The present landscape was moulded more recently by a couple of glaciations, the last one ending about 10,000 years ago. During the Last Glacial Maximum, about 22,000 years ago (i.e the time of maximum ice accumulation), Ireland was covered by the Celtic Ice sheet (more or less a continuation of the Scandinavian Ice Sheet). Glacial ice plucked and scoured the hard limestone, leaving rounded hills and in places a veneer of ice-carried debris. It is a glacio-karst landscape, and it is the latter to which most visitors are drawn.

Limestone is naturally prone to dissolution, a process where calcium carbonate (CaCO3) dissolves slowly in rain and percolating groundwater. Over time (many 1000s of years), unique ‘karst’ landscapes evolve including rugged surface structures, sinkholes (also called dolines), and subterranean caves and streams. Karst landscapes are common in limestones exposed to humid tropical and warm temperate climates. The Burren is unique because its karst followed the retreat of glacial ice.

Scramble over any part of the Burren and the first thing you’ll comprehend is the need to watch your step. The limestone slopes and pavements are cut by myriad fractures, enlarged and deepened by the slowly dissolving limestone – these are the grykes, that separate the intervening limestone blocks, or clints. Clints and Grykes; the word clint is old Scandinavian (Viking) meaning summit or cliff; gryke (a variant of grike) – not much is known about this word other than it is from 18th century northern English.  Grykes focus surface water and hence are home to a wonderful array of flowers (including many orchids), grasses, small scrubby bushes and creepers (like ivy). Surface waters from rain and snow melt rapidly descend the gryke-riven limestone to become part of an extensive network of small, underground fracture flows and seepage, groundwater that emerges lower down the slopes, in valleys and along the shoreline.

Burren limestones contain many fossils, and the best place to see these is along the shore, where waves have smoothed the rock surface. Two stretches of coastline we walked during our sojourn there, were Flaggy Shore near New Quay (and close to a Linnane’s, a delightful pub on the estuary), and Black Head, a promontory that is fully exposed to the Atlantic. Limestones at Flaggy Shore belong to the Tubber Limestone Formation; those at Black Head are the Burren Limestone Formation and are a bit younger. The limestone formation at Black Head appears step-like along shore and hillslope; the steps reflect the well-developed laying, or bedding.  Some of these beds are separated by thin layers of muddy or shaly rock. Back-stepping of layers at Black Head may have been accentuated by post-glacial uplift of the coast, a product of isostatic rebound following retreat of the ice sheet.

The most common fossils include brachiopods (sometimes called ‘lamp shells’, are invertebrates with two shells, superficially like clams, but having a completely different animal inside), corals, and gastropods (sea snails). There are also crinoids (distant relatives of star fish) but I did not see any of these. Corals are common along the coastal exposures, mostly in clusters of skinny branched colonies. Most have circular cross-sections, but we did find one example of a colony where individual columns had hexagonal cross-sections – a now extinct group known as Rugose corals. The exposures seen along the coast are like a slice through these colonies; this also applies to the brachiopods – rarely do we see the complete shells.

The Burrens are unlike anywhere else in Ireland. The expression ‘Emerald Isle’ doesn’t really apply here. Green fields and woodlands, the idyllic Irish scenes in tourist brochures and poetic verse, are replaced by grey hues that merge with cloud and sea. Even the dry-stone walls disappear in the tide.  As far as landscapes go, the Burrens are a bit understated; there’s none of the vertigo sensed at the Moher Cliff edge, no brooding mountains, just the quietude of a landscape that has withstood Atlantic battering. The Burrens are unique.

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Atlas of Unconformities

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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

Stratigraphy is a cornerstone (sic) of the earth sciences. With it, we unravel earth’s history, the sequence of events and processes that have given us the world we live in. It is the story, written in rocks and fluids, of the physical, chemical, and biological world.  Perhaps we should now include the social and psychological spheres of our existence, as part of the latest geological period, the Anthropocene – layer upon layer of human thought, actions, consequences.

Unconformities are a fundamental part of Stratigraphy.  They are that part of the rock record in which time and rock are missing – periods of time in which rocks either did not form, or if they did form were subsequently removed. In both cases, the “missing” information tells us that something happened; the ‘something’ may have been local, confined to our own backyard, or of global extent such as extinction events, the construction of mountains or destruction of oceans. So, geologists who find unconformities don’t throw up their hands in despair; they rub their hands in glee at the promise of so many possible explanations.

