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Messages - johnd

Geology / Geology Group Diary (02)
October 15, 2015, 01:41:25 pm
Geology Meeting on 14 October 2015.  Notes on THE JURASSIC COAST OF DORSET

The Jurassic Coast is a World Heritage Site. It stretches from Orcombe Point near Exmouth in East Devon to Old Harry Rocks near Swanage in East Dorset, a distance of 96 miles. There are three key coastal areas where the Jurassic rocks can be studied in Dorset.

1. The cliffs around Lyme Regis and Charmouth.
During the 19thC Dorset quarrymen used the term Lias (a corruption of the word layers) to describe the Lower Jurassic (205 - 180 Ma) sequences of alternating thin limestones and bluish grey clays that form the cliffs in West Dorset. Here the Lias reaches a thickness of over 130 metres. It represents the sediments deposited on the margins of the Tethys Ocean which spread over much of the Permo-Triassic landscape of Britain. The Lias sea contained a rich fauna of ammonites and marine reptiles such as Ichthyosaurs and Plesiosaurs made famous by Mary Anning, the Victorian fossil collector who lived in Lyme Regis. Dark bituminous shales also occur in the Lias which suggest that the sea deepened from time to time. The shales contain iron pyrites, a mineral formed where there is a lack of oxygen in deep waters. The Bridport Sands occur at the top of the Lias sequence and mark a change in environmental conditions as sandy sediment was washed into the area.
2. The Isle of Portland (Upper Jurassic; Portland & Purbeck; 150 - 142 Ma)
The Isle of Portland forms the southern flank of the Weymouth anticline (eroded down to the Oxford Clay) and it is highest in the north and dips gently down to sea level at Portland Bill in the south. Although the Portland Stone extends across most of the area 75% of it is covered by the lower Purbeck Beds (quarry overburden). The Portland Stone is a fine grained white limestone that is well jointed and easily dressed into rectangular blocks. It became popular as a building stone in the 17th C when St Paul's Cathedral was rebuilt after the Great Fire of London. Most of the quarries are on or near the coast so that the stone could be exported by sea. A fossiliferous limestone known as the Roach occurs at the top of the Portland Stone and contains 'Portland screws and osses 'eds ' otherwise known as Aptylexia (gastropod) and Laevitrigonia (bivalve). Large ammonites such as Titanites have been extracted from the Portland beds.
Chesil Beach is a 16km long shingle ridge (storm beach) that links Abbotsbury at the head of the Fleet lagoon to Portland. Longshore drift moves south eastwards bringing coastal material from the west.
3.; The coast around Lulworth Cove.
The coast from Durdle Door to Lulworth Cove and Mupe Bay is formed of a wall of Upper Jurassic strata that form a barrier protecting the softer Wealden Beds (Lower Cretaceous) from the continuous erosion of the sea which has already broken through in several places. Stair Hole adjacent to Lulworth Cove provides a good example of the way that the sea is excavating the soft Wealden beds behind the Portland/Purbeck wall. The famous Lulworth crumple in the Purbeck beds shows the effect of the Alpine earth movements (25 Ma) during the Oligocene. Lulworth Cove itself has been cut back through the sands and clays of the Wealden beds to the high cliffs of the massive Chalk ridge that extends eastwards into the Purbeck hills. Note the exposure of Upper Greensand (at the base of the chalk) near where the road reaches the cove and here the greensand really is greenish in colour! On the east side of Lulworth Cove is the Fossil Forest that consists of silicified boles which mark the base of conifers that grew during Purbeck times.

4.  The Isle of Purbeck.
The most important structural feature of the Isle of Purbeck is the Purbeck monocline produced by the northward thrust of the Alpine orogeny. The chalk ridge of the Purbeck hills can best be seen around Corfe Castle that stands on a knoll between a twin water gap. Here the chalk is almost vertical hence the width of the Purbeck hills is relatively narrow Rule of thumb...the steeper the dip of a stratum, the narrower the outcrop. The underlying Wealden, Purbeck and Portland beds all dip steeply north near Corfe but then become horizontal across the Isle of Purbeck forming the southern limb of the monocline.The best place to see the actual curvature of the monocline is in St Oswald's Bay to the west of Lulworth.
On the coast around St Alban's Head the Kimmeridge Clay (154 -150 Ma) appears below the Portland Stone cliffs. Kimmeridge Bay is the type locality for this formation, consisting mainly of black shales and clays with some bands of limestone that form the Kimmeridge ledges. Some of the shales are bituminous but they have a high sulphur content giving off an obnoxious smell when burnt.
Swanage was the centre of the Purbeck Stone trade in the 18th & 19th C but in Medieval times Purbeck 'marble' was much in demand  for cathedral interiors, eg.Westminster Abbey and Salisbury Cathedral. A beautiful example of 'marble'columns can be seen in Eldon Memorial Church near Kingston.
Geology / Re: Geology Group Diary
September 19, 2015, 09:30:50 pm
Here are a few images used to illustrate the Geology of Devon and Cornwall.
Geology / Geology Group Diary
September 18, 2015, 09:20:28 pm
Meeting on 9 September 2015 Notes on Geology of Devon and Cornwall

