Geology

Cornwall’s Geology and Scenery, Colin M Bristow

2015-01-24 19.47.49Apologies dear Reader if you have been waiting for this post all day.  I had nothing planned for today other than finishing this book and writing a review for you, but then E awoke with a burning desire to go and see the dinosaurs at the Natural History Museum in London.  She was so intent that I thought it prudent to agree, on the condition that while we were there we also went to the rocks section so that I could take some rock photos with which to illustrate this post.  So, here we are a slightly delayed but highly beautified post for you. I’ve also added another geology quiz at the end!

Colin M Bristow should have been a publicist, not a geologist.  Take, for instance, this paragraph from the first page:

Four hundred million years of turbulent geological history have led to the Cornwall of today; it involves volcanoes and prehistoric animals, baking hot arid deserts and equatorial tropical forests, deep seas and one of the most important mountain chains that this planet has seen.  Cornwall’s importance in the geological world is confirmed by the fact that it has more hard rock geological Sites of Special Scientific Interest than any other British county.

Are you salivating too? It certainly got my geological heart racing.  Volcanoes! Deserts! A semicolon!  But is also raises so many questions. How can one mountain chain possibly be more important than any other?  Does hard rock geology come with a soundtrack? How can there be an equatorial forest when Cornwall is nowhere near the equator?  I guess we will have to read on and find out.

This book is a beautiful fit with the previous book as much of the geology of Cornwall has been explored as part of the excavation of the tin and copper mines. Why, we already know what China clay is – we are ahead of you, Colin!  But I hadn’t heard of the Cornubian Massif before. The book is really good at explaining in more detail why the conditions led to Cornwall being so rich in different minerals (largely due to heat and water, as far as I can work out).

Fossils!

Fossils!

The book has a great section on the development of our understanding of Cornish geology.  It begins in the 18th century with many geologists mainly interested in looking for evidence of Noah’s flood, to the setting up of the Royal Geological Society of Cornwall which provided a hub for all those interested in this  fascinating subject.  There is also a great story about an early 20th century geologist called EML Hendriks who

found the fossil plant Dadoxylon in rocks along the Roseland coastline thus showing that most of the rocks of west Cornwall… were around 100 million years younger (than previously thought).

She discovered that they were out by 100 million years!! 100 MILLION years.  What insane science is this that can be ‘give or take 100 million years’.  No wonder E is late all the time; after 4 years studying this it is a miracle she still wears a watch!

Chapter 2 has some useful descriptions of various geological concepts like faults and folds and

A beautiful example of folding

A beautiful example of folding

types of sedimentary rocks, which will be useful to those of you who haven’t yet read the Field Guide to Rocks and Minerals. We also learn that Cornwall isn’t great for fossil hunters, so you need to look elsewhere to scratch that particular itch. There is also a great description of how the earth’s crust is formed, which I can verify after my trip to the Natural History Museum.

A geological history of Cornwall in nine bullet points (for those insufficiently interested to read the book)

  • Around 408 million years ago, in the Devonian period, a great sea covered Cornwall and laid down a lot of sedimentary rock on top of a base of pre-Cambrian (stupidly old) rocks.
  • Slowly, the sea retreated northwards towards Wales and there was some volcanicity resulting in “large squashed blobs of lava” (p47) and Cornwall was slowly stretched North-South.
  • We then head into the Carboniferous period when mountain ranges began to rise in the South of Cornwall causing a deep sea trench to appear in front of it – there was another build up of sedimentary rock which filled the marine trench.
  • Then the tectonic plate carrying France crashed into us (very slowly) and all the sedimentary layers were scrunched forming another mountain range ‘the Variscan Orogeny’,
  • The mountain range then ultimately began to collapse and the granites intruded as the sedimentary rock was melted and cooled. This was when the modern granite was formed.
  • The earth around these granite lumps was rich in minerals caused by the rocks heating and cooling at different times as the granite heated and cooled with different fluids circulating within the granite.  As the minerals were mixed together by this process, we got the rich veins of minerals we have today.
  • Nothing interesting happened in Cornwall for the 180 million years of the Mesozoic, apart from the dinosaurs who lived through climates from arid desert to soupy jungle. They think Cornwall was an island surrounded by sea at this point.

    Gratuitous dinosaurs

    Gratuitous dinosaurs

  • Then Cornwall slowly sank into the sea again because of tectonic plate movement and was covered by chalk.
  • Cornwall rose again in the tertiary period and cooled a bit for the ice ages, in the quaternary, but nothing too exciting happened.  As the ice from the ice age melted some more exciting minerals were deposited.

I have to be honest, a lot of this book went into much more detail than I was interested in.  There were so many different seas, faults and mountains that I longed for a scale model of Cornwall that I could see it happening to.  It was hard to visualise a lot of it.  Still, it was a good overview and more dedicated students would get a lot out of it, relatively painlessly as it is really clearly written.

THE LONG AWAITED SECOND GEOLOGY QUIZ!

1) The Cornubian Massif is:

1) The name of the latest gang in Penzance

2) The remains of a mountain range that once stretched from Alabama to Poland

3) A huge chunk of rock in the middle of Dartmoor.

