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photo of the coast of Lewis with title "A Lewisian Perspective - the basement of the Earth"

A Lewisian Perspective: The basement of the Earth

Professor Frank Rennie of UHI

Professor Frank Rennie explains Lewisian geology

On March 6th Frank Rennie, Professor of Professional Rural Development at Lews Castle College, UHI, gave a fascinating lecture and comprehensive introductory tour of the geology of the Isle of Lewis.

The geology of the Lewisian gneisses is complicated, but is so much more than simply boring monotypic banded rocks, and this lecture highlighted features to look out for in the landscape.

Despite being a long-time member of the Edinburgh Geological Society, Prof. Rennie had never made it to one of our lectures so far. It was our pleasure finally to welcome him, even if it took him to be the presenter himself in order to make it here



Right at the start we’d like to invite our readers to have a look at the slides Professor Rennie provided for this talk and kindly shared online:

PowerPoint presentation – A Lewisian Perspective: The basement of the Earth


Events at the Roots of the World

“Far, far below the deepest delving of the Dwarves, the world is gnawed by nameless things. Even Sauron knows them not. They are older than he. Now I have walked there, but I will bring no report to darken the light of day.” [1]

If there are such ‘nameless things’, and if they have been down at the roots of the mountains since the beginning of time,  at about 3.2 billion years ago they would have witnessed (and likely munched on) the earliest forms of what we now know as Lewisian gneisses.

These rocks formed in the root zone of a gigantic mountain range. What we see today are the worn-down roots of a once-mighty mountain belt. This is the end state of mountain building which, millions of years in the future, even the young Alps and the middle-aged Appalachians will approach.

The rocks formed 35 – 40 km deep in the crust in a highly metamorphic environment, at temperatures of ~ 900° C and a pressure of ~10 kbar. At the time of their formation, the rocks were buried in the Earth’s crust positioned at the South Pole.
But while their formation started at 3.2 billion years ago, the gneisses as a whole spanned a period of roughly 1.5 billion years, a detail often overlooked when the immense age of these rocks is mentioned.

In order to make this inconceivable time span more tangible, Professor Rennie used one year to represent the whole of Deep Time. In this comparison the Outer Hebrides formed between February and June, while the Lewisian gneisses appeared around May. Fortunately, we can be quite sure that no creature had them for elevenses, for on this time-scale even the first fish appeared only in November.

Lewisian Gneiss and Where to Find Them

Three types of Lewisian gneisses can be found: metasediments, granitic gneisses and undifferentiated gneiss which cannot be placed in either category, and is often referred to as ‘banded’ or ‘grey’ gneiss. This latter is a highly dense rock that doesn’t break or weather easily. The thin section (see presentation) shows crystals elongated by pressure on the matrix. Other tension-related phenomena, for example boudins, or pods, occur as well.

The gneiss may be the rock on which Lewis rests for the most part, but most of it is hidden under grass, peat and other surface covers. Outcrops are mostly found along rivers or at the coast.
At Ness, a flaggy and fractured metasedimentary assemblage can be seen, while at Scourie a dyke shows in the form of its absence – the intrusion building the dyke weathered easier than the grey gneiss, which is much less susceptible to erosion. In Dail Beag Precambrian granites cut through the gneiss.

The landscape of the Outer Hebrides both directed and was shaped by ice flow during the last Ice Age. Signs of this can be seen in Sough Galson or in the interesting raised beach over glacial till at Habost.

No More Mr Gneiss Guy

The geology of Lewis and its gneisses is much more complicated than many are aware of or care to learn about. On the other hand, there is demand for education – local people as well as visitors show their interest. Professor Rennie told of rocks brought to him for identification, and requests to lead geology tours around Lewis.
Maybe, with the help of the geological institutions in Scotland and the new Scottish Geology Trust, geological ‘wildlife safaris’ and / or rock identification events similar to those held for mushrooms (albeit with a less dangerous background) could be organised in the future.

It’s no use going back to yesterday, because I was a different rock then…

Many are aware that  Lewisian geology is more akin to that of Greenland than mainland Scotland. The question, however, why the Atlantic opened to the West of Lewis, instead of separating it from Scotland and keeping it nice and tidy with Greenland, could not be answered. This is a conundrum for future geologists to research and resolve.


Our heartfelt thanks go to Professor Rennie for investing time and effort in both this highly interesting lecture and his long-awaited visit to Edinburgh.


