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.