Peach and Horne – the Memoir at 100

Full text

feature
Rob Butler* dusts down his copy of the North-West Highlands Memoir to find that it still talks to 21st Century
geoscientists a century later…
Peach and Horne -
On a small hill overlooking Loch Assynt in NW Scotland is a famous yet simple stone memorial
to a remarkable geological double act: Ben Peach and John Horne. Generations of students
have driven past this site, on their way to another, often rain-cooled day of field training,
perhaps regaled by their lecturers of the exploits of their forebears and the golden years of
NW Highland geology, at the end of the 19th Century.
Much has been written over the years of these times and the exploits of the various protagonists.
Visitors to the nearby Inchnadamph Hotel will have seen the famous photograph of the
pair (below) and perhaps posed on the bench outside the bar. This Bicentenary year also
marks the centenary of Peach and Horne’s most remarkable publication, the Memoir The
Geological Structure of the North-West Highlands of Scotland. Not only was it an instant classic,
the 1907 Memoir remains a masterpiece of regional geoscience.
Famously Edward Suess, the Alpine
tectonicist and synthesizer, remarked
that Peach and Horne’s work had
rendered the mountains transparent.
The Memoir made key statements
about structural geology, opening
debates that remain unresolved
and establishing NW Scotland as
a key site in world geology. This
importance continues today, with the
recent designation of the region as a
UNESCO Geo-Park. So what’s in the
1907 NW Highlands Memoir?
The geology of NW Scotland is highly
varied, so the Memoir covers a lot of
ground. The basic sedimentary units
and their relationships are of course
fully described. Perhaps I betray my
Title image: Photograph of the Glencoul area, looking NNE
from the summit of Spidean Coinnich (Rob Butler).
Left: 1912 portrait of Peach and Horne outside the
Inchnadamph Hotel.
Top right: Peach and Horne’s cross-section of the Moine
Thrust Belt at Glencoul, from the classic Assynt Special
Sheet. The eponymous thrust is illustrated truncating the
imbricate slices of Cambrian sediments beneath Elliott and
Johnson subsequently modified this interpretation to show
no such truncation. The section also shows the Moine
Thrust (Stack of Glencoul). Courtesy of British Geological
Survey IPR/80-17C
Right: Peach’s watercolour (from his field notebook) of the
Glencoul Thrust on the Aird da Loch peninsula and, in the
background, the Moine Thrust on the Stack of Glencoul.
Courtesy of British Geological Survey IPR/80-17C
*Reader in Orogenic Geology in the Institute of Geophysics
and Tectonics, The University of Leeds. Rob continues to
appreciate the cooling and rehydrating properties of
Highlands fieldwork. Mike Johnson is thanked for comments
on an early draft. E: butler@earth.leeds.ac.uk

Volume 17 • No. 1 • 15
feature
background but it’s for the structural geology that I find the
Memoir most perceptive. There are substantial discussions
on the geology and evolution of the Lewisian complex,
much of which we take for granted today. Peach, Horne and
colleagues mapped out basic field relationships and used
these to deduce a long and complex tectono-metamorphic
history. The terms Scourian and Laxfordian were not
coined until the 1950s by John Sutton and Janet Watson.
But the Survey geologists had established that the meta-
basic sheets within the Lewisian represented dykes and
sills at different states of deformation and established the
workflows for building tectono-metamorphic histories in
basement rocks. What Sutton & Watson did was to make
more detailed maps, but they followed essentially the
same workflows.
Using the Survey maps for context, Peach and Horne’s
colleague Jethro Justinian Harris Teall made detailed
petrological studies to deduce links between deformation
and metamorphism. This fired the starting pistol for
research on reaction-related softening mechanisms in the
continental lithosphere that continues today. By recognising
that “the old lines of movement which existed before the
Torridon Sandstone was laid down have in some places been
used again in post-Torridonian time” (p. 251), the team were
fully aware of the notions of structural inheritance, an
issue again that is alive today.
