St Agnes is part of an area of continental crust  that includes the present-day area of land south of a line from Perranporth to Start Point in Devon,  that has traveled many thousands of miles and passed through many climatic zones in the last 350 Million years  Before the Present ( MBP) 


The oldest rock - the 'Killas'

The geological history of  what was to become the future St Agnes  began some  350 MBP,  south of the equator, in a deep sea basin on one of  the many areas known to-day as   'continental plates' that, like pieces in a constantly moving jigsaw, make up the earth's crust.

Our  'plate' was moving north relative to other 'continental'  and intervening 'oceanic' plates in the area, in a process known as Plate Tectonics, and it continues to move to this day.

In the deep  sea basin, where the geology of St Agnes  begins, sands and clays were deposited that later hardened  to form the modern sandstones and shales  known to geologists as the  'Porth Towan Beds' of Upper Devonian age. To the Cornish miners they are  'killas', a general term for the local bedrock into which over many centuries they dug looking for valuable minerals.

Killas forms the cliffs, the Beacon and underlies almost the whole of St Agnes. The only exceptions are small areas to the north where  relatively younger granitic  rocks of some 285-270 MBP form the cliffs of Cligga Head, and to the west of the Beacon  (Cameron  Quarry) where the granite again occurs, but is here decomposed and forms what is locally  known  as 'candle clay' used by local miners before the 2nd world war to affix candles to their helmets and mine walls. 

As our 'plate' moved north the relatively cold granite of to-day began to rise as  huge domes ('batholiths') of  hot magma  in the earth's crust. This caused doming of the killas  forming a  NE - SW trending mountain chain as the  low density magma  rose and intruded into the cold  thick sedimentary layers of the 'killas'. Pressure and heat 'cooked' the latter into hard slate and sandstone, a process known as metamorphism.

The eroded 'stumps' of these mountains  still exist forming  the killas-covered St Agnes Beacon, and the other high ground areas where  the granite 'roots' of the mountains are now exposed, as at St Austell and  the Land's End peninsula. Hot gases and superheated liquids rising through the cooling granite produced the rich mineral veins of  tin and copper that made St Agnes world famous in the 19C, and the 'killas' was much quarried to build the engine houses and other mine buildings as well as  local domestic houses and  stone 'hedges'.

Sea levels rose and fell  and climatic conditions  changed throughout  the time our 'plate' was moving north across the equator  to its present position.

The St Agnes Formation (5.2 - 23.3 MBP)  

These deposits known as  the 'Beacon Sands' and the 'New Downs Clays'  are unique in British Isles geology in having  fossil  pollen in their clay of  Miocene  age. The pollen suggests a Mediterranean  climate at that time with conifer forest, mixed woodland and scrub but no evidence of proximity to the ocean,  but with an inland dune scenery  with seasonal flooding of otherwise dry river valleys like wadis in  a modern desert. These deposits are now  extensively quarried at New Downs.

The 'Beacon Cottage Farm' Beds, possibly deposited in an inland lake and later dug by  miners  for  candle clay  and pottery, are of Mid-Oligocene  age  c.28MBP. These beds  form a crescent shaped outcrop on all but the eastern side of the Beacon. Fossil pollen in the clays include those from  palms and  suggest a climate with  frost free winters  but a cooling climate with  a mean annual  temperature of  about 12° C.

As the climate cooled by 1.8MBP  the Pleistocene epoch began. This  consisted of a series of 'Ice Ages'  separated by warmer interglacial periods with high rainfall. Unlike other  areas however, this did not result in a glacial cover to our region.

Following  the last Ice Age  our area was tundra  with  a very sparse vegetation cover like modern  Northern  Siberia  but with sea ice sometimes abutting the cliffs  (and covering parts of Scilly) and may have locally pushed up some river valleys.  Successive periods of  freezing and thawing of our now  permafrost- dominated area, led to breakdown of the  killas rocks  producing  angular gravels, sands and clays known as 'Head'. These loose deposits moved gradually downhill  under gravity (gelifluction) and  formed thick deposits in valleys such as  Chapel Porth.

Seasonal summer melting of snow and ice caused local floods and the rapid run-off produced deep channels, known locally as 'girts'  in the loose Head deposits  as near  Trevellas,  Chapel Porth and Mingoose.

Sitting on St Agnes Head today  looking out to sea you may think you are looking at a 'timeless' scene but as we have seen these rocks started life many thousands of miles to the south millions of years ago in a deep sea basin; the Atlantic Ocean  did not exiSt

Even as geologically recently as 7,000 - 5,500 years ago when Mesolithic  hunter-gathers sat on the  headland knapping flints, the scene, from  what are now the cliffs of St Agnes, has changed dramatically.    

Then, the first human colonizers of this area would have been surrounded by forest and would not have looked immediately over the Atlantic. At that time the shoreline was many miles away to the west. They would have been sitting on the high ground that now forms our modern day cliffs  gazing west over an extensive  plain covered by  a vast  luxuriant deciduous forest of  oak, hazel, birch and alder. Where now the surfers surf and holidaymakers swim and the fishermen, dolphins and grey seals  and gannets hunt for fish, they would have hunted and  built seasonal camps .

The remains of this now submerged forest are sometimes exposed at low tide at Portreath and elsewhere along the Cornish coast  after storms remove  the overlying modern sea sands.

Knapped flints and cores  are still occasionally found amongst the heather and gorse-covered coastal heathland. The pebbles of flint  so beautifully fashioned  into arrowheads, scrapers  and knives using antlers and stones, came from local beaches. The flint pebbles are all that remains in the local area of a once thick cover of chalk  deposited in  a warm sea about  80MBP  covering the 'killas' and other rocks over a wide area of southern and eastern United Kingdom.

Our tectonic 'plate' and with it one of its geological  'passengers' - the St Agnes area - has come a long way, but the  tectonic process continues with global warming and sea level rises as the last of the Arctic glaciers melt.

© Roger Burrows