64 | Great Geologists
some cases — before moving to a shepherd’s cottage at
Heilam near Loch Eriboll. There, he found an outcrop at An
t-Sròn that could act as a stratotype for the region (in a similar
manner to the function of Dob’s Linn in the Southern Uplands).
Unlike the Southern Uplands, the succession in the Northwest
Highlands is poorly fossiliferous, so Lapworth was forced to
focus on detailed lithology to build up his understanding of the
stratigraphy.
Mrs Elizabeth Grey, 1922. Her extensive fossil collections proved
invaluable to Lapworth. Provided by and used with permission of
The Natural History Museum.
of the leading centres for geological teaching and research in
Britain.
Notwithstanding his location in the Midlands, Lapworth
continued to focus his research on Scotland, now turning his
attention to the Northwest Highlands. This region had been
the subject of a long-standing controversy, with Murchison
once again on one side of the debate. Murchison considered
the succession there to be effectively continuous, despite
metamorphic schists overlying unmetamorphosed limestones,
shales and sandstones. He claimed much of this succession for
his Silurian system. Others, such as James Nicoll of Aberdeen
University, considered the contact at the base of the schist to
be tectonic. Lapworth decided to see if his detailed mapping
methods employed in the Southern Uplands could be used to
shed light on this problem.
Lapworth carried out two intense field seasons in 1882 and
1883, in the area in the far north-west of Scotland, around
Durness and Loch Eriboll. In his mind were some of the ideas
on mountain building, including what would become known
as thrust tectonics, being promoted in continental Europe by
geologists, such as Escher, Heim, Suess and Brøgger. He
was familiar with reverse faulting from his observations in the
Southern Uplands (such faults are present at Dob’s Linn, for
example).
As was his practice, Lapworth began mapping around Durness
at a very detailed scale — twenty-five inches to the mile in
Lapworth soon recognized that the contact between the
metamorphosed schists and the underlying sediments was
tectonic. Indeed, the contact was often characterized by
a specific new rock type formed by actions of shearing —
mylonite. In a letter to his friend and colleague, Thomas Bonney,
he describes such rocks as follows: “Conceive a vast rolling and
crushing mill of irresistible power, but of locally varying intensity,
acting not parallel with the bedding but obliquely thereto; and
you can follow the several stages in imagination for yourself.
Undulation, corrugation, foliation and schistose structure – slaty
cleavage are all the effects of one and the same cause…..Shale,
limestone, quartzite, granite and the most intractable gneiss
crumple up like putty in the terrible grip of this earth-engine
– and are all finally flattened out into thin sheets of uniform
lamination and texture.”
By 1883, Lapworth was able to publish a preliminary account
of his interpretation, noting the structure to be dominated by
“gigantic overfolds” and “gliding-planes, along which the rocks
have yielded to the irresistible pressure of the lateral Earth-
creep during the process of mountain-making.” This was a first
description of large-scale thrust tectonics in British geology.
Lapworth called it, “The Secret of the Highlands.”
Towards the end of his 1883 field season, Lapworth suffered
what appears to have been a nervous breakdown. The popular
story is that he imagined the great Moine Nappe grating over
his body as he lay tossing in his bed at night. Perhaps, he
was concerned about another intellectual fight with Geikie,
who, as ever, supported the simplistic Murchison view of the
succession (i.e. all conformable). How true this is uncertain,
but after 1883, he left research in the region to others. Having
clarified the large-scale structure, the details would be fully
realized by the Geological Survey team led by the legendary
team, Peach and Horne.
The later part of Lapworth’s career focused on the Paleozoic
geology of the Midlands and Welsh Borderlands, where his
application of detailed biostratigraphic zonations extended
downwards into Cambrian strata.
Lapworth’s research on the graptolites, which began during
his work in the Southern Uplands of Scotland, continued
throughout his career, and he became world renowned as the