[1] J.R.R. Tolkien, The Lord of the Rings, The Two Towers, “The White Rider”

Holyrood Park image - Deep Time Walk

Deep Time Walk at the Science Festival

Holyrood Park image - Deep Time Walk

Deep Time Walk at the Science Festival – Thursday 11, Saturday 13 and Monday 15 April 2019

The Edinburgh Geological Society is delighted to be contributing to this year’s Science Festival with a unique event in Holyrood Park, exploring deep time, in association with the team behind the award-winning Deep Time Walk mobile app. Join us in the Deep Time Walk to travel across Earth’s 4.6 billion year timeline at a rate of one million years per metre. Starting at the creation of Earth, the walk covers significant events in history, including the formation of the Moon, plate tectonics, the early evolution of life, dinosaurs and much more.

Download the award-winning Deep Time Walk app at

Full details and tickets –

Buy the Deep Time Cards direct from EGS – a set of 58 beautifully illustrated cards, giving 4.6 billion years of Earth history in a tactile, easy to use format. Each card provides a 100-million-year summary of the key transitions that have occurred across Earth’s deep history, with associated reference data and inspiring quotes.

Hutton's Section, Salisbury Crags

HOGG comes to Edinburgh on 11 and 12 July 2019

James Hutton by Sir Henry Raeburn, on display in the Scottish National Portrait Gallery

The History of Geology Group of the Geological Society of London and the Edinburgh Geological Society are organising an open meeting, Aspects of the History of Geology in Scotland and the North of England, at Surgeons’ Hall, Nicolson Street, Edinburgh.  This meeting will include presentations on Thursday 11 July, followed by a field day on Friday 12 July. Friday will feature a walking tour of sites linked with the history of geology around the city, such as Hutton’s Section on Salisbury Crags. Further details can be viewed at meeting.


Excursion Programme 2019

Location map for Excursion Programme 2019

Our bumper Excursion Programme for 2019  allows you to experience the superb range of geology that southern Scotland has to offer.   The programme starts on Wednesday 24 April with a short evening excursion around Corstorphine Hill, and runs right the way through to 20-22 September with the weekend excursion to Stonehaven and Highland Boundary Fault. In between these trips, there will be 16 others Wednesday evening (7-9pm), Saturday (all day) and Introductory excursions for new members (x3). Do try and take this opportunity to learn some field geology from our expert leaders.

Field Excursion to Ardross in Fife by Neil Mackenzie

Visit our website page Excursions to find out more details and how to book them.


EGS Strategy – Members’ Views

EGS Council is reviewing our five-year Strategy and would welcome views from the membership. The current strategy for 2015-19 is available online at

Please complete a short online survey (it has only 4 questions) that asks for your feedback on what we do and your ideas for the future. There is a modest prize draw! You can find the survey at

Deadline for responses – 31 March 2019.

Clough Medal

Clough Medal Lecture – Sideways views of Scottish Garnets: Insights into Metamorphic Processes

The Clough Medal is presented annually to a geologist whose original work has materially increased the knowledge of the geology of Scotland and/or the North of England, alternatively someone who is Scottish and has significantly advanced the knowledge of any aspect of geology. Find out more about Charles T Clough.

This year’s recipient is Dr Tim Dempster from the University of Glasgow School of Geographical & Earth Sciences in recognition of the exceptional contributions he has made to advancing the understanding of the geology of Scotland, particularly the metamorphic geology of the Highlands, where he has employed a wide range of technologies to better understand the processes; and his work as a dedicated and popular teacher of undergraduate students, using the Highlands of Scotland as a ‘natural laboratory’.

In this joint lecture with the Geological Society of Glasgow Dr Dempster talked about garnets, the workhorse of metamorphic petrologists.

Being capable of recording original compositions during growth, garnets allow determination of pressure-temperature paths and durations of metamorphic events.

Concentrating on the contact zone between the garnet and its surrounding minerals, a 3D image of the garnet rather than a thin section – which would offer too small an area for clear analysis – was created. Then the team projected a ‘map’ of the adjacent minerals on the garnet’s surface and variations in its chemistry, e.g. the Calcium content, were recorded.

This way of analysing the garnet showed that the crystal’s growth itself can change the chemistry of the surrounding matrix and hence the reaction path during growth, leading to a ‘false’ temperature recording. Dr Dempster’s studies of garnets from the Scottish Highlands therefore question some key concepts of metamorphic equilibrium.

Background reading: Dempster, T. J., La Piazza, J., Taylor, A. G., Beaudoin, N. and Chung, P. (2017) Chemical and textural equilibration of garnet during amphibolitefacies metamorphism: The influence of coupled dissolution-reprecipitation. Journal of Metamorphic Geology, 35, 1111-1130.

Central Scotland Regional Group of the Geological Society

The Central Scotland Regional Group of the Geological Society organises regular events at various locations in Glasgow and Edinburgh. All event details are available on their web page. Meetings generally start at 6:15pm with tea and coffee from 5:30pm. All welcome.

