Two 4-Day Field Trips

Two 4-Day Field Trips in South West England

Nick Chidlaw is offering two field trips in the south west in the second half of September. He provides some background information followed by more particular details.

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Examination of the geology of south west England indicates that it was largely developed within a zone of plate tectonic collision. A key feature is the intensely folded and faulted nature of many of the rocks; these structures indicate that the rocks were compressed from a generally S to N direction in the geological past. This caused them to buckle up into rows of folds aligned approximately east-west, in many cases overturned to the north. Faulting of the rocks mostly took the form of high-angle ‘reverse’ and low-angle ‘thrust’ dislocations, along which extensive sheets (‘nappes’) of crust were carried over the top of others in a dominantly northerly direction. There is also evidence of major ‘strike-slip’ faults, where the crust either side of a near-vertical fault has moved in opposite directions relative to one another. One such fault locates north Devon some 400 km south east of its present position, in the Ardennes region, prior to such movement. It is now recognised that Devon and Cornwall are part of two discrete areas of crust that have highly contrasting geology to each other and that, although now juxtaposed, originated in different places and were created under different conditions. These are the ‘Cornubian Terrane’ (occupied by most of Devon and Cornwall), and the ‘North Armorican Composite Terrane’ (the northern margin of which occurs on shore in parts of south Devon and Cornwall). The rocks of the former terrane in the study area are mostly a variety of sedimentary rocks of Devonian age (419 – 359 Ma) which have been mildly metamorphosed; the latter terrane in the study area is mostly composed of rocks of a similar age, but which were originally ocean crust, and have been metamorphosed to a much greater degree. The highly deformed geology of south west England was created by plate tectonic collision, culminating at the end of the Carboniferous period in the ‘Variscan Orogeny’.  

 

Rocks formed subsequent to the Variscan Orogeny are widespread in south west England, and were created under crustal tension. In Permian times, mountainous land created by the plate collision, underwent extensive weathering and erosion, depositing coarse-grained sediments on slopes. De-compression of the orogenic belt allowed crustal melting at depth and the rising up into the higher crust of silica-rich magmas to form extensive granite intrusions together with smaller intrusions and volcanics, and causing alteration and metalliferous mineralisation of the adjacent rocks.    

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Please note that these field trips are independent of one another - you can decide to attend either or both, according to your interests and availability.

 

Some potential attendees may wish to attend both trips; there is a gap of 5 days between these trips, creating the opportunity for those people to stay in the south-west, and explore Devon and Cornwall ‘on holiday’ during this time.  

 

The two trips visit sites that are mostly coastal, and due regard to tidal conditions has been made, to enable maximum time available to study the rocks safely. 

 

Tuition fee is £85.00 per person per field trip.

 

If you / anyone else you know would like to attend either or both of these courses, please forward cheque payable to me to:

 

Dr N Chidlaw

8 Silver Street

Dursley,

Glos.

GL11 4ND

 

Bank transfer can be arranged if you do not use cheques - let me know.

 

Please provide me with your postal address - I will then be able to send you your trips handout(s) in good time before the trips run, should they become viable.

The deadline for the minimum number (10) of enrolments for both trips is Tuesday 13th August.  Maximum number of enrolments is 30. If the minimum of enrolments is reached by this deadline, the arrangements will continue; if not, the course will be cancelled and fees received will be returned to those who have sent them in, soon afterwards.

Please note that for each course you will need to:

Arrange your own transport and accommodation (such as B&B) if you live beyond commuting distance - tutor can advise

Bring your own packed lunch(es) each day, and any refreshments (e.g. flask of coffee, fruit juice, mineral water, high energy snacks etc.)

Wear strong footwear with good tread and ankle support, and warm waterproof clothing if weather is poor

Wear hard hats when below rock faces (bring one, or request from the tutor in advance of the trip(s) to borrow one)

You would be insured against accident for the duration of both trips.

Please see details of the two trips below. I hope this proposal is of interest to you, and to hear from you soon.

Regards,

NICK CHIDLAW.

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GEOLOGY BETWEEN THE BOLT PROMONTORY AND DARTMOOR, SOUTH DEVON (Thursday 19th – Sunday 22nd September; 10.00 am – 5.00 pm each day)

 

This field trip focuses on the varied geology of the Bigbury Bay area, and at Plymouth and the nearby south west margin of Dartmoor. We examine rocks of both the North Armorican Composite Terrane and the Cornubian Terrane, and the boundary between; post-orogenic sediments are studied, as well as the south west margin of the Dartmoor granite and its effect on the adjacent ‘country’ rocks when it was intruded.

 

View of Outer and Inner Hope villages, Bigbury Bay west of Salcombe. Here, the North Armorican Composite Terrane (which continues south into France) and the Cornubian Terrane (occupied by most of Devon and Cornwall) meet. The boundary, (‘Start Boundary Fault’) lies on the left (north) side of the promontory extending out to sea on the left of this picture. The rocks south of the fault in this picture are heavily sheared, leading to the creation of the cove.  

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FIELD GEOLOGY ON THE LIZARD PENINSULA, CORNWALL  (Saturday 28th September – Tuesday 1st October; 10.00 am – 5.00 pm each day)

This field trip, located on the Lizard peninsula to the south of Falmouth, includes the southern margin of the Cornubian Terrane, and the northern margin of the North Armorican Composite Terrane. The nature of the boundary between the two will be examined. Deep water ‘turbidite’ sediments, highly deformed, and rocks formed by undersea landslides, both mildly metamorphosed, characterise the Cornubian Terrane rocks here. The North Armorican Composite Terrane rocks are composed mostly of the Lizard Complex, a much deformed and dismembered fragment of Devonian oceanic ‘lithosphere’ (crust and uppermost mantle).  Such remnants of oceanic rocks, that became driven up onto continents through plate tectonic activity, are known as ‘ophiolites’.   

View of Kynance Cove on the south west side of the Lizard Peninsula. Here, peridotite rock from the Devonian earth’s upper mantle has been extensively altered to variably-coloured serpentine. The least-altered peridotite on the Lizard is black on colour (due to finely-disseminated magnetite). Much of the alteration may have occurred in Devonian times, when the ocean crust was under tectonic tension, allowing sea water to penetrate down into these upper mantle rocks, causing the ‘serpentinisation’.