West Cornwall and Lizard Geology
From our research we have come up with this highly simplified version of our local geology.
Sandstone is compressed sediment and so the majority of Cornwall was an ancient sea bed lifted up by tectonic movements. Where continental plates meet there are deep fissures into the mantle of the earth and magma (molten rock) can often well up to form volcanoes
Igneous rocks are formed from magma (melted rock) that has cooled and solidified, either within the Earth's crust or on the Earth's surface. Magma that solidifies within the Earth's crust cools slowly and result in coarse-grained (large crystal) rock; for example, granite. Magma that solidify at or near the Earth's surface cools rapidly and result in fine-grained (small or no crystal) rock; for example, basalt and obsidian. Igneous rock is composed of mostly silicate minerals
What Makes the Lizard Unique?
It is very unusual for
rock that normally lies beneath the outer earths crust to become part of the
crust and be visible on the surface. The Lizard is unique in Europe because this
is precisely what has happened. Serpentine is a
metamorphic rock (ie: formed when other rocks are altered by heat and/or pressure)
found in the upper mantle of the earth. Serpentine is often associated with
ophiolites, sections of the oceanic crust and mantle that have been shifted from
deep underground onto continental margins when continental plates have collided.
In the case of the Lizard Peninsula a pocket of magma from the mantle
(below the earths outer crust) was allowed to rise to the surface, solidify, and
was pushed onto the mainland 375 million years ago. Owing to a high level of toxic metals
and a deficiency in nutrients, serpentine outcrops sustain only certain types of
plants. These relatively barren lands are ecologically unique and home to rare
plant and animal species.
Other continents where serpentine is on the surface include North America and Africa.
The flat landform and absence of drainage, because of the dense bedrock, gives
rise to slowly permeable, seasonally waterlogged soils. In the Winter pools form within the wet heaths. From the highest point, Goonhilly Down, there is a radial drainage pattern. Streams find their way to the sea by narrow valleys opening to small coves. The water often runs just beneath the topsoil. In Summer most of the pools dry up and the soil becomes hard. In the spring and Autumn the cliffs blaze with colour. Other types of rock appear alongside the serpentine , notably Gabbro which is a grey fine grained igneous rock. Gabbro is quarried near St Keverne..Many Geologists and Botanists visit the Lizard every year to study its unique qualities. The Botanists tell us that many of the usual UK plant species do not establish well because the soil lacks zinc than for any other reason. We have noticed that grasses and palm species as well as maritime pine do very well. The only large deciduous tree that is native is the elm. Unfortunately Dutch elm disease is still common and claims young trees suckering up from older root systems once the trunk diameter reaches over 6 inches. In many areas there are few tall trees
If you would like to contribute to this page or correct any
errors please email us on lrmltd@btopenworld.com