Geology and Engineering - the Severn Crossings

OUGS at the Severn crossings

Dave Green, our leader for the day, met us at Old Passage near the eastern end of the "Old Severn Crossing" built in 1966 as part of the M4 from London into South Wales. He outlined the plan for the day: a morning on the beach here at Aust, then over the bridge to Beachley for a lunch stop at the Old Ferry Inn before going down onto the beach there, and then finally to Black Rock picnic area near the "New Severn Crossing" where again there is access to the foreshore.

The 1960s road bridge between Aust and Beachley; the near
pier rests on Carboniferous Limestone

The first crossing built here was the rail tunnel in the 1880s; a problem with this was a freshwater spring cut into midway through. The tunnel still has to be pumped because of this, and the soft water is used for paper manufacture.
The site of the old bridge is a narrow point at the river; narrow because of the geology. The geology ranges from Carboniferous to Jurassic. To the NW are folds west of the Malvern fault system, and a block of Carboniferous limestone east of the mouth of the Wye in the Beachley area.
The Second Severn Crossing more or less follows the line of the railway tunnel: both are mainly in Triassic rocks.

Aust Foreshore between Old Passage and the old Severn Crossing

Rock covered with tufa, Part of the old river cliff at Old Passage.

Unfortunately at this point it began to drizzle a bit, but it didn't last long: not a serious inconvenience and had almost stopped by the time we set off down the lane towards the estuary. A good causeway, built for access to one of the electricity pylons that carry wires across the Severn, took us most of the way, with a pause to look at a rock exposure almost completely covered in tufa. Debate about the source of the calcium carbonate for this: possibly from a spring in the Lilstock Fm.

There's an information board along the track

This leads down onto a sandy and pebbly foreshore, with quite a lot of fallen blocks lying around from higher up the cliff face.

Triassic rocks exposed in the cliff go from the red Mercia Mudstone at the base to the Lilstock Fm (Penarth Gp) at the top of the cliff and include the 'Bone Bed'.

Aust foreshore with the Old Severn Crossing in the background

The Mercia Mudstone was deposited over a huge area stretching from here to Yorkshire but is pretty much unstratified and contains no fossils apart from dinosaur footprints though it could be heavily bioturbated. There's debate about the depositional environment: wind blown dust, a saline lake perhaps. In some places (e.g. Clevedon) there is oolitic limestone. It might have been an environment similar to the Red Sea or Persian Gulf today. Laterally the mudstone grades into Dolomitic Conglomerate (not dolomitic and not conglomerate) deposited in palaeovalleys around a lake or floodplain.

The base of the Mercia Mudstone contains much gypsum: some
appears to be deposited in joints but although there could be
some alighment of these, many seem to be randomly
orientated.

At the top of the Mercia Mudstone is a grey/green coloured layer: no change in lithology so perhaps reduction or leaching?Change in pore water chemistry maybe? eH/pH may have changed. Some indication of environmental change perhaps as the setting veered towards marine (there are marine fossils in the dark grey strata above. The whole is capped by the younger, cream coloured, Lilstock Fm.  Various finds here included a large piece of bone from the Bone Bed, many bivalve fossils from the Lilstock Fm, also Estheria minutae, a crustacean (clam shrimp).

Reduction spot surrounding a particle, possibly
organic matter; there are also reduction rims
along the veins of gypsum.

Horizontal layers of nodules probably represent the original gypsum
deposit but there are also horizontal bands of
satin spar, deposited later. 

Retracing our steps, we drove over the bridge (£6.40 toll) and through Chepstow to Beachley where we stopped at the Old Ferry Inn for lunch.

Beachley

The Severn Crossing from Beachley looking towards Aust

From Beachley we had a good view back to Aust beach with the clearly visible reduced zone at the top of the Mercia Mudstone.