What better example to begin with than one of James Hutton’s classic localities on Arran, west Scotland (image above).  This is the unconformity at Lochranza where Carboniferous sandstone overlies Late Precambrian Dalradian schist.  The unconformity here represents about 240 million years of time, seemingly missing, and yet it also represents a period of mountain building, where deeply buried metamorphic rocks were uplifted many kilometres, exposed and worn down by the vagaries of ancient weather systems, and buried by sand shed from the rising mountains.  This tale of the evolving earth is encapsulated in the seemingly innocuous contact between the two different groups of rock.

The images:  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.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

An uncluttered view of Hutton’s unconformity at Lochranza (same location as the image above)

 

 

Basal conglomerate of the Carboniferous succession that onlaps Dalradian schist at Lochranza. Hammer is at the unconformity.

 

 

 

 

The unconformity between Archean metavolcanic and plutonic rocks at Cobalt, Ontario, and the Proterozoic Gowganda Formation, is marked by a regolith of blocky granodiorite and granite, that is overlain by diamictites deposited during Early Proterozoic glaciation.

 

Portskerra: Old Red Sandstone (ORS) on Moine schists, north Scotland

The ORS is a mixed bag of sedimentary rocks, mostly Devonian, but extending into the late Silurian and early Carboniferous. Their importance lies in the direct association with Caledonide tectonics, where sediment was shed from the rising mountains into adjacent foreland basins. The ORS is sometimes compared with the younger Molasse foredeep successions of Europe. The unconformity at Portskerra is an erosional surface, where the ORS fills paleotopographic lows and drapes the intervening highs.

Numbered sites refer to the thumbnail images below.

 

Sites 1 (left) to 4 as shown in the general view above. Moine rocks were exposed during Caledonian uplift and subsequent erosion that removed many kilometres of overlying rock. Much of this sediment was deposited as ORS.

 

A coarse ORS breccia, consisting almost entirely of fragmented Moine schist, overlies the unconformity.

 

The ORS beds contain shallow trough crossbeds and ripples, and occasional pebble-cobble lags that mark the base of channels.

 

Rippled sandstone in beds a metre above the unconformity.

 

Typical, strongly foliated Moine schist.

 

The NW coast, towards Portskerra and the distant Orkney archipelago.

 

Loch Assynt, northwest Scotland

Lewisian gneisses and migmatites (Archean) are overlain unconformably by Torridonian sandstone (Proterozoic).  The roadcut adjacent Loch Assynt is west of the Moine Thrust complex; both rock assemblages are part of the ancient Laurentian continental block.  The three thumbnail images below are from the same general location.  At this locality there is subdued paleotopographic relief on the unconformity.

 

 

Expedition Formation, Canadian Arctic

The Campanian to Middle Eocene Eureka Sound Group on Ellesmere and Axel Heiberg Islands represents the last gasp of sedimentation in a thermally subsiding Sverdrup Basin. In the central part of the basin, The Expedition Formation contains two stratigraphic sequences separated by a disconformity where most of the Maastrichtian is missing. Along the basin margins Sequence 1 is commonly missing such that Sequence 2 onlaps Paleozoic bedrock.

 

The Campanian-Lower Paleocene unconformity at Hot Weather Creek, Ellesmere Island.  Throughout the basin, the base of the Paleocene is characterised by thick quartz-rich sandstones deposited in estuaries, sandspits and bars.

 

 

 

Lower Paleocene Sequence 2 along the basin margins commonly onlaps Paleozoic rocks – here Ordovician carbonates. The earliest sediments infilled a karst paleotopography. Mt. Moore, Ellesmere Island.

 

This Lower Paleocene – Ordovician unconformity has a well developed regolith in the carbonates. Mt Moore area.

 

Left: Lower Paleocene Sequence 2 on Devonian sandstone-limestone. The trace of the unconformity coincides with the stream (lower right).  Right: Lower Paleocene Sequence 2 on Permian limestones-grainstones near Canon Fiord. The trace of the unconformity coincides with the stream (center).

Buchanan Lake Formation, Canadian Arctic

This is the youngest formation in the Eureka Sound Group. Its deposits record inversion and dismembering of Sverdrup Basin by thrust-dominated tectonics during the Middle Eocene.  Deposition took place in several foredeeps, that also were involved in the deformation.

  

Syntectonic, Middle Eocene Buchanan Lake strata disconformably overly Lower-Mid Eocene delta deposits (Iceberg Bay Fm, Sequence 4). Sediment was derived from uplifted Late Paleozoic and Triassic rocks. They were subsequently overthrust by Late Paleozoic anhydrite and Permian mudstone-sandstone. North of Whitsunday Bay, Axel Heiberg Island.

