"There are plenty of worlds still to conquer in Cornwall - worlds which succeeded each other in
time. Out of the ruin of these
worlds Cornwall is built, and it is for young geologists of our county ... to set themselves with
patience and perseverance to the
task, and rest not until the stratigraphical puzzle has been solved, the true sequence of
Cornish sedimentary rocks discovered, and
the conditions of their formation clearly realised."
Howard Fox, 1894
The geology of the Lizard peninsula in Cornwall has been a matter of study, discussion
and argument for nearly two and a half centuries. In fact the debate was started by
Borlase in 1758, who first saw the geological interest of the Lizard peninsula. The first
detailed research was undertaken by De la B_che in 1839. Since then many a famous
geologist has looked into the subject and until recently all were unable to give the origin
of the formation of the peninsula. For the Lizard peninsula has a totally different
sequence of rocks than the rest of the Cornish peninsula. And although geologists soon
managed to establish this sequence, the actual origin of the formation of the Lizard
complex remained uncertain. However over the last ten years, geologists, using newly
acquired knowledge have now managed to give a satisfactory explanation of the
formation and origin of what was one of England's greatest geological enigma.
In a first part we shall discuss the geological originality of the Lizard peninsula as well
as the reasons for which its origin remained so long obscure. In a second part we will
present the knowledge which was acquired during previous research and especially the
rock sequence of the peninsula. Finally we will look at the hypothesises, and then the
discoveries which led to the breakthrough in the last ten or twenty years.
At a first glance the Lizard peninsula could look to a geologist as an ideal subject of
study. It presents a coastal section nearly forty kilometres long, of very varied
geological nature. This peninsula itself is constituted by a large platform (figure 1),
sloping only slightly towards the sea. This platform is the direct result of erosion in the
Pliocene age whilst the area was submerged beneath the sea.
However a closer look at the situation reveals the many problems that geologists have
been confronted with since De la B_che in 1839. One of the most important of these is
the absence throughout the peninsula of variations in the relief which would clearly
indicate variations in the rock type. However geologists soon learnt to tell these by the
different kinds of vegetation, reflecting the differences in surface soils and thus in
rocks.
Large areas of the land are covered by heather and thorn, revealing the sterile nature of
the soil beneath them. Other parts are used for agriculture, this rich land contrasting
with the one mentioned earlier.
Although the coast is in many respects quite spectacular with its sudden cliffs (rising up
to 200 ft), inland the scenery is surprisingly monotonous, due to the absence of
particular relief. Thus only the coast presents an interest for the tourism industry with
many coves, among the most impressive in Britain (such as Kynance cove).
One of the many interests along the coast is the existence of a buried forest in MountÆs
Bay. It can at times be exposed after heavy storms have shifted large quantities of sand.
although this buried forest, and the raised beaches that can be found along the coast are
obviously much more recent compared to the great geological events leading to the
formation of the Lizard peninsula, they reflect the climate of this part of Cornwall
during the Ice Ages, when alone in Britain it was not covered by Ice. This climate has
been compared to the one now experienced in Greenland.
As a map clearly shows the peninsula can be divide in two along a boundary consisting
in a series of faults, running from Polurrian cove to Porthallow (figure 2). To the north
of this boundary lies the Meneage; a series of sedimentary Palaeozoic rocks. To the
south is the larger Lizard complex; a succession of metamorphic and highly crystalline
rocks whose, age we will see, varies greatly.
Early on geologists were able to determine the rock sequence of the Lizard complex. In
1901 Harford Lowe suggested the following sequence, which was generally accepted :
Kennack gneisses and granite gneiss
Epidiorite dykes
Troctolite and Gabbro
Serpentine
Hornblende-schists
Treleague quartzite
Man of War gneiss
Old Lizard Head series
The Old Lizard Head series are the oldest rocks of the peninsula, visible only as small
outcrops, the main igneous complex having been injected into them. They consist of
mica-schists, granulites and green schists. Their age remains uncertain because of the
scarce fashion in which they appear but also because of their state of great alteration.
The Old Lizard Head series could either be pre-Cambrian or Devonian.
The Man of War gneiss appears only as a group of reefs off the Lizard Point. It is very
distinct from any other rock of the Lizard. It is composed of a grey gneiss, green
hornblende and chlorite.
The Treleague quartzite is restricted to a small outcrop north of St Keverne. The grey
quartzite forms the majority of the rocks, with however noticeable quantities of garnets.
There is some controversy as to its place in the rock sequence. But as it contains sub-
angular pebbles, as opposed to the highly metamorphic granulites of the Old Lizard it is
clear that the Treleague quartzite was deposited after its the metamorphism. Its precise
age remains unknown.
The Hornblende-schists can de divided into two groups. Firstly the Landwednack
Hornblende-schists constitute the main part of the outcrops of Hornblende-schist. They
present a medium-grained texture, with blue-green hornblende prisms. They are
intruded by peridotite, basaltic dykes and Kennack Gneiss.
The Traboe Hornblende-schists are lithologically different from the Landwednack type.
They are grey, coarse-grained with bands of feldspar. They are slightly less
metamorphic than the Landwednack, as traces of the original igneous rock may be
found.
The Serpentine constitutes nearly half the rocks of the Lizard complex. It has become
its most famous and characteristic rock. It can be divided into three different kinds of
rocks; the dunite-serpentine, the tremolite-serpentine and the bastite-serpentine (figure
2). This serpentine provided at one time an industry for the Lizard peninsula, producing
items such ornamental slabs, pillars etc..., however it soon appeared that the cracks in
the rock, which were decorative, also meant that it could not be used in constructions,
and the industry dwindled. Geologists now agree more or less that the rock dates from
the Upper Devonian.
