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Messages - johnd

21
Here are a few images of Freshwater West.
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FIELD TRIP TO FRESHWATER WEST & WEST ANGLE BAY

We assembled at Freshwater West car park at 10.30am. 14 members were present. A short walk brought us to Little Furzenip headland close to the B4139 road.  From here we had an excellent view of the rock platform exposed at low tide. The steeply dipping Old Red Sandstone sequence youngs to the south forming the southern limb of the Castlemartin-Corse anticline. The foreshore is bisected by the Flimston Bay Fault, a Variscan dextral wrench fault with a displacement of about 120 metres. This fault is a major tectonic feature extending southwards through the Pembroke peninsula, across the Bristol Channel and through Devon where it is known as the Sticklepath Fault.
From Little Furzenip we followed the track down to the beach where the ORS strata was exposed in a series of isolated outcrops to the east of the Flimston fault. Here we looked at conglomerates, sandstones and calcrete bearing mudstones usually arranged in fining upward sequences each with an erosion surface at the base of the unit followed by lag gravel, coarse sandstone and then grading upwards into fine sand and mudstone. This graded bedding indicated the way up of the rock sequence. We also examined some beautifully displayed ripple drift bedding sequences. These are typical of highly sinuous meandering streams that must have flowed across the arid alluvial plains in Devonian times.  On the eastern side of the fault gap at Little Furzenip there are thich layers of calcretes. These represent fossil soils produced where alternating wet and dry seasons result in leaching followed by evaporation and the precipitation of calcareous minerals within the soil.  The multiple calcrete  profiles at Freshwater West are of regional importance and are known as the Chapel Point Calcretes from the type area on Caldey Island.
After a rather slow lunch at the Hibernian Pub we assembled on the front in West Angle Bay and observed the line of the synclinal axis that trends WNW-ESE through the bay. We noted the dark grey limestones and shales of the Avon Group (formerly known as the Lower Limestone shales) at the base of the Carboniferous Limestone sequence that outcrop on either side of the bay with the younger Black Rock Limestone occupying much of the area beneath the sandy beach. Next we walked along the foreshore to the first cove that lies below a ruined limekiln on the north side of the bay. Here we saw the Avon Group strata dipping towards the south and resting discordantly upon the almost vertical beds of Black Rock Limestone.  There has been a strong northerly thrust movement pushing the Avon Group rocks northwards over Black Rock Limestone. The line of this thrust fault can be traced through Cove 2 and into the south side of Cove 3 where a narrow zone of brecciated limestone marks the lower side or footwall of the thrust. These broken and shattered rocks are the result of movement along the thrust fault during the Variscan earth movements.
After a rather slow lunch at the Hibernian Pub we assembled on the front in West Angle Bay and observed the line of the synclinal axis that trends WNW-ESE through the bay. We noted the dark grey limestones and shales of the Avon Group (formerly known as the Lower Limestone shales) at the base of the Carboniferous Limestone sequence that outcrop on either side of the bay with the younger Black Rock Limestone occupying much of the area beneath the sandy beach. Next we walked along the foreshore to the first cove that lies below a ruined limekiln on the north side of the bay. Here we saw the Avon Group strata dipping towards the south and resting discordantly upon the almost vertical beds of Black Rock Limestone.  There has been a strong northerly thrust movement pushing the Avon Group rocks northwards over Black Rock Limestone. The line of this thrust fault can be traced through Cove 2 and into the south side of Cove 3 where a narrow zone of brecciated limestone marks the lower side or footwall of the thrust. These broken and shattered rocks are the result of movement along the thrust fault during the Variscan earth movements.
Next we clambered over the rocks into Cove 3 where there are several interesting geological structures on the foreshore including two periclinal folds that appear as shallow elongated domes rather like the upturned hull of a boat. The axes of the folds are orientated parallel to the main synclinal axis of West Angle Bay. The rocks are also cut by a series of en echelon veins that are where the rock has been sheared and the resulting tension gashes have later been filled with the mineral calcite. You may also notice some small circular holes with radiating fractures about 30 cms long. These are not natural features, rather they result from blasting operations during quarrying many years ago. Many of the limestone faces contain fragmented brachiopods, corals and crinoids and some calcareous mudstones are bioturbated showing infilled burrows. Some of the latter were preserved in chert that had been precipitated from silica rich waters. The burrows are referred to as trace fossils since the original organism that produced them has long since been destroyed. Finally we reached the far northern side of the cove where the Avon Group mudstones were faulted against the limestone. This is a normal fault with a downthrow to the south. The mudstones are orange brown due to downwash from the overlying iron rich glacial drift. Note that the rock fragments (clasts) in the drift are very angular since they have been shattered under periglacial conditions.                  John Downes
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Geology / Re: Geology Group Diary (31)
May 09, 2018, 09:30:46 pm
These are the final images for Yorkshire Pennines!