2) The Royal Geological Society of Cornwall is:

1) The oldest geological society in the world

2) Probably the second oldest geological society in the world

3) The only geological society to focus exclusively on hard rock

3) The most characteristic igneous rock in Cornwall is:

1) Granite

2) Very gneiss

3) Basalt

4) Why should you be careful of moving to Cornwall

1) Because of the fierce nature of the scrumpy

2) Fierce locals

3) Because many local houses are built of granite and it is highly radioactive

5) The official term for the study of fossils is

1) Palaeontology

2) Paleaontology

3) Paloeantology

6) What are you likely to find on surfaces where the slaty cleavage and bedding are parallel?

1) Fossils

2) Smut

3) Jokes that aren’t as funny the second time

7) China clay is

1) Made in China

2) Delicious on toast

3) Now Britain’s second most important mineral export after petroleum

8) What caused the ice age?

1) Aliens

2) Nobody knows

3) The collision of India with the southern area of Asia threw up the large high altitude Tibetan plateau which modified the world’s climate

9) This is a picture of

20150124_135011

1) A dolerite intrusion in granite

2) A black rock sandwich

3) China clay

10) Geology quizzes are

1) Freaking awesome

2) Freaking brilliant

3) All of the above

Answers

Question 1: 2) The remains of a mountain range that once stretched from Alabama to Poland

Question 2: 2) Probably the second oldest geological society in the world

Question 3: 1) Granite

Question 4: 3) Because many local houses are built of granite and it is highly radioactive

Question 5: 1) Palaeontology

Question 6: 1) Fossils

Question 7: 3) Now Britain’s second most important mineral export after petroleum

Question 8: 2) Nobody knows

Question 9: 1) A dolerite intrusion in granite

Question 10: 3) All of the above

So, how did you do???

Cornwall’s Geology and Scenery, 2nd Edition, Colin M Bristow, Cornish Hillside Publications, 2004

7 thoughts on “Cornwall’s Geology and Scenery, Colin M Bristow

  1. Finally, my wait is over and the much anticipated review of ‘Cornwall’s Geology and Scenery’ has hit the blog. I thought it would never come!
    Can’t wait for the third in the trilogy, ‘Another 1001 Things You Didn’t Know About Cornwall.’

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  2. Woohoo! I got some right! None of the really intellectual ones though – thanks for making this one easier. I hope you had a scone with jam and cream while reading – it’s the law that scones must be eaten whenever Cornwall is mentioned…

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  3. The dolerite intrusion appears to be in gneiss–note the flattened phenocrysts (orb-like due to heat and pressure of the rock after it had cooled–metamorphosis). Certainly it could be granitic-gneiss but gneiss none the less. One needs to be able to look more closely and perhaps chemically analyze a sample before presenting the name of the mineral.

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    • Hi Larry, thank you for your comment and I hope you enjoyed the quiz. It is great to have someone with real knowledge of the topic visiting the blog. I took the picture of the dolerite intrusion at the Natural History Museum in London – I’m pretty sure they will have checked the name of the mineral and done any necessary analysis. It is a beautiful piece of rock, isn’t it? Best wishes, Kate

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      • Kate, yes it is a nice piece of rock, produced by the processes God put in-place when he formed the world we live on. Perhaps you, like I, have discovered “labeling issues” with museum pieces. In the case here, there may be a larger issue involved. Namely, many across the world incorrectly name every intrusive rock “granite.” (I even observed counter tops made of dolerite, gabbro and granodiorite being sold as “granite” or “black granite.” Granite is light in color (felsic) and never dark/black (mafic). I understand the mis-labeling but that doesn’t make it correct–it makes granite a marketing term instead of rock term. Incidentally, I didn’t mean to imply I am “someone with real knowledge [an expert] of the topic.” That’s certainly one reason for my disclaimer at the end of the comment. I don’t have the means to chemically analyze a rock and certainly not just a picture of one. Thanks for the REPLY. Before I close here, I’m sharing another very, very confusing label for “SHE WOLF,” a broken piece of sculpture in the Uffizzi Galleria in Firenze, Italia. “…ONE SQ. METER OF PORPHYRY REQUIRED 150 WORK-HOURS TO BE SMOOTHENED, COMPARED TO THE 5-6 HOURS REQUIRED FOR ITS EQUIVALENT IN CARRARA MARBLE. CARVING CALLED FOR ABRASIVE POWDER RICH IN QUARTZ KNOWN AS EMERY.” The last sentence is troublesome for many. Emery is aluminum oxide (like ruby and sapphire) and contains no quartz. Quartz is silicon dioxide. It’s certainly not clear, however, if “emery” is being used generically–consider EMERY BOARDS used on our nails and EMERY PAPER/CLOTH used to make wood and such smooth. The abrasive I’ve seen of both these examples is often sand, not emery. Sand is certainly far more available than emery. Sand is also hard enough to cut our nails and wood at far less cost for abrasive makers. My art history teacher, my geology professor and I have had some discussions about “PORPHYRY” also. Would you and/or others be interested in what I’ve learned about that intrusive? Another fine discussion item: “ARKANSAS STONE” Renzo

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