The first 2,500 million years of the geological history of Britain are stored in the gneisses of the Lewisian Complex of North West Scotland. This book explores the long journey of discovery in which this history has been gradually deciphered since the end of the 19th Century when these rocks were first investigated in detail. The usual tools of stratigraphic investigation were of no value in dealing with such a complex assemblage of highly deformed and metamorphosed rocks; there was no fossil evidence and few signs of recognisable sedimentary strata.
This book charts the increasing sophistication of the geochronological and geochemical techniques used to decipher the complex. The first important breakthrough was the recognition that a set of intrusive metamorphosed dykes could be used, perhaps, to separate episodes of deformation and metamorphism that occurred before the dykes were intruded, from those that occurred subsequently.
Geochronological dating methods evolved from the first relatively crude potassium-argon and uranium-lead dates in the 1950s to the present amazingly accurate lead isotope dates. Geochemical techniques have also advanced to the point when mafic igneous assemblages can be identified as having oceanic volcanic arc signatures or were the products of intra-continental magmatism. Thus, from a stratigraphy composed of three events, Scourian, dyke intrusion and Laxfordian, has grown a complex history covering many separate events of igneous, metamorphic and tectonic activity spanning 2,500 million years of Precambrian time.
Much of the extensive literature on the Lewisian is highly specialised and not easily accessible to the general reader; this book is an attempt to distil the most important results of this research into a more user-friendly form. It will appeal to many geologists including students, geological visitors to the North West of Scotland and academics seeking a readable account of remarkable and significant advances in earth science.
1 - Introduction
PART I: THE PIONEERS
2 - Early ideas
3 - The 1907 Memoir
4 - Sutton & Watson 1951: the Scourian and the Laxfordian
5 - Mapping of the Outer Hebrides
PART II: GATHERING THE DATA
6 - The Loch Maree Group and the Inverian
7 - The 1960s re-mapping of the Mainland
8 - Problems of correlation and nomenclature
9 - Geochronology: initial steps
10 - Nature and origin of the ‘Fundamental Complex’
11 - Re-mapping of the Hebrides
12 - The 1971 Lewisian Conference: a summary of progress
13 - The Mainland revisited
PART III: MODELS and HYPOTHESES
14 - Introduction of the shear zone model
15 - Kinematic models
16 - Petrogenesis I: the Scourian Complex
17 - Petrogenesis II: the Proterozoic
18 - Towards a Lewisian chronology
19 - The wider picture: tectonic models. Glossary
Graham Park has studied the Lewisian complex for over 60 years and is in a unique position to provide this account in the scientifically sound but readable manner familiar from his earlier books such as Mountains and Breakthroughs in Geology.
"[...] this is an outstanding major new contribution to the geology of the Lewisian that will stand the test of time and will be of use to anyone interested in the Lewisian, or Precambrian geology in general, and in the development of ideas in structural and metamorphic geology and geochronology. Graham Park is to be congratulated on producing a work of the highest academic standard that will remain a key reference and is a tribute to his life's work. I recommend it wholeheartedly, it has been a pleasure to read it, and I will doubtless re-read it several times over."
– The Edinburgh Geologist