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Geology / Re: Geology Group Diary (31)
May 09, 2018, 09:17:22 pm
6 more images.
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Geology / Geology Group Diary (31)
May 09, 2018, 09:13:20 pm
The Geology Group met at Merlin's Bridge Village Hall at 10.30 am on Wednesday 9 May 2018. The topic for the meeting was
BRITISH LOWER CARBONIFEROUS ROCKS.
By the end of Devonian times the Caledonian mountains had been eroded down to lowlands and the Old Red Sandstone continent was inundated by the shallow shelf seas of the Rheic Ocean that separated Avalonia from Gondwana. This ocean was closing during the Carboniferous and continental collision would produce the Variscan Fold mountains extending from the Appalachians through the Pyrenees and the Atlas to the Urals.
During the Lower Carboniferous Period (363-325 Ma) Britain lay in the equatorial region and so experienced hot wet tropical conditions suitable for the growth of coral reefs on the margins of the seas in which carbonates were deposited on offshore ramps. St George's Land  formed a land barrier stretching from Wales across to Belgium; whilst marine limestones were laid down in the Mendips and South Wales, across much of Ireland  and in the Pennines to the north. The shallow shelf seas were rich in marine life particularly corals such as Caninia (single coral) and Lithostrotion (colonial coral) and numerous brachiopods including Productus and Spirifer. Crinoids also grew in profusion. In North Yorkshire, Northumberland and the Midland Valley of Scotland deltaic sediments were deposited from rivers flowing off the northern landmass.
South West England.
The Mendip Hills lie to the south of Bristol and extend WNW-ESE from Weston super Mare to Frome in Somerset. This axial trend reflects the influence of the Variscan earth movements that produced a major asymmetrical anticlinal structure plunging eastwards. Since the main thrust came from the south as the Rheic Ocean closed, the dips on the north side of the Mendips are much steeper than those on the south. Since the oldest rocks lie in the core of an anticline we find the Upper Old Red Sandstone in the centre of the Mendips where it forms the upland known as Black Down. The Carboniferous Limestone has been eroded over the centre of the anticline and now outcrops on the flanks of the Mendips. The northern outcrop is well seen in Burrington Combe which is one of several valleys cut into the limestone. However, during Triassic times flash floods would have dumped vast quantities of limestone debris and sand into these valleys. Today erosional remnants of the Dolomitic Conglomerate (which in fact is a breccia) can be seen resting unconformably on the sides of Burrington Combe. Thus much of the Mendips have been exhumed from beneath a Triassic cover.  The steeply dipping limestone beds are well seen at the Rock of Ages which is where the Rev.Montague Augustus Toplady is said to have sheltered in a cleft during a storm in 1763. According to the apocryphal story he received divine inspiration after which he wrote the classic geological hymn 'Rock of Ages'! Aveline's Hole is a fine example of a swallow hole that follows the inclined bedding plane and leads down to two large chambers where the remains of a Mesolithic cemetery have been found.
Cheddar Gorge on the south side of the Mendips was cut by meltwater during cold periglacial phases of the Pleistocene Ice Age when permafrost would render the limestone impermeable. In warmer interglacial times the waters would descend into underground passages and caves leaving the gorge dry, as it is today. The caves contain the remains of animals that lived in Devensian times and the bones of 'Cheddar man' in Gough's Cave have been dated to around 7150 BC.
Ebbor Gorge lies about 8 kms to the south west of Cheddar where the Ebbor Thrust has forced the Carboniferous Limestone over Namurian quartzite that is exposed by a stream near the entrance to the gorge. The thrust zone extends along the SW margin of the Mendips and demonstrates the powerful effect of the Variscan earth movements. Wookey Hole near Wells is a magnificent swallow hole leading to some 25 underground caverns through which flows the River Axe. Human and animal remains indicate that the caves have been used since Palaeolithic times. Wookey Hole is a SSSI but like Cheddar, it is highly commercialised and a tourist' honey pot'.
The Avon Gorge which is spanned by the Clifton Suspension Bridge, cuts through a complete sequence of Lower Carboniferous rocks. In 1905 Vaughan pioneered the use of an assemblage of brachiopods and corals to create fossil zones K,Z,C,S & D to subdivide the strata [Cleistopora, Zaphrentis, Caninia, Seminula,& Dibunophyllum]. Today these zones have been superceeded by a classification based on 6 cycles of marine sedimentation during the Lower Carboniferous.