Geikie
In his preface, Archibald Geikie predicted that the value of the
Memoir lay in: “the full description and illustration which it
contains of the remarkable tectonic structures, the discovery of which
has made the north-west of Scotland a classic region for the study of
some of the more stupendous kinds of movement by which the crust
of the earth has been affected”. Geikie was referring to what we
now know as the Moine Thrust Belt. Indeed it is for this that the
Memoir is most famous, even though only about a fifth of it was
concerned with thrust belt structures. John Horne’s introductory
chapter to this part of the Memoir is a startling synthesis. For
the first time it gave basic descriptions of imbricate slices,
thrust sheets, the folds they contain and the thrusts that delimit
them. There follows an account of the field localities that
remains valuable today. The geometries were displayed on
cross-sections, with text descriptions that outline the rationale
behind the interpretations of structural geometry. Apart from
structural geometry, the Memoir recounts the use of kinematic
indicators to determining the direction of thrusting from the
alignment of sheared worm burrows and related mineral
fabrics. These insights build on the work on Peach and Horne’s
contemporary Charles Lapworth in relating fault rocks,
especially mylonites, to thrust evolution.
These interpretations were remarkable for the early 20th Century.
But most of the thrust belt concepts were first published almost
20 years earlier by the team in their 1888 paper in the
- the memoir at 100

16 Geoscientist
feature
Geological Society’s Quarterly Journal. An expectant world had
nearly a 20-year wait for the definitive article. Geikie
commented that: “some geologists find literary labour more
irksome and arduous than field-work, and would rather survey
many square miles of complicated ground than write a few pages
descriptive of them”. It is a sentiment since echoed, perhaps in
more colourful language, by many a PhD supervisor.
It is a curiosity through much of science that apparently
definitive statements on a subject commonly result in
research activity moving elsewhere. So following publication
of the 1907 Memoir there was scarcely any investigation of
thrust system geometry in NW Scotland. In the middle of
the century there were a few attempts to apply thrust-type
interpretations in the complex world of ductile deformation
in the Moine, notably by Kennedy and Watson, and much
later by Geoff Tanner. Otherwise structural geologists
focused on detailed examination of deformation fabrics and
indulged in labyrinthine numerations of fold phases.
Finally in 1980 things got going again, with Dave Elliott and
Mike Johnson’s reinvestigation of structural evolution in the
northern part of the belt. Elliott brought the balanced sections
approach from the oil and gas industry exploring in the
Rocky Mountain foothills. This represented new technology,
with the interpretations of structural geometry shown on
cross-sections tested geometrically through the ability to
graphically restore the sections to pre-deformed state. But
the application to NW Scotland relied heavily on the Survey’s
century-old field observations. Jack Soper and Tony Barber
took this further. Their geometric model for the deep
structure of the thrust belt is arguably the first crustal-scale
balanced section.
All the new ideas, together with the acquisition of deep seismic
profiles (MOIST - BIRPS’ first profile was explicitly designed
to image the Moine Thrust on a crustal scale) prompted a
wholesale re-examination, by a team led by Mike Coward.
This involved substantial new mapping of the region and,
although the maps themselves were not published at the
time, they are a key component of the new compilation of
Moine Thrust Belt maps being published currently by BGS.
One of the key issues concerning those of us who have
worked in the Moine and other thrust belts over the past
twenty years has been the sequence and nature of tectonic
Clough’s field map for the head of Loch Glencoul, illustrating the map pattern for the truncation of imbricated Cambrian sediments by the Glencoul Thrust. The basic mapping
scale, seen here, is 1:10,560 and formed the basis for the regional 1” (1:63,360) compilation. The scene reproduced here represents about 1 km2. BGS ©NERC

Volume 17 • No. 1 • 17
contacts. It provides an illustration of how the Memoir retains topicality.