The geology of the south west peninsula of Devon and Cornwall is dominated by the granite intrusions (plutons) that extend from Dartmoor through Bodmin Moor, St Austell, Carnmenellis and Land's End to the Scilly Isles. These granite outcrops are linked underground since they are part of the same large batholith that was intruded some 290 Ma during the Variscan earth movements. When the magma cooled it slowly crystallised at around 600ÂșC producing crystals of quartz, mica and orthoclase feldspar. The feldspar crystallised first and formed large crystals called phenocrysts. The heat from the intrusion baked the surrounding rocks (known as killas in Cornwall) forming a metamorphic aureole or zone of baking. During the later phase of magma cooling, hot mineralising fluids emanated from the magma chamber and produced hydrothermal veins as both gangue and metallic  minerals were precipitated. These minerals crystallize out at different temperatures...tin at 5500C; copper at 5000C; lead-zinc at 4000C; Gangue minerals including quartz or calcite form at circa 2000C and are often formed in layers within the metallic vein. High temperature minerals occur nearest to the granite whilst copper, lead and zinc were precipitated at successively lower temperatures farther away from the granite. Another form of mineralisation known as pneumatolysis led to the alteration of feldspars in some granites by rising hot water vapours during the later stages of  magma crystallisation. The hydrated feldspars in the granite produced kaolin or china clay which is mined extensively around St Austell.
The granite landscape is dominated by tors, outcrops of weathered jointed rock that form the summits of the granite moorlands. The physical appearance of the tors can be described as accumulations of large irregular blocks resting on the granite outcrops, as for example at Haytor on Dartmoor. Large blocks litter the slopes having been detached from the main outcrop. Many researchers consider that the tors evolved under periglacial conditions during the late Devensian when the area would have been just beyond the maximum ice limit. On the other hand, Linton (1955) proposed that tors were formed by a two-stage process, involving deep tropical weathering during the Palaeogene followed by solifluction and mass movement during the Pleistocene.
During early Devonian times, around 400 Ma, a slice of the Earth's mantle-crust boundary was slowly thrust upwards and northwards and welded on to the ORS continental margin bordering the Rheic ocean.. This fragment of ocean crust (known as ophiolite) now forms the Lizard peninsula, composed of ultra mafic rocks including gneiss, gabbro and serpentine. The latter is a coarse grained igneous rock derived from peridotite, the mantle material. When polished, serpentine shows an interlocking mosaic of green and black olivine crystals.
The sedimentary rocks of north and mid Devon and NE Cornwall are Upper Carboniferous in age and are known as the Culm Measures. The term culm derives from a local name for thin sooty coals that occur within thick sequences of mudstones and sandstones. The sediments were deposited in the Rheic ocean and later deformed by the Variscan orogeny as illustrated by the chevron folds at Millook on the north coast of Cornwall.
Two more photos (images resized with a little help from Geoff). The Penclegyr quarries provided dolerite that was crushed at Porthgain, graded and stored in the hoppers alongside the harbour. The dolerite is a massive intrusion which developed columnar jointing as the magma cooled.
Here are a few photos of Abereiddi bay. Photos 1&2 show the Caerhys Slates on the south side of the bay, photo 3 shows Trwyncastell on the north side and photo 4 shows the U3A group ready to go!
We looked at the Caerhys slates in the southern side of Abereiddi Bay where the well known tuning fork graptolites are preserved on the steeply dipping slates. The graptolites are known by the wonderfull scientific name of Didymograptus murchisoni !
Many thanks Colin for your excellent write up on our trip to Porthgain and Aberieddy. One technical point to clarify. As we approached along the old tramway to the dolerite quarries at Penclegyr we passed through the Lingula Flags (Upper Cambrian) which were baked by the dolerite intrusion. The dolerite quarry exposed some columnar jointing formed as the lava cooled and contracted. Note also the glacial till (boulder clay) covering the rocks in the cutting. The Llanvirn volcanics  are seen on the Trwyncastell headland at Abereiddi but not around Porthgain.
Good to know that you will be coming along. Just seen your comment of 26 July. Who is Welshcol. Do I know you? Regards John
U3A Geological Field Trip to Porthgain and Abereiddi

Saturday 12 September 2015    Low Tide at Fishguard 14.10 BST

Meet at 10.30 am outside the Sloop Inn
at Porthgain SM815326

In the morning we will look at the 19th C. harbour facilities for loading crushed stone and then walk along the coast path to Penclegyr where there are disused quarries in a large dolerite intrusion. The Ordovician slates are well displayed in the Porth Ffynnon inlet.

Lunch at the Sloop Inn or bring a packed lunch.

In the afternoon we drive to Abereiddi and examine the shales containing 'tuning fork' graptolites. Then walk over to Trwyncastell headland where the Llanrian volcanics outcrop.
Finish by 3.30pm

Form more information CLICK HERE
I am planning the next U3A geology field trip as shown below. It will be advertised in the next Newsletter.

U3A Geological Field Trip to Manorbier and Lydstep Point

Saturday 11 July 2015  Low Tide 8.52

Meet at 10.30 am in the beach car park at Manorbier [SS063976]

In the morning we will walk along the coast path on the east side of Manorbier Bay as far as Rook's Cave. We will be looking at the features of the Old Red Sandstone including volcanic tuff bands and calcrete formations.

Pub lunch in Lydstep or bring a packed lunch.

In the afternoon we start at the NT car park on the Lydstep headland [SS088977] and walk around the headland examining the Carboniferous Limestone including the spectacular Lydstep Hole and the Gash Breccia at Whitesheet Rock. Finish by 3.30pm