Mosses and lichens, Botallack, St Just Mining District. © HES.
 

 

 

Geology and Wildlife

The geological history of the geo-cultural region in which the nominated Site is located begins nearly 400 million years ago (Ma). Sand and mud settled on the floor of a Devonian sea, and molten rock formed submarine lavas and intrusions within the sediments. Around 320 Ma, during Carboniferous times, continents collided and caused a major earth-movement. This subjected the earlier rock formations to folding, faulting and cleavage on a general axis with an east-north-east to west-southwest trend. It is this alignment that accounts both for the orientation of the granite emplacement and the main tin and copper lodes. Mudstones became slates, which together with subordinate bands of sandstones have long been known collectively by the Cornish term ‘killas’.

Between 300-270 Ma, during the late Carboniferous and Permian periods, continental collision generated considerable heat and pressure which melted the crust to form granite, a coarse crystalline igneous rock formed deep in the earth. Separate granite masses intruded into the rocks above them between 290-270 Ma. They merged to form an elongate body of granite, known to geologists as a batholith. The intense heat also caused water to circulate within the granite, producing the main tin, copper and tungsten mineralisation around 270 Ma.

Around 250 Ma, during the late Permian, a mountain chain was created during a period of considerable uplift. The rocks which once covered the granite were then gradually removed by deep weathering and erosion, exposing the tops of the granite domes. Around 236 Ma, during the Triassic, the cross-course lead-silver-zinc mineralisation formed in a north-south structural orientation. This alignment, perpendicular to the main tin and copper mineralisation, was due to changes in geological stress regimes.

Within the past 4 million years, marine erosion created a relatively flat surface (the 130-metre planation surface), as well as wave-cut platforms and raised beaches. It is likely that tin placer deposits were formed within the same period, and went on being formed until relatively recent times. The sea level fell during the Ice Ages of the past 1 million years, (ending around 10,000 years ago) and rose in recent times by about 15 metres. River valleys (known as ‘rias’) were cut and subsequently flooded by these events, including the River Tamar and the Fal estuary.

Mining and the Natural Environment

Mining and the natural environment have always been inextricably linked. Geological and geomorphological processes which took many millions of years to develop determined the resources available for mining and the sites where they awaited discovery and exploitation. Long ago, miners learnt the tell-tale signs of mineralisation - the characteristic greens of secondary copper minerals, the reds of iron-bearing rocks, the hard resistant whiteness of quartz stringers and reefs, the local softening and erosion of other altered rocks that signalled the presence of valuable ores. Elsewhere they began to realise that common plants were stunted or absent where such minerals occurred, or that some species - indicator plants - alone thrived where certain minerals lay not far beneath the surface. They dowsed, tasted the water, learned the smells of pyrite and mundic - developed a sense of geology that was instinctual long before it was written down or scientifically analysed - picking up subtle hints from their natural environment - clues that unerringly guided them to what it was they sought.

In turn, their activities changed the environment. Rocks whose weathering products were far more acidic or toxic to plant life than those experienced in the landscape - softened and tamed by long exposure to wind, water and bacteria - were brought from deep beneath the earth in vast quantities, broken into fragments, crushed to fine sands, burnt so that they turned to toxic gases, discarded as waste and spread across its surface or spilled into its watercourses. Across its landscape, environments were created which had not existed in Cornwall or West Devon for tens of millions of years. The few plants which could live there are very specialised - pioneer species which can gain a tenuous foothold in such dangerous habitats and after many decades create the conditions where other, less tolerant species could, perhaps, build on the shallow, poor soils they had painstakingly created. A slight change in their habitat - the disturbance of the surface of a waste dump, the spreading of a mere inch of nutrient-rich topsoil, the removal of a mineral-rich input to a stream - can undo the work of centuries and destroy such habitats for ever.

These are special places - rare not only in Cornwall, but worldwide. Some are so free-draining that they resemble miniature deserts, others are so utterly saturated with acidic water that only the most primitive species can survive, many are rich in freely-available toxic minerals whose closest comparisons are lava flows. The plants and animals that survive - and in many cases thrive here - are often unusual and find these conditions nowhere else in a landscape which agriculture has slowly modified over thousands of years - these are wild, primitive and important places in our landscape - but also vulnerable places - for their inhabitants are often small and undramatic, their value often unrecognised until they have gone. These are the homes of rare mosses and lichens, of stunted variants of common plants, of bare sands and clays, exposed rocks and the insects, beetles and other animals which are found here and which can survive nowhere else. Many generations of such plants and animals must have lived out their lives in islanded areas like these, utterly isolated from contact with other such colonies, that subtle changes brought about through specialisation and inbreeding may have occurred. Other species rely on chains of sites like these, spread throughout the landscape, moving from one oasis to another in what is to them a sterile and inhospitable desert of farmland and townscape. Remove enough of these sites, and they are trapped.

The contents of the spoil heaps, hacked as they have been from deep below the ground, are also extremely important resources for the geologist and mineralogist. These are types of rocks and minerals which simply do not occur at the surface, where millions of years of exposure to air and water chemistry, coupled with the effects of some of the smallest, yet most abundant life forms on the planet have changed them into the stable, familiar materials which make up most of our environment. They provide rare and valuable glimpses into the formation of our planet and the way it has developed. Seventy one globally-rare species of minerals were recorded in Cornwall up until 1992 from such sources - twelve had never been recorded anywhere in the world before that date, and it is certain that many more await discovery. The words "waste" and "spoil" are often so wrongly applied to such sites - these are treasure houses which may prove to be as important for the knowledge of the natural world which they provide us as the copper and tin from which they were once separated and discarded.

For further information, try the following sites:

Cornwall Wildlife Trust, English Nature, Devon Wildlife Trust, Cornwall RIGS Group, The Russell Society, Camborne School of Mines

 
 

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