Mercia Mudstone is exposed in the low cliff at the back of the foreshore here too, with the added bonus that there were some "Bristol Diamonds" to be found - potato sized nodules which, when broken open reveal quartz and other crystals. It is likely that these formed when anhydrite nodules were replaced by silica. 

Eastern side of the small beach at Beachley Point shows
Dolomitic Gonglomerate deposited unconformably
on Carboniferous Limestone

To the SW of this exposure, at the back corner of an area of saltmarsh, there was a sudden change to older Dolomitic Conglomerate which we identified as a fault by the drag on the beds of Mercia Mudstone, and by slickensides. The fault is, unusually for this context, a reverse fault with downthrow on the NE.

Another view of the Carboniferous Limestone/
Dolomitic Conglomerate unconformity

Finer material in the centre of the beach

To the SW again we came to Carboniferous Limestone beds: these have been thrust over the Trias. They are folded into a SE plunging antiform. Dave gave an impromptu tutorial on dips and strikes here for the benefit of those who are just starting geology.At the end of the point we came to a small beach with Carboniferous Limestone either side, but Dolomitic Conglomerate deposited against that. The conglomerate (actually more of a breccia, and it fizzed well with hydrochloric acid) is made up of coarse, matrix supported clasts at the edges, but finer material nearer the centre of the beach. 

This feature is a palaeovalley in the Carboniferous Limestone landscape which was infilled with a poorly sorted wadi deposit of angular clasts in a matrix support of fine silt/sand.

Black Rock

Black Rock Picnic Site overlooks the estuary SE of Portskewett and just a little way north of the Second Severn Crossing (and rail tunnel).

The foreshore NE of Black Rock picnic area

Mercia Mudstone exposure

Plenty of parking at the picnic site (at least before we arrived!) Then an easy walk past a viewpoint with interpretation panels down to, and along, the back of the saltings took us to a cliff.










The exposure is mainly red in colour, with some yellow bands and at first glance we wondered if the yellow was sandstone, interspersed with the red mud. However, closer examination revealed that both yellow and red areas include sandy beds but the red areas also include muddier sediments. The yellow bands are patchy when you get close up, rather than completely continuous.

Yellow sand is angular, fine, well-sorted, and no visible sedimentary features.

Close up

Sandstone a short way to the NE

Sandstone close up

A puzzling feature is curved joints which cut across the beds and there was quite a bit of discussion as to whether the horizontal layers were bedding or diagenetic, and whether the joints were in fact folded bedding planes.

There was a small amount of evidence of burrowing or perhaps roots.

Walking NE along the cliff line, the beds do begin to dip in that direction and at the end of the cliff we came to beds which were entirely yellow sandstone and presumably either a fault contact or younger and above the Mercia Mudstone.







At some levels there are small (cm sized) very sub-rounded to very well rounded, quartz pebbles, perhaps reworked from the sediment source rather than rounded in transport.

Also from this location we could see downstream to the second Severn crossing which is built mainly on piers placed on bedrock: this being mainly Mercia Mudstone, some of the piers needed caissons as a good platform beneath them to carry the weight. Also, at one point, near the eastern end, the rail tunnel passes within 15m of the neighbouring caissons so these had to be specially constructed with deeper piles so that the weight of the bridge is taken below tunnel level.

OUGS Annual Symposium - Canterbury 2014

In Canterbury this year, with the topic "Marine Geotales", so lots of interesting lectures on a marine science theme. Then on Sunday morning we split up into groups for field trips - I went on the Canterbury Building Stones trip with Geoff Downer

Here we are, about to set off from the coach drop off point in Lower Chantry Lane, Geoff in the check shirt on the right. From here we walk to St Augustine's Conduit House on King's Park.

First stop - St Augustine's Conduit House

Originally roofed, this was where water was collected from several springs and then fed along a pipe to the Abbey

 Three archways where the spring tunnels feed into the conduit house

Geoff explaining it to us. Although the Abbey was destroyed in the 16th Century the conduit house continued to be used, supplying water to a brewery, up until the 19th C.

