82 | Great Geologists
to complete! Peach and Horne led a team of geologists that
progressively mapped the region at the extremely detailed
scale of 1:10,560 (six inches to the mile), beginning on the
north coast in 1883, and completing the mapping on Skye in
1897. It took ten more years to write-up their seminal memoir,
“The Geological Structure of the North-West Highlands of
Scotland.”
The 1907 memoir is more than an exceptionally detailed
description of regional geology. It explains how mountains
form by the long-term deformation of the continental crust.
It describes deformation structures at a variety of scales and
explains how the recognition of these in the field can be used
to understand broader tectonic processes. It is more than
the first major synthesis of thrust belt structure — Peach and
Horne provided the basis for understanding fault and shear
zone processes, and methods for unraveling tectonic histories
in metamorphic basement. It established the North-West
Highlands as a key location for training in structural geology, a
situation that remains unchanged today. Moreover, it provided
an impetus for the understanding of tectonic processes in
Europe, North America and beyond.
Structural cross-sections from the 1907 North-West Highlands
Memoir.
Peach and Horne were the leaders of a team, and each team
member contributed to the research in their own way. For
example, in 1889, one of the team members, Henry Cadell,
developed a ‘squeeze box’ — a method of mixing layers of
plaster of Paris with wet sand and clay in a box to which a
horizontal force could be applied. This caused deformation
of the ‘strata’ within it, mimicking some of the folding and
thrusting seen in the North-West Highlands. This greatly helped
Peach and Horne understand the processes that had led to the
large westward translation of the Moine Thrust Sheet, by what
we now know to be as much as 100 km.
The closing chapter of Peach and Horne’s work on the
North-West Highlands was leading a field trip for the British
Association for the Advancement of Science in 1912. This was
attended by 30 notable geologists from Britain and Europe,
including the great Swiss structural geologist, Albert Heim.
The party enjoyed an exceptional trip and passed the evenings
composing and singing La Chanson du Moine Thrust (the
Moine Thrust Song). They also paid extensive tribute to the
brilliant work of their leaders. Heim remarked, “They are a
couple of scientists, Investigator-Twins, such as I never have
seen before in my life, two men so delightfully developed in a
wonderful common work of research.”
The Arnaboll Thrust (part of the Moine Thrust Complex) at Ben
Arnaboll with Lewisian Gneiss thrust over Cambrian quartzite. It
was outcrops such as this that enabled Lapworth and Peach and
Horne to appreciate the true nature of the rock succession in the
North-West Highlands. Geikie introduced the term “thrust plane”
after visiting this outcrop with Peach and Horne. Photograph
courtesy of Prof. Rob. Butler, University of Aberdeen.
The North-West Highlands is now a Geopark (www.
nwhgeopark.com) with abundant opportunities for anyone with
an interest in following in the footsteps of Peach and Horne.
Geologists from across the world regularly visit to be amazed
at the detailed mapping and unravelling of complex structural
geology by one of the most famous pairings in the history of
geological research.