Geology and Local History

If you are just beginning to be interested in the history of the area or region in which you live, you may be surprised at this recommendation – start by looking at geology. If your vision of local history is one of battles, families, villages, costumes and – above all – of people, you may never have considered geology. And yet Shakespeare said it all, didn’t he – ‘All the world’s a stage, and all the men and women merely players:’ And so we are, and our stage is the land.

The generations who have lived here before us have had to do so on a part of the earth's crust that has been formed millions of years ago, and only its very top surface has been shaped and changed by people who have been using what they first found only thousands of years ago. To begin to understand local history, you need to know your area – or stage – so start at the beginning and look at its form, its natural boundaries and the forces that have shaped its hills, mountains, moors, and cliffs.

Get an atlas and locate your chosen area. Let’s say Penwith. Here it is on the furthest west end of Cornwall, itself the furthest west county of England (despite the inhabitants, who maintain that England only starts at the Tamar). Look at its latitude - 50.3 degrees north, that’s quite a long way north of the Equator, about the same as north Newfoundland, northern Mongolia and the far north of Japan. And yet we do not have the frozen winters and very hot summers of those places that we see on TV programmes. You may have to have a quick look back at those school topics of the circulation of the ocean currents and winds to remind yourself how very lucky we are to live on the west side of a continent, surrounded by the sea on three sides, so that we have relatively warm winds and waters all year round and only very rarely need a sheepskin coat in the winter, or stand pipes in the summer – at the moment. Of course the Atlantic brings us some pretty good winter gales, but nothing like the hurricanes elsewhere in the world.

How we got to this shape and latitude is all very interesting and there are plenty of geology books and magazines anxious to explain it but for local historians it is probably sufficient to have a good general knowledge of the basic geology of West Cornwall, that is, the main types of rocks and minerals, and the general features of relief and resulting drainage (rivers and streams). The best way to do this is to obtain a geological map. An atlas can give a very simplified picture but the best sources are the maps Simplified geology map of Cornwall, courtesy of Dr Nick le Boutillier, by the British Geological Survey, at many different scales; they are all listed on the internet and one should be enough. There is always a key for the colours, showing the relative ages of the different rock types and it is fairly easy to see that there are 4 largish areas running down the length of the county, labelled granite and the key says helpfully that they are intruded. This means that all the rest of the main colours represent much older rocks, and that the granite has pushed up into them fairly recently (in geological terms, which means long before humans were here). The older rocks, from the Devonian period, are generally slates, sandstones and mudstones. Lean over the wall by the swimming pool in Penzance and all the rocks below are Devonian.

These granite intrusions have a very interesting effect on the landscape which human inhabitants have taken note of. Firstly, they represent areas of higher land, with the granite weathering down into rather Granite cliffs at Tol-Pedn-Penwith below Gwennap Coast Watch Station. The tough granite rock stands almost vertical against the power of the Atlantic while the faulted face provides good climbing, this pitch is called the Chair Ladder. Picture by Glyn Richards

The granite cliffs of Tol-Pedn-Penwith, south of Land's End, where the hard rock cliffs rise almost vertically from the sea and the faulted granite makes for good climbing. This pitch is called the Chair Ladder and you can just see climbers near the bottom if you look closely. (Photo by Glyn Richards)

poor soils, often difficult to plough. Secondly, their height, combined with the incoming Atlantic clouds, means rain runs off them in all directions, giving the pattern of streams and rivers. Thirdly, when the hot granite pushed up into the Devonian rocks, and then cooled, various minerals such as tin and copper were deposited in long cracks surrounding each intrusion, which became the basis of the Cornish mining industry. The geological process explains the modern pattern of surviving mine chimneys, circling each granite mass.

It is clear that geology affects every feature of the current landscape. Sit on Penzance Promenade and look out over Mounts Bay. The hungry sea has worn this big bay back because the land in the centre is the relatively soft Devonian rocks, while the granite mass on the Newlyn side (note the hill behind the little port) and the ancient hard schists and serpentines of the Lizard on the east, have resisted and now form the arms of the Bay. The resulting beaches made boat building easier and the variety of available rocks were later useful for the construction of harbour walls, all very handy for a fishing industry. Geology can also be seen in the older buildings of Penwith; many are granite built and it is interesting to see in the lower walls of Penlee Park, a mixture of Devonian slates, granite and volcanic rocks from Penlee, showing how these were all available close by (rock is too heavy and expensive to carry very far, except by ship).

The lower Kenidjack Valley, St Just showing the granite water wheel pit and mine dumps. Surface rock here is mostly killas (slate) while the dumps are granite brought from underground blow the killasA good local historian will use both books and maps for research, but nothing beats going out walking – and looking. Old quarries in fields and on cliffs can be identified and their rocks traced in old field boundaries. It is essential to walk the area that you are interested in, more than once, at different times of the year and the day. Have a good walking stick and do not hesitate to poke into green hedges to find rocks; climb over mine dumps, peer at the sides of channels worn in rocks by streams, examine the sands or rocks and pebbles of beaches, and look closely at cliffs. The more you look, the more you will see and be able to recognise. Geology has not determined the use of the land in Penwith because different generations have different needs and technologies, but it is always there, and is a constant factor, sometimes important, in the decision making of ourselves and our ancestors.

Dawn Walker

PLHG publications can be purchased online here or perused in the Morrab Library.

Local History Primer:

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Penwith Local History Group
Local History Primer:

Growing Up in West Cornwall. A Publication by the Penwith Local History Group

"Growing Up in West Cornwall"

Edited by
Sally Corbet

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The Morrab Library showing the new extension
The Morrab Library showing the new extension.
Photo Glyn Richards