The Pennines
Whilst there is a conformable transition from the ORS into the Lower Carboniferous strata to the south of St George's Land; to the north there is a marked unconformity between the Carboniferous and older rocks. This suggests that whilst the sea advanced steadily in the south providing an unbroken record of sedimentation; in the north uplift of the basement blocks (Askrigg and Alston blocks) meant that the sea did not cover them until midway through the Lower Carboniferous. Only in the basin areas between the blocks was there continuous cyclic sedimentation of Yoredale mudstones, sandstones and thin limestones.
The West Riding of Yorkshire includes most of the Central Pennines that are dissected by several rivers which are tributary to the River Ouse. These rivers flow through and form the Yorkshire Dales from Swaledale in the north; Wensleydale; Nidderdale; Wharfedale; Airedale to Calderdale in the south. For the most part the rocks are Carboniferous Limestone, and Yoredale beds and they form a faulted tilted structure known as the Askrigg Block formed of Lower Palaeozoic basement rocks.. The block dips gently north eastwards but the higher western side is marked by steep scarps along the line of the Dent Fault and the South Craven Fault. The maximum upthrow along the Craven faults is about 1600 metres. Giggleswick Scar (NW of Settle) is a good example of a fault scarp along the line of the South Craven Fault. The younger Namurian gritstones of the Craven lowlands lie on the downthrow side of the fault whilst the older Carboniferous limestone forms the upthrow side of the fault.
Malham Tarn also rests on impervious Silurian slates, but notice that its outfall disappears underground when it reaches the limestone at the North Craven Fault. Malham Beck issues as a Vauclusian Spring at the base of Malham Cove, a natural amphitheatre with a well developed limestone pavement above it. The clints (blocks) are cut by rectilinear grykes (fissures) that form as solution weathering widens the joints in the horizontally bedded limestone. Cawden Hill near Malham village is one of several reef knolls that lie along the southern margin of the Mid Craven Fault. They are formed of calcite mudstones with some shelly limestones rich in corals, crinoids and brachiopods. These reef knolls clearly formed  on the margins of the Lower Carboniferous seas where the water was shallow above the Askrigg Block. Another spectacular karstic feature is Gaping Ghyll, a large swallow hole that takes the water of Fell Beck off the slopes of Ingleborough. This peak is capped by impervious Millstone Grit and Yoredale beds but as the water drains off on to the underlying limestone it disappears down numerous sink holes (swallow holes) including Gaping Ghyll which has the highest underground waterfall in Britain (98 metres).
The strata in the Yorkshire Dales are gently dipping to the northeast so the Great Scar Limestone is well exposed in the Craven District but farther north the overlying Yoredale beds are more common. Yoredale is an older name for Wensleydale where alternating thin limestones, shales and flagstones occur in rhythmic succession. Such cycles of sedimentation reflect the fluctuating sea levels above the Askrigg Block towards the end of Lower Carboniferous times. Differential erosion of the strata on the valley sides has produced a stepped topography with the limestone forming prominent terraces. Also where the rivers cross the harder rocks there are often spectacular waterfalls. Hardraw Force can be visited through the Green Dragon Inn in Wensleydale and lower downstream on the River Ure are a series of rapids known as Aysgarth Falls.
Swaledale was a major lead-zinc mining area reaching peak production in the late 18th C. The most important mineralised zone occurred on the north side of Swaledale, particularly around Gunnerside Gill. The veins run approximately east-west along pre existing faults in which hydrothermal minerals were precipitated. Gangue minerals such barytes and fluorite also occur in the veins and some old mines have been reworked in recent years in order to obtain these minerals. The source of the hydrothermal fluids is considered to be a large granite intrusion deep below the Askrigg Block and the rising granite also provided the necessary buoyancy to elevate the block above the surrounding lowlands that occupy 'basin' structures where great thicknesses of sediment accumulated in Carboniferous times in contrast to the relatively thin cover over the Askrigg Block.          John Downes 

26
Geology / Re: Geology Group Diary (30)
April 12, 2018, 10:27:09 pm
A further 6 images relating to the Old Red Sandstone.
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Geology / Re: Geology Group Diary (30)
April 12, 2018, 10:16:34 pm
Here are 6 more images relating to North Devon and South Wales.
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Geology / Geology Group Diary (30)
April 12, 2018, 10:10:21 pm
The Geology Group met at 10.30am on Wednesday 11 April 2018 at Merlin's Bridge Village Hall. This month's topic was the
DEVONIAN GEOLOGY OF BRITAIN.