The general challenge might be framed today as understanding the
patterns of strain localisation in continental crust. Deducing the relative
activity of faults is a key part of this work. Twenty first Century
tectonicists might use sophisticated monitoring campaigns in
tectonically active regions to examine slip rates on faults, backed up
with absolute dating using the multitude of geochronological tools at
their disposal. Nineteenth Century field geologists used cross-cutting
relationships displayed on maps and sections, and these still provide
critical tests for modern data. It is not surprising, but none the less
sobering, to find John Horne discussing the “Probable sequence of
movements” in Chapter 32 of the Memoir. His general conclusion was
that the structurally higher thrust planes moved last, slicing across
earlier, more outlying structures. Consequently it was this style of
structural relationship that appears on the cross-sections of the time.
Field slips
It is worth investigating where the notion came from. Carefully archived
at Murchison House, BGS have retained Ben Peach’s notebooks and
field slips. At an early stage in their investigations the Survey team
sought an explanation for the repetition of stratigraphic units as
encountered on transects across structural trend. In a world without
thrusts such repetitions would classically be interpreted as due to
folding. But Peach saw that this predicted a symmetrical alternation of
rock units, not the asymmetrical pattern he found. So he proposed
imbricate faulting. Field notes include a modern-looking sketch in
which imbricate thrusts repeat stratigraphy, alongside the restored
geometry. It’s almost the world’s first balanced and restored cross-
section. The figure even illustrates how different erosion levels
through the structure generate different outcrop patterns. No wonder
his contemporaries held Ben Peach in such high regard.
Peach’s sketch from the 1880s is informative because it also shows
the limitations, albeit viewed with 120 years of hindsight! For his
restoration attempts to explain neither the relationships between the
imbricate thrusts themselves, nor with any major thrusts that might
contain them. And it is in these parts that the key lies in resolving the
issues of fault sequencing. In the Memoir John Horne notes that
imbricate thrusts are generally overstepped by whichever major thrust
plane overlies them, arguing that this requires truncation. Thus
higher thrust surfaces were believed to postdate the imbricates below.
The duplex model applied by Elliott and Johnson in 1980 treated these
relationships differently, arguing that the imbricate thrusts merge
into the overlying thrust planes. This model, generally termed
“piggy-back thrusting” is widely believed to be the more appropriate
nowadays.
Horne’s Memoir chapter 32 indicates that there was no model of
thrust sequences that worked universally in the thrust belt, hinting at
debates between Peach on one side and Lapworth and Cadell on the
feature
The cross-section through the Dundonnell structures from the 1907 Memoir, based on the
fieldwork of William Gunn. Courtesy of British Geological Survey IPR/80-17C
Dave Elliott
Mike Johnson
Mike Coward
Jack Soper and Tony Barber

the Dundonnell structure, including being cited as an ideal demonstration
of a specific duplex type, it is ironic that it lies in an area of perhaps the
poorest exposure in the entire thrust belt.
The two models of thrust sequences make different predictions of how
the stratigraphic units are distributed against thrust surfaces. In
other words, different geological maps demand different structural
interpretations. A common conceit is that field maps should be
objective documents. Edward Bailey in his Tectonic Essays maintained
that Peach and Horne “merely reproduced what they saw in Nature, and left
it at that”, working in “an atmosphere of detachment”. Yet it is clear from
Horne’s chapter 32 in the Memoir that thrust relationships had been a
topic of discussion throughout the mapping campaign. And Peach’s
hypothetical illustration of imbricate structures made clear predictions
of stratigraphic relationships.
The mapping was by no means objective and free of hypothesis – of
course it never is. No field geologist can ever hope to record everything.
What is recorded depends on what hypothesis is being tested at the
time. So it’s as well to have a hypothesis to test! Consequently it
means that in areas of complex structure, new hypotheses making new
interpretations generally require new mapping. Fortunately, attempts
in recent years to generate standardised workflows and methods of
field investigations, beyond the basics of recording simple data and
observations are no longer fashionable as they stifle new ideas and
fossilize perhaps erroneous hypotheses. Any objectivity they might
provide is entirely illusionary.