 

Syntectonic Buchanan Lake conglomerate (brown hues) overlies unconformably Triassic sandstone.  Stang Bay, Axel Heiberg Island.

 

New Zealand Paleogene-Neogene basins

The Plio-Pleistocene Wanganui Basin occupies a position between the Hikurangia subduction zone and the Late Cretaceous – Miocene rift-passive margin succession comprising Taranaki Basin. Along its eastern margin, Wanganui Basin strata onlap much older greywacke-greenschist basement, shown above at Otupae Station (about 30km SE of Waiouru, along the west flank of the Ruahine Ranges.

 

Marine terraces eroded into Middle Pliocene Tangahoe Mudstone are exposed on the south Taranaki coast. Here there are excellent examples of shallow, shore platform channels and potholes, filled by pebbly sand of the Rapanui Formation.  Pollen assemblages indicate that shallow marine-beach and dune deposition took place during interglacial conditions in the late Pleistocene.

 

The Late Eocene-Oligocene Te Kuiti Group (New Zealand) contains cool-water carbonates and associated mudrocks, that accumulated on a broad platform during a period of relative crustal stability. The deposits gradually onlapped  eroded greywacke basement (Torlesse-Waipapa terranes), as shown in this quarry, west of Te Kuiti town.  The limestone unit is the Otorohanga Limestone. This stratigraphic pinchout is unconformably overlain by Early Miocene, deeper water Mahoenui mudstone.

 

Waitemata Basin

The Lower Miocene Waitemata Basin extends from greater Auckland into Northland, New Zealand. The fill is dominated by turbidites deposited at bathyal water depths. The basin mainly overlies Mesozoic greywacke.  In what is a remarkable contrast in water depth, the basal few metres consists of conglomerate, fossiliferous sandstone and limestone that were deposited in shallow shelf and pocket beach settings.  The pre-Miocene surface has considerable paleotopographic relief. Along the Early Miocene coastline this was manifested as greywacke islands, sea cliffs and sea stacks.

The cartoon below shows a rough reconstruction of the Early Miocene environment (drawn more than 30 years ago). Panels a and b show shoreline, beach, subtidal facies, complete with cliff rock-falls and landslides. Panel c depicts the early stages of draping and blanketing by bathyal turbidites and debris flows.

Brian Ricketts, Peter Ballance, Bruce Hayward, and Wolfgang Meyer, 1989. Basal Waitemata Group lithofacies: rapid subsidence in an Early Miocene interarc basin, New Zealand. Sedimentology v. 36(4): 559 – 580

The unconformity in the shore platform below Leigh Institute of Marine Sciences. Intensely deformed greywacke below the red line, is overlain by flat-lying, shallow water calcareous and fossiliferous sandstone. Fossils include abundant barnacles, bivalves (including large oysters), gastropods, solitary corals, bryozoa, calcareous algae (Lithothamnion rhodoliths), foraminifera, and trace fossils.

 

The unconformity at Matheson’s Bay. The steep paleosurface (just left of hammer) is overlain by angular boulders and cobbles of greywacke. Some boulders contain evidence of pre-Miocende weathering.

 

Paleo-seastacks of greywacke that, following rapid subsidence to bathyal depths, were draped by turbidites. Left: North end of Matheson’s Bay. This sea-stack has remnant pholad borings (bivalves that bore into hard rock). Right: Omana Bay, south Auckland. Here, drape folds over greywacke sea-stacks have been exhumed in the modern shore platform.

 

Panorama of lower Waitemata Basin strata, looking south from Takatu Point. The unconformity on the small island is overlain by boulder conglomerate and well bedded calcareous sandstone.

 

Kariotahi, Pleistocene dune-barrier bar complex

There are several very large barrier island-bar systems along the North Island west coast. during the Pleistocene, they effectively straightened the coastline, blockading harbours and estuaries with shallow marine and subaerial dune sands, with entrance and egress of water through narrow tidal inlets.

The coastal exposure at Kariotahi beach, west of Auckland city, contains a nice example of an ancient valley cut into older dune sands, that was subsequently filled with a new generation of dune sands and stream deposits, only to be exhumed much later in the Pleistocene. The unconformity between the original valley margin and the infilling dunes is shown below. The unconformity also shows signs of old soils and weathering.

The valley margins (outlined) are overlain by younger dune sands. The present valley has cut into both of generations of Pleistocene dunes. Kariotahi, west Auckland.

 

Closer views of the Pleistocene valley unconformity. The older (brown) deposits occur below the steeply dipping surface; the younger dunes above.  The irregular, rust-coloured resistant layers are iron-pan; iron oxides that have precipitated during groundwater seepage. Kariotahi, west Auckland.