When the Iapetus Ocean finally closed at the end of the Silurian Period continental collision between Laurentia, Avalonia and Baltica produced the Caledonian Mountains along a suture line that extended from Greenland and Scandinavia through Scotland and Wales, to Newfoundland and New England. Since these continental areas were situated near to the equator and cut off from rain bearing maritime influences, the climate became arid with low humidity and rivers fed by torrential storms often produced flash flooding. The rapidly eroding mountains provided a source of coarse sediments that were deposited in lakes and rivers in the  inter- montane basins; these muds, sands and gravels eventually formed the rocks of the Old Red Sandstone. These eroded continental deposits were first laid down in Scotland in mid Silurian times but in the Brecon Beacons in South Wales deposition of the ORS did not begin until the end of the Silurian. However, whilst continental conditions continued throughout the Devonian Period (409-363 Ma) over most of Britain, a shallow shelf sea covered South Devon on the northern margin of the Rheic Ocean that separated the ORS landmass from Gondwana.
South West England. It is of interest to note that the Devonian Period was named by Sedgwick and Murchison in 1839 when they were working on the marine sediments in Devon and Cornwall. It was later realised that the continental ORS was of a similar age since in North Devon it is interbedded with Devonian marine sediments. The Foreland Grits (basal ORS) form steep cliffs near Lynmouth but these are succeeded by marine shales and then the Hangman Grits (Middle ORS)  east of Combe Martin. Around Ilfracombe to the west, marine sediments outcrop but these are overlain by an Upper ORS outcrop that extends to the coast near Morte Point. In South Devon the Torquay Limestone is a shelf sea deposit formed of reefs containing corals, brachiopods and gastropods. In North Cornwall marine mudstones have been converted into slates by the Variscan orogeny and the famous 'Delabole butterfly' is an example of a brachiopod Spirifer verneuili that has been deformed by intense earth movements.
South Wales and the Welsh Borders. A large triangular outcrop of ORS extends from Shropshire south to the Severn estuary and west through the Brecon Beacons to Pembrokeshire. Up to 3000 metres of mudstones, sandstones and conglomerates accumulated on river flood plains and coastal lowlands bordering the Rheic Ocean. In the semi arid tropical climate thick beds of calcretes formed as evaporation brought carbonate minerals to the surface. Spore bearing plants grew near the rivers where primitive fish were living. The well known Ludlow Bone bed at the base of the ORS in Shropshire contains fish scales and spines in a condensed sequence only 4 cm thick. Cephalaspis is a typical armoured jawless fish found in the Lower ORS of the Welsh borders. In Pembrokeshire at Red Cliff, Marloes, there is a conformable junction between the Gray Sandstone Group ( Silurian) and the Lower ORS. Thick calcrete beds are present at Freshwater West and Chapel Point, Caldey Island.
Scotland. The ORS derived from the eroding Caledonian mountains, covers large areas in Scotland.  In the Midland Rift Valley up to 9000 metres of sediment was deposited by  braided streams rushing down from the surrounding highlands and forming alluvial fans that spread out at the foot of the valley sides (Highland Boundary Fault & Southern Uplands Fault). Extensive vulcanism also occured as evidenced by the basalt lava flows inter bedded with the Lower ORS sediments. The Ochils and Sidlaw hills are formed of these lavas
In NE Scotland the Orcadian Basin covered the area around the Moray Firth through Caithness to the Orkney Islands. This was the site of several large shallow lakes surrounded by alluvial plains and along the edge of the highlands alluvial fans developed where rivers deposited thick beds of sand and gravels. The Old Man of Hoy is a much photographed sea stack on Orkney where the ORS cliffs rise sheer from the sea. On the mainland the Caithness Flagstones are one of the most famous rock formations within the Middle ORS. They consist of banded and varved (seasonal deposits) lake sediments; rhythmic alternations of mudstones, dolomitic limestones and cross stratified sandstones. The carbonate horizons contain beautifully preserved freshwater fish remains, particularly in the Achanarras fish band. In 1839 Hugh Miller, a Cromarty quarry worker, discovered calcareous nodules within the ORS that contained armoured jawless fish like Pterichthys.  He spent many years collecting fish specimens for local museums and in 1847 published an account of his work entitled 'The Old Red Sandstone'. A small outlier of Middle ORS occurs at Rhynie in Aberdeenshire where volcanic waters rich in silica entered a peat bog and preserved a remarkable array of primitive vascular plants in chert.
Evolution at the cross roads?  In Devonian times aquatic plants invaded the land and fish with lungs and bony fins developed into amphibians. The irony is that the Devonian has often been called the Age of Fish yet at that time 90% of Britain was occupied by the arid ORS continent.
John Downes





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Geology / Re: Geology Group Diary (29)
March 14, 2018, 09:22:20 pm
These are the final 6 images.
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Geology / Re: Geology Group Diary (29)
March 14, 2018, 09:17:41 pm
The first two images of this group are of the Skomer volcanics which unfortunately were omitted from the sequence shown this morning.