Through the early 1980s Mike Coward, his students and colleagues set
about remapping swathes of the Moine Thrust Belt. Like Ben Peach,
Coward was a great one for developing ideas in the field, returning
from regular forays in the thrust belt with new hypotheses and novel
structural geometries. The mapping uncovered relationships between
rock units that had gone unrecognised by Peach and Horne that were at
least consistent with Elliott and Johnson’s notion of piggy-back thrusting,
But there were entirely new geometries too, where higher thrusts were
re-imbricated along with the rocks that underlay them. And Coward
found sites where low angle thrusts truncated the imbricates below, as
proposed by Peach and Horne. The reality is that there probably is no
general sequence of thrusting. Indeed other thrust belts where there are
stratigraphic constraints on timing show synchronous thrusting and
that any strictly sequential models are illusory.
other. A famous exception to the then-preferred
overstep model is the Dundonnell structure, found
just south of Ullapool. The Survey’s section shows
higher thrusts, including the Moine, folded by what
might now be called a detachment fold developed
on the Sole Thrust. This part of the thrust belt was
mapped by William Gunn somewhat later in the
campaign than the classic Assynt and Eriboll
sectors where the overstep model was erected.
There must have been some interesting discussions
convincing the more senior members of the team
of the geometry. A century later Elliott and
Johnson seized on the Dundonnell structure as
their key representation of piggy-back thrusting,
reinterpreting the fold at its core as a stack of
imbricate thrusts. Given the subsequent fame of
feature
Image below: The critical ground at the head of Loch Glencoul, looking
NNE with the direction of thrusting inferred to be from right to left
(i.e. WNW). The visible hillside is approximately 450m high.
Photo: Rob Butler).

Volume 17 • No. 1 • 19
Coward proposed that many low-angle, truncating structures
such as proposed by Peach and Horne were actually extensional
and found other such structures within the major thrust
sheets. The debate still rumbles on as to the significance of
truncating structures, so these debates are by no means settled.
Indeed they probably cannot be resolved definitively in the
Moine Thrust Belt where there is little direct evidence for
the orientation of the rocks at the time of deformation nor
synorogenic sediments. However the key point is that each new
interpretation has been based on a new map the construction
of which falsified the previous map interpretations.
Each interpretation has its own suite of relationships between
the various geological boundaries, be they stratigraphic or
structural (or igneous). The challenge facing each new
interpreter is to create a topologically robust three dimensional
model and then to show the outcrop pattern of this structure
in the landscape. And even if Coward’s proposition of
extension cannot be tested definitively using the 400 million
year old structures of NW Scotland, the concept has been
applied and tested successfully in other parts of the world.
So the Moine Thrust Belt has continued to inspire structural
research internationally, in the same way that Peach and
Horne’s descriptions did.
The geometric and spatial distribution of geological materials
remains a fundamental part of understanding geological
processes, especially in tectonics. And these relationships in
three dimensions are important constraints on models for
their formation. If we don’t understand the product, how can
we understand the processes? So understanding structural
geometry will remain a critical issue for solid Earth science.
The need to challenge geometries of previous interpretations
suggests that field mapping will continue to be important too.
However, in the 21st Century, with increasing use of 3D seismic
and digital landscape data, there is a tendency to imagine that
virtual world is the prime environment for developing structural
geology understanding. Leading structural geologists in the oil
industry, who routinely work 3D seismic, tend to disagree with
this notion that field mapping is a thing of the past.
As Steve Matthews, Global Geoscience Adviser at BP told me:
“Having worked in industry for over 23 years, I can think of
no better training than field mapping projects for maturing
ability in effective interpretation of 3D geological structure”.
Field mapping, like seismic interpretation, rarely delivers a
unique structural interpretation. Understanding the range of
options is important, and attempting to map out structural
geometry can quickly identify those hypotheses that are
geometrically implausible.