St Martin's Church

We visited here to see a couple of tombstones in the churchyard, before moving round the back where there are the remains of an early chapel

 Walter Cozens, a local builder, has a nice lump of granite, but also, unusually, a sarsen!

 Around the back is Mary Tourtel's grave - she was the creator of Ruper Bear!

Built in 597 it is the earliest Christian Church - the first church founded in England, the oldest parish church in continuous use, and the oldest church in the entire English speaking world. It forms part of Canterbury's World Heritage Site along with the Cathedral and St Augustine's Abbey.

Moving around the east end of the church to the north side we could see Roman bricks - thinner and flatter - mixed in with the flints etc of the later church, and sandstone quoins

 The Roman bricks

 Checking out the stonework in the church

A reconstruction of the original "Bertha's Chapel" on which the east end of the church is based.

"St Martin's was the private chapel of Queen Bertha of Kent in the 6th century before Augustine arrived from Rome. Queen Bertha was a Christian Frankish princess who arrived in England with her Chaplain, Bishop Liudhard. King Æthelberht of Kent, her pagan husband, allowed her to continue to practise her religion by renovating (ca. AD 580) an existing church which the Venerable Bedesays had been in use in the late Roman period but had fallen into disuse. As Bede specifically names it, this church was dedicated to Saint Martin of Tours, a city located near where Bertha grew up.Upon Augustine's arrival he used St Martin's as his mission headquarters, immediately enlarging it (AD 597), and King Æthelberht was soon baptised here. With the quickly subsequent establishments of Canterbury Cathedral and St Augustine's Abbey, St Martin's lost prestige but retains its priority and historical importance."  

http://en.wikipedia.org/wiki/St_Martin%27s_Church,_Canterbury  accessed 21/1/15 

 

 

A selection of the Church's building stones

The old prison

Returning to Longport Street we stopped to look at the old HM Prison. This is an early "panopticon" (all-seeing) style prison (see http://en.wikipedia.org/wiki/Panopticon ) designed by Jeremy Bentham in the late 18th C.

The prison has now been bought by the University, apparently for student accommodation. (Maybe a future symposium?!)

 We were more interested in the ooids and bioclasts in the Portland Stone!

St Augustine's Abbey and a wall game

 Around the corner, in Longport Lane, we had a view of St Augustine's Abbey. The abbey was founded shortly after AD 597 by St Augustine and, with the Cathedral and St Martin's Church, forms the World Heritage Site.

Large blocks of Caen Stone in a wall by the Abbey.
From here we tried our hand at recognising different types of local building stone in an old wall.

 This Thanet Sandstone has, unusually, been bored by modern piddock shells

A selection of stones in the wall. Amongst the more common Caen Stone, Flint, Ragstone and Thanet Sandstone we spotted rarer Marquise Stone, Quarr Stone (which came from the Isle of Wight and had been worked out by the 12th C), Tufa and Purbeck Marble.

Something different in Monastery Street - "Brick tiles" - a way of facing a wall to appear like brickwork

In Lady Wootton's Green we found an information board with some more detail about Bertha, whose chapel we had seen earlier, and from where we had a good view towards the Cathedral, over the city wall.

Thanks to Geoff Downer (who I understand has just taken over as South East Branch Organiser) for a particularly interesting take on building stones with a lot of history included.

Volcanoes on a Shoestring - Fuerteventura and Lanzarote October 2013 - First two days

Our third visit to the islands and 16 of us, plus organiser Jan and myself, met up in Caleta de Fuste on the 19th. We stayed at Castillo Playa once again: the 2-person bungalows really suit our groups plus the management is so helpful and friendly. My own aim this time, as well as giving the group a good understanding of the islands' varied volcanic geology, was to explore around 10 new potential field locations and 'road test' them.