 

Typical dune cross bedding in the younger valley fill.  The muddy, concave layer near the bottom of the image is thought to have formed in an interdune pond. Kariotahi, west Auckland.

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The Moine Thrust: An idea that unravelled mountains

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Preamble
I first heard about the Moine Thrust (northwest Scotland) during my undergraduate studies at Auckland University. Our department structural geology guru spoke of it with a kind of reverence – “the principles of thrust faulting discovered in the Scottish Hebrides provide us with the tools to unravel the history of mountain belts. At some time in your career you must visit this iconic geological treasure”. It’s taken 48 years, but here I am, courtesy of my good friend and colleague, Randell Stephenson (Aberdeen University). The Moine – Old Red Sandstone unconformity at Portskerra, Lewisian gneisses and Moine schists near Tongue and Durness, and the Moine Thrust, complete with duplexes and mylonites at Eriboll, Glencoul, and Knocken Crag. And a quick chat with bronzed Ben Peach and John Horne.

My structural geology mentor would be pleased, but might have quipped “what took you so long?”

 

There have been times in the history of geology when a simple explanation of a complex problem has not only been proven correct – it has revolutionized the way we think about earth processes. The horizontal (or nearly so) transport of thick slabs of the Earth’s crust over large distances is one such problem. We now know that the movement of panels of rock takes place along faults – thrust faults, and that mountain belts past and present owe their existence to a process we refer to as thrusting. The discovery of thrust faults in the latter part of the 19th century is an intriguing tale with two interwoven threads: a scientific thread of astute geological observation and field mapping in the Scottish Hebrides that led to the formulation of a revolutionary idea, and the tension among members of the professional geological fraternity who were confronted not only with a complicated scientific problem, but also had to contend with professional arrogance and institutional bias.

The beginning of such a story is not always easy to pin-point, but we’ll start with one of its main characters, Roderick Murchison, a Scottish geologist who counted among his acquaintances, Adam Sedgwick, Charles Lyell, and Charles Darwin. His forté was the Silurian System (now known to be about 444 – 419 million years old). The Scottish Hebrides contain some of the oldest rocks in Europe – highly metamorphosed Precambrian schists and gneisses as old as 3.1 billion years. In the 18th and 19th centuries they were referred to as the ‘Primitive’ series; they now are collectively called the Lewisian. The Primitives are overlain by younger rocks, including fossiliferous Cambrian sandstones and limestones, but in places these younger rocks are in turn overlain by slices of Primitive metamorphic rocks (Lewisian). Herein lies a fundamental conundrum; the oldest Primitives should not overlie younger fossiliferous rocks. Murchison, during his field excursions in the late 1850s, recognised this sequence of strata, but determined that the Primitives were not as old as previously thought, and in fact were younger than the fossil-bearing rocks.

James Nicol, another Scot, a Fellow of the Geological Society of London, and clearly a better field geologist than Murchison, continued the Hebridean researches and discovered that there is a significant break in the stratigraphy, in particular between the fossiliferous beds and the primitive metamorphic rocks. In other words, Murchison was incorrect in his interpretation that the whole sequence of strata continued, one group of rocks upon the other, in an unbroken sequence.  It seems Murchison took exception to this, and for Nicol it became a case of dodging slings and arrows from the gentrified professionals.

Enter Archibald Geikie, well known in Victorian geological circles (eventually sporting a knighthood, and directorships of both the Scottish and British Geological Surveys). Geikie’s primary task in conducting his own field work was to verify Murchison’s version of the story (published 1861). He did just that by ignoring, as a recent paper by John Dewey and colleagues suggests, some basic geological and stratigraphic principles (in fact Dewey et al. refer to Geikie as a Murchison “acolyte” and “sycophant” – it doesn’t get more denigrating than that in scientific circles).

The Murchison view prevailed for a few years until James Callaway, in the late 1870s-80s, conducted some detailed mapping in the Glencoul area, and discovered that a slab of primitive rocks (Lewisian gneiss) had been carried over younger limestones, and that there was a definite discordance between the two sequences marked by highly deformed and fractured rock. About the same time, Charles Lapworth mapped in detail similar rocks around Loch Eriboll, and he too found primitive schists discordantly overlying younger limestones and sandstones (the discordances in both areas are now known to be part of the Moine Thrust).