The 1907 Memoir and the research it describes, portrayed in
the spectacular series of 1”geological maps published by the
Survey, is then a stunning achievement. It represents arguably
the starting point for thrust belt research worldwide and
established the international status of Peach and Horne. Above
all it bears testimony to the importance of field mapping in
areas of complex structure as the key tool for tectonic
research. But field maps are highly interpretative. They
illustrate hypotheses and marshal the spatial relationships
between rock units that are explained by these hypotheses.
This may sound horribly subjective, but at least the rocks
remain to be visited. The experiments can be re-run to
inspire new models and concepts. Within the NW Highlands
Memoir it is the inspirational qualities of fieldwork that shine
through the years.
feature
Further reading and comments
Belt or zone?
Many geologists use Moine Thrust “Belt” and Moine Thrust “Zone”
interchangeably to mean all the thrust structures between the undeformed
foreland and the sheets of Moine metasediments (what the Survey called the
“zone of complication”). I prefer to use thrust “belt” for this – because that
is what other thrust systems around the world are generally termed. Thrust
“zone” to many conjures images of a single structure (e.g. shear zone). So in
this regard it may be with using Moine Thrust Zone to refer to the Moine
Thrust itself, perhaps with its associated mylonites.
There is no substitute for getting hold of the Memoir itself, the Assynt Special
geological map (see below) or, for the northern part of the thrust belt, the
preliminary report:
Peach, B.N., Horne, J., Gunn, W., Clough, C.T., Hinxman, L.W. & Teall, J.J.H.
1907, The Geological Structure of the NW Highlands of Scotland.
Memoirs
of the Geological Survey of Great Britain, p. 668.
Peach, B.N., Horne, J., Gunn, W., Clough, C.T., Hinxman, L.W., & Cadell, H.M.
1888. Report on the recent work of the Geological Survey in the north-
west Highlands of Scotland, based on field notes and maps by Messrs.
B.N. Peach, J. Horne,W. Gunn, C.T. Clough, L.W. Hinxman, L.W. and
H.M. Cadell.
Quarterly Journal of the Geological Society of London,
44, 378-441.
Otherwise the spectacular summary of Peach and Horne’s mapping is still
available on the Assynt Special Sheet, published by BGS at 1” to 1 mile,
complete with cross-sections (check out the BGS on-line bookshop).
Historical perspectives are given by:
David Oldroyd’s 1990
“The Highlands Controversy: constructing geological
knowledge through fieldwork in nineteenth-century Britain”
(Chicago
University Press, 1990),
précised
in Chapter 2 of Nigel Trewin’s “
The Geology
of Scotland
”(4
th edition, The Geological Society, 2002).
The first major re-interpretation of Moine Thrust Belt structure after the 1907
Memoir was:
Elliott, D. & Johnson, M.R.W. 1980. Structural evolution in the northern part
of the Moine thrust belt, NW Scotland.
Transactions of the Royal Society
of Edinburgh
,71, 69-96.
The crustal-scale balanced sections appeared in:
Soper, N.J. & Barber,A.J. 1982. A model for the deep structure of the Moine
thrust zone.
J. Geol. Soc. 139, 127-138.
An account of Mike Coward’s researches, including a full reference list is in
the following publication in his memory, which includes some modern papers
on the Moine Thrust Belt too:
Ries,A.C., Butler, R.W.H. & Graham, R.H. (eds).2007, in prep.
Continental tectonics:
a multidisciplinary approach
. Special Publications of the Geological Society
Many of the geological maps based on the 1880s work are still available for
purchase from BGS (at 1:50,000 and 1:63,360). These include the classic
Assynt Special Sheet – a real eye-opener!
A selection of Peach, Horne and their colleagues work, including cross-
sections, images from field maps and note books is to be found on the Assynt
Geology website: http://earth.leeds.ac.uk/assyntgeology
The “bandits of Inchnadamph” – part of the Geological Survey team on location at the
time of the main mapping campaign in the NW Highlands. Courtesy of British Geological
Survey IPR/80-17C