The 2013 Shoestringers at Degollada de Agua Ovejas
with the lateral collapse scar of the Southern Shield Volcano in the background

Sunday 19th October

An introductory day when we had a look at some aspects of Fuerteventura's "basal complex" and gave the group a chance to settle down. We started off with a quarry face exposure of olivine pyroxene phyric basalt flows near the village of La Matilla: this is an opportunity to introduce the island's shield volcano origins and their current 'post erosional' state where only remnants remain of the original 2000 m high mountains. These remnants take the form of narrow, sharp crested ridges "cuchillos" separated by wide, U-shaped valleys typical of erosion of the near-horizontal basalt flows.

Central Shield Volcano flows in the quarry at La Matilla

From here we moved on towards Puertito de los Molinos, stopping above a barranco east of the village to examine the basal complex exposures of seamount series rocks cut by dykes, which have been eroded and subsequently overrun by an Upper Pliocene olivine basalt flow.

Los Molinos barranco - dyke cutting through Basal Complex

We enjoyed looking at more stunning exposures of the dykes at the mouth of the barranco down in Los Molinos itself and established that they show a strong NNE-SSW alignment. The literature suggests that there is evidence here of the emergent stage of the original Fuerteventuran seamount some 22 Ma years ago but we failed to find the reef material and subaerial weathering evidence that is referred to. The unconformity showed up really clearly in the cliffs however, with young flows and littoral deposits interbedded.

Unconformity in the cliffs north of Puertito de los Molinos

The day finished with a visit to Tindaya, a strikingly pale coloured, conical peak formed of quartz-trqachyte some 18.7 Ma ago. This felsic dome was formed early in the history of the Northern Shield Volcano and is more resistant to erosion than its surroundings. It has been quarried for ornamental stone and on our last day we saw some beautiful examples of the liesegang rings that make it so attractive when we visited Gran Tarajal. Surrounding Tindaya (which was a Majo sacred mountain) we found plenty of shelly material and had fun deciding what could be evidence of Pliocene raised beach and what was a recent 'paella deposit' of limpets and mussels! The problem about the raised beach is that it is much higher than average so various hypotheses were considered with a landslip generated tsunami the winner.

Tindaya

Liesegang rings in decorative slabs, Gran Tarajal. These may have come from Tindaya quarries

Monday 21st October

After stopping in Tiscamanita to look across to Gairia, one of the early post-erosional scoria cones, we visited Pajara to see the fascinating western facade of the church of Nuestra Senora de Regla which was built in 1687 - 1711 and incorporated many Aztec style motifs - suns, jaguars, birds and the "orobus" - the snake eating its own tail which symbolises the circle of life and death. 

Nuestra Senora de la Regla, Pajara, 

Along the road to Ajuy we stopped to take a look at an exposure of Jurassic deep sea sediments, 180 Ma old, which were deposited on the Mid-Atlantic Ridge during the earliest stages of Atlantic rifting. Although metamorphosed, hydrothermally altered and cut by numerous dykes the light and dark striped strata are completely recognisable and a very unusual exposure of this type and age of sediment.

Early Jurassic ocean crust - some of the oldest in existence - cut by Basal Complex dykes
on the slopes of Morro del Moral

On Ajuy beach there are some stunning exposures of similar rocks which have been worn smooth by wave action so we had a look at these before enjoying some garlic prawns at one of the restaurants at the back of the beach.

Jurassic ocean crust sediments cut by basal complex dykes at the north end of Ajuy beach

There's a convenient path from the beach up onto the north cliffs where it took us along the marine erosion platform to a viewpoint across Caleta Negra. There are 5 Ma Pliocene limestones deposited on the platform and these are overlain by younger lava flows from Morro Valdes.

At Caleta Negra the Jurassic marine sediments and dykes of the Basal Complex are unconformably overlain by coastal sediments over which the Morro Valdes lava flowed into the sea forming pillow lava

Morro Valdes lava has deposited blocks on the caliche that coats the marine platform