Not to be outdone, Geikie, who by then was director of the British Geological Survey and who probably had the last say in anything geological, charged some of his geologists to map the Assynt area in detail (of course Lapworth and Callaway had already done this). Unfortunately for Geikie, his team confirmed the conclusions of Lapworth and Callaway. In what is now a classic in geological literature, Ben Peach and John Horne in 1884, and later in 1907, concluded that the Primitive rocks had indeed been tectonically pushed over the younger strata. They coined the term “thrust” for this process. Geikie, in an about face, also published this same conclusion in 1884, but made little or no reference to the previous (correct) interpretations of Nicol, Lapworth, or Callaway. The main thrust, and the one that caused all the discord, is the Moine Thrust. It stretches almost 200km along northwest Scottish Highlands.

Our understanding of thrust faults and the processes of thrusting has advanced over the decades since the opus of Peach and Horne. The basic geometric relationships are shown in the cartoon below. Fundamental to the formation of thrust faults are ‘pushing’ forces that act roughly horizontally. Prior to the late 1960s – 1970s the existence of such forces were problematic for earth scientists; there was no obvious large-scale mechanism that could generate them. But the advent of plate tectonic theory, based as it is on the surface-parallel movement of great slabs of the Earth’s crust and mantle, helped solve this dilemma. Particularly helpful in plate tectonic theory are the gargantuan horizontal and oblique forces that are generated when continental plates collide.

As the cartoon shows, a thrust fault will form when the pushing forces acting on the crust exceed the strength of weak rock layers. Rock failure, in the form of a ramp, allows the moving panel of strata (which may be 100s of metres thick) to ride up the ramp and thence across a relatively flat surface. The thrust sheet is commonly folded over the ramp. If the pushing forces continue, a second thrust fault may form beneath the first, resulting in a second panel of strata that carries, piggy-back style, the first thrust panel.  This process can be repeated several times, such that the end-product is a stack of thrust sheets, one on top of the other, 100s to 1000s of metres thick. Such stacks are commonly called duplexes. The culmination of this process is a mountain belt.

One of the main identifying traits of thrusting is the repetition of strata: older rocks structurally imposed over younger rocks, that in turn are overlain by younger rocks etc. etc. For Murchison and all those who followed, this was the fundamental problem.  Lapworth recognised this at Loch Eriboll, where several thrust panels are stacked one upon the other. Peach and Horne referred to it as a “zone of complication”. Ancient Lewisian gneiss was emplaced in the topmost thrust panel, and beneath this is a repetition of much younger Cambrian sandstone and limestone.  This is a classic example of a thrust duplex.

Callaway’s mapping in the Glencoul area showed what has become another classic example of thrust repetition. From the lake shore, Lewisian gneiss is unconformably overlain by Cambrian quartzites (quartz-rich sandstones) and related sedimentary rocks; this is a stratigraphic contact, albeit one in which a great deal of time is missing. However, the gneisses and their overlying Cambrian rocks reappear above the Cambrian strata; the contact here is the Glencoul Thrust (part of the Moine Thrust complex).

The iconic stratigraphic section and exposure of the Moine Thrust at Knockan Crag is designated a Geo-Heritage Site. Peach and Horne, bronzed and looking on approvingly, point the way to a short hike that takes one through the key elements of the Cambrian stratigraphy to the Moine Thrust and overlying panel of ‘Primitive schist’.  The thrust occurs above the Durness Limestone where there are excellent examples of mylonite, rock that was ground up and fractured as the thrust fault formed – giving us some insight into the massive forces that generated these structures.

 

(Click on the images below for a larger view of Knockan Crag stratigraphy)

 

 

The Moine Thrust was an integral part of a protracted period of mountain building from 475 to 405 million years ago, when three continental plates collided as the intervening Iapetus Ocean was consumed by subduction: Laurentia (now North America and Greenland), Baltica (Scandinavia and northern Europe), and Avalonia (southern England and east Newfoundland).  The ancient mountain chain is called the Caledonides, the Roman name for all those unruly Celts. Remnants of these once lofty peaks can be traced from Scandinavia to northwest Scotland and Ireland, eastern Greenland, and the Appalachians of eastern North America.

Our understanding of the Caledonides and mountain belts in general, took a great leap forward with the field work of Nicol, Lapworth and Callaway; the icing on this thrust-cake was provided by Peach and Horne. This story, like many others in the history of science, is interwoven with personal feuds and institutional bias. Arguing against accepted wisdom is always fraught. It is worth remembering the trials of the folk who wage, in their search for truth, against the establishment. So, the next time you put your finger on a thrust fault, spare a thought for those whose common sense and sound scientific reasoning ultimately prevailed.

 

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