The Troctolite and the Gabbro are situated on the east side on the peninsula. The gabbro
is green to grey in colour, but it weathers deeply, producing a yellow clay. The
Troctolite has a deep red colouring, due to the weathering of one of its minerals,
olivine. Once more the precise age of the rocks is uncertain.
The Epidiorite dykes are also named the æblack dykesÆ. They are fine-grained and are
usually vertical within the gabbro and the serpentine. These rocks are probably in a way
the most ænormalÆ of the whole of the peninsula, being the least metamorphic.
The Kennack gneisses and the granite gneiss are thought to be the youngest rocks of the
peninsula, but for some geologists, they do not have a clear position in the rock
sequence. They appear both as sills and dykes, penetrating the epidiorite dykes.
As we have seen, although the sequence was well established, geologists could not
solve some of the problems that the Lizard peninsula brought. Amongst these problems,
was that of the origin of the peninsula...
Working on the rock sequence given previously geologists tried to reconstitute the
history of events leading to its formation. In 1964 Green gave the following geological
evolutionary history, which was unchallenged until 1979 (Kirby).
1. Deposition of muds and sandstones, volcanic tuffs and basaltic lavas (Old Lizard
Head Series, Landwednack and Traboe Hornblende-schists).
2. Local intrusion of acid sills (Man of War gneiss).
3. Regional metamorphism, with intrusion of diapiric peridotite.
4. Intrusion of gabbro.
5. Intrusion of basaltic dykes and sills.
6. Intrusion of acid microgranite (Kennack gneiss)
7. Orogenic movements of Permo-Carboniferous age, faulting.
8. Post-orogenic normal faulting.
However in his thesis Kirby pointed out the many incoherences about the above history.
For instance if the gabbro was intruded after 1, 2 and 3, yet in the field examination
reveals that only the peridotite is intruded. Also the dykes are concentrated in the
gabbro, which cannot be explained by the above history etc...
Kirby, with the knowledge that geologists now had of the ocean crust also pointed out
the similarities with the rock sequence of the Lizard complex. Hence a new theory was
to be produced and later confirmed. The Lizard peninsula is of a oceanic crust nature
and formed at a sea-spreading centre.
It is now therefore known that the complex is the remains of an ophiolite sequence of
Devonian age, obducted onto the continental crust of Cornwall. The age of this
happening probably being the Variscan orogeny. Furthermore a study of the faults
within the Lizard rocks reveals that they were produced at a slow spreading center,
there dip being in average 30 , these angles are known to be characteristic of these
kinds of spreading centres.
The Lizard peninsula is thus a result of tectonic activity, developing at the mid-Atlantic
ridge. Its geology is now, as we have seen, fully understood and accepted. Geologists
such as S. Roberts are now using the advantages produced by the peninsula to get a
better understanding of these tectonics events at the ridge-axis. The advantages being
that the complex enables land-based studies, which offer more possibilities of field
work and sampling, than the deep ocean would, despite the new technology.
It took geologists over two centuries to discover the real nature of the Lizard peninsula.
During this time the issue of the geology of the Lizard led to many arguments in
between the different geological societies and the geologists themselves. And though in
the end the answer more than confirmed the suspicions of Borlase, that the Lizard
complex was totally different from the rest of the Cornish peninsula, it was also one
that no one had imagined beforehand. The reason for this being mainly of course that
the phenomenon of mid-Atlantic ridge spreading was not known or not understood well
enough.
So now that the enigma is no more, one could think that the peninsula is no longer a
matter of debate and intensive research for the geologists. However, despite the fact
that the oceanic nature of the Lizard is now well established, the peninsula is still a
center of scientific study. Geologists are using its nature to pursue research on slow sea-
spreading ridges the advantages of the Lizard being numerous.
References
An introduction to the Geology of Cornwall by R.M. Barton, published in 1964 by
Worden printers Ltd., æThe Lizard and MeneageÆ p 11-38.
British Regional Geology; south-west England by E.A. Edmonds, M.C. MsKeown and
M. Williams, published by Her MajestyÆs stationery office in 1975, p 14-20.
Fluid evolution during tectonic exhumation of oceanic crust at a slow-spreading
paleoridge axis: evidence from the Lizard ophiolite U.K. by L. Hopkinson and S.
Roberts, published in Earth and planetary science letters in 1996, 141.
Geology of the Lizard and Meneage, Memoirs of the Geological survey of Great Britain
by J.S. Flett and J.B. Hill, published in 1946 by His MajestyÆs Stationery Office.
Present views of some aspects of the Geology of Cornwall and Devon edited by K.F.G.
Hosking and G.J. Shrimpton, published by the Royal Geological Society of Cornwall in
1964. æA re-study and re-interpretation of the geology of the Lizard peninsula,
CornwallÆ by D.H. Green, p 87-114.
Report on the Geology of Cornwall, Devon and Wets Somerset by H.T. de la Beche,
published by Longman, Orme, Brown, Green, and Longmans in 1839.
Slow-spreading ridge-axis tectonics: evidence from the Lizard complex, U.K. by S.
Roberts, J.R. Andrews, J.M. Bull and D.J. Sanderson, published in Earth and planetary
science letters in 1993, 116.
The Petrochemistry of rocks of the Lizard complex, Cornwall by G. A. Kirby,
published by the University of Southampton (PhD thesis), in 1978.