The next event is a lecture by Mark Hudson on Sub-surface Laser Scanning, Multi Beam Sonar Surveys & Void Surveys
Date: 12 February 2019
Venue: Edinburgh Centre for Carbon Innovation

Mark Hudson presents on recent developments in sub-surface scanning and void surveys including ongoing research and development in the area, Mark will draw on his many years of experience in investigation of underground voids with numerous case studies from the UK.

Should you have missed or are not able to attend one of the CSRG events, presentations can be found on the past meeting resources webpage or their YouTube channel.

Evidence of a large explosive silicic eruption on Skye

An article recently published in Nature Communications has suggested a connection between the Sgúrr of Eigg and a distant rocky outcrop Òigh-sgeir, with a major volcanic eruption on the island of Skye and a significant climate event 55 million years ago, the Palaeocene-Eocene Thermal Maximum (PETM).

Valentine R. Troll et al compared mineralogy and isotope geochemistry of the pitchstone on Eigg and Òigh-sgeir, and the results suggest that the two outcrops represent a single, pyroclastic deposit. Prior to this study, David Brown and Brian Bell (in a paper published in 2013) had suggested a connection between the outcrops and an volcanic eruption on Skye – the new paper confirms these results and proposes a connection with the PETM.

The magnitude of the Skye volcanic eruption was estimated to 3.9- 15 km3 DRE (dense-rock equivalent) and a 5-6 on the Volcanic Explosivity Index scale, which compares with historical examples such as the 1991 Pinatubo eruption ( ~ 5 km3 DRE). The results imply that large-scale explosive silicic eruptions have likely been common during the opening of the North Atlantic. This paints a more violent picture of the rift to drift transition of the North-Atlantic region between 61 and 56 million years ago than previously assumed.

Source and run-out distances of the proposed Skye volcanic event, Troll et al. 2019

You can read more about the Sgúrr of Eigg and different ideas proposed in the past to explain its formation by some of Scotland’s well-known geologists (including Hugh Miller and Archibald Geikie) in the Geology of Eigg (2016), by John D Hudson, Angus D Miller and Ann Allwright, published by the Edinburgh Geological Society.

Valentin R. Troll, C. Henry Emeleus, Graeme R. Nicoll, Tobias Mattsson, Robert M. Ellam, Colin H. Donaldson & Chris Harris: “A large explosive silicic eruption in the British Palaeogene Igneous Province”. Scientific Reports volume 9, Article number: 494 (2019) Published in Nature

A summary of the article was included in the BBC news website

The original article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format. Creative Commons licencing


2018 EGS Public Lecture – What the Ice Age ever do for us?

At this time of year, its good to reflect on our successes as a Geological Society. This year we held our 2018 Public Lecture event at Dynamic Earth, Holyrood Road, Edinburgh on 21st November 2018. The event attracted around 150 people of all ages. Our topic was the Ice Age and we heard from three excellent speakers – Prof Emrys Phillips and Dr Carol Cotterill (both BGS Scotland) and Dr Tom Bradwell (University of Stirling). This event was our 2nd major public lecture in recent years

Landscape and the human psyche – by Dr Carol Cotterill

During the interval at this event, a rolling slide show was presented giving an artistic interpretation of the landscape.  The work was put together by one of the speakers, Dr Carol Cotterill.  We thought that people may not have had a chance to view this in full and so a pdf download is now available.  Please note that the contents are copyrighted to Dr Cotterill, we would therefore encourage anyone who wants to make use of the material to contact Carol directly.

Thanks again to all who contributed to this year’s successful 2018 Public Lecture.

Secrets of the Philosophers’ Stone unveiled by Physicians’ College

The Royal College of Physicians of Edinburgh is revealing the secrets to finding the Philosophers’ Stone, as part of its new public exhibition – Searching for the Elixir of Life: The mysteries and legacies of Alchemy – which looks at the ancient tradition of alchemy that stretches back thousands of years and was a mysterious early version of science and spiritualism. This is the first time in 337 years that the college’s fascinating collection of alchemical books and manuscripts has been put on public exhibition. The display will include the College’s Ripley Scroll – one of only 23 surviving copies anywhere in the world, and the only one in Scotland. The scroll uses symbols and illustrations to reveal the steps needed to create the Philosophers’ Stone.

The exhibition is free to attend and open to the public from 10am-4.30pm Monday to Friday, until summer 2019, at the Royal College of Physicians of Edinburgh, 9 Queen St, Edinburgh EH2 1JQ.

Further information about the exhibition on the Royal College website.