I'd offered to lead my Severnside branch of the OU Geological Society on a walk around the Bay, looking at building stones - nothing like a leap into the dark as I'd never been there but I did have a South Wales GA leaflet which was a good start point (and which is quoted extensively here). That got augmented by a reccy (nice way to spend my birthday!) and some research online (see list of sources at the end).
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| We met up in Havannah Street car park, by the Wetlands area created as part of the Cardiff Bay development |
The Bowline Knot sculpture is by Andrew Rowe: the knot, used to moor ships, reflects the coal port heritage.
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| View across to Penarth at the far end of the barrage, our final goal for the walk |
The first geology we encountered was on one of the new apartment blocks, Ocean Reach, where there are two contrasting types of natural stone cladding:
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| A medium grained sandstone, angular grains, well cemented but apparently quite porous and with marked Liesegang rings on some pieces |
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| What I believe to be Upper Ordovician green slate from the Lake District. This would have been a volcanic tuff and we could see graded bedding from ash to lapilli sized tephra. Alteration has changed some of the minerals to greenish-blue chlorite |
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| In this piece rip-up clasts of fine grained mud-sized material is likely to indicate that deposition was underwater |
From here we walked up the side of the old Mount Stuart dry docks where everything from warships to British Rail ferries was overhauled, past Techniquest (on the site of an old repair shed) and on to the Pilotage Building, probably the oldest building in the Bay area and built of thin slabs of a fine sandstone.
Then on up Bute Street where we singled out two buildings in particular, the old National Provincial Bank at 113-116, and also No 54.
The old Provincial Bank building which dates from 1926 is built of white Portland
Stone (top Jurassic,
Tithonian – 145 to 152 Ma). This stone is a very white oolitic, bioclastic limestone and was more desirable and prestigious
than Bath Stone. It comes from quarries in the
Isle of Portland in Dorset.
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| 113-116 Bute Street - a 1920s building in prestigious Portland Stone. |
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| 54 Bute Street - a plethora of different stone types in this 19th Century building |
Built
in 1875 for the Powell Duffryn Coal Company, Pascoe House is an interesting
example of one of the older buildings in Cardiff Bay.
The main part is grey sandstone, most probably Pennant Sandstone, from the Valleys of
South Wales. Mid Pennsylvanian
(Moskovian) 315 – 307 Ma (Westphalian D).
The yellow buff stone, used for carved decoration, is Bath Stone, another bioclastic
oolitic limestone but darker coloured than the Portland Stone. It is quarried near
Bath: “Box Ground” was popular in the 19th C from Hartham Park underground
quarry. Bath stone was taken from the Bath
Oolite Member and the Combe Down
Member of the Chalfield Oolite
Formation, part of the Great Oolite
Group; 163 – 170 Ma (M
Jurassic).
The red stone, used along the bottom, the entrance steps and
in the arches, is local Radyr Stone, quarried at Radyr, just north of Cardiff, between 1850 and 1910. This breccia contains irregular fragments of white, brown and grey rock and was deposited in wadi mouth fans by flash flooding. It is of Upper
Triassic (Norian) age, 210 Ma
old. Radyr Stone
crops out at Radyr, northwest of Cardiff.
The polished granite in the columns by the entrance is
called Bessbrook granite. The GA leaflet
says it is 60 Ma years old i.e. from the unnamed subvolcanic felsic ring dyke which was
intruded into the 400 Ma Newry granodiorite during the Palaeocene, forming the Slieve Gullion complex in the Mourne
Mountains. However, it is more likely from the granodiorite itself: a description of Bessbrook Granite reads: “Irish Bessbrook Granite, quarried at
Altnaveigh in County Armagh from the 399 Ma Newry Granodiorite, intruded during
the Caledonian mountain building phase. This is the porphyritic facies of this
granite with sparse, white feldspar phenocrysts in a medium to coarse grained
groundmass of plagioclase, microcline, quartz, hornblende and biotite. Small (1
cm), black xenoliths of hornblende-rich metasediments are abundant in this
granite and give an appearance of being flecked with black marks.” Ruth
Siddall, UCL, Urban geology in London. It certainly matched this description in appearance.
From here we made our way back down The Flourish, pausing at Craft in the Bay to look for (and find) pyrite crystals in the dark grey Welsh slate bands in the paving and to look at the granite bollard in front of the building which could be Cornish granite, perhaps from Bodmin Moor as that has easy access to a north Cornish port at Padstow. However we discussed that it might be from further afield, even Norway with which country there was substantial trade.
At the Wales Millenium Centre we admired the variously coloured Welsh slate cladding but deplored the fact that it was too high up to get a good look at. Five different colours, and different textures (rough-hewn
and cut) in the blocks of cladding which are all slate waste from North- and Mid-Wales.
Green and purple
slate from Penrhyn quarry (Bethesda, Lower Cambrian)
Dark grey rustic
rusty slate from Corris quarry, north of Machynllyeth. (U Ordovician)
Plain dark grey slate
from Blaenau Ffestiniog. (U Ordovician)
Colours reflect slight compositional differences.
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| The Wales Millenium Centre with its coloured slate cladding seen from Raold Dahl Plass. |
In Raold Dahl Plass, an oval feature created out of the old Oval Basin which gave access, via two locks to Bute West Dock, These docks, opened in 1839. were financed by the wealthy Earl of Bute and mark the start of Cardiff's great coal port era.
We had an interesting detective hunt in the Plass, looking for the Radyr Stone repairs to the dock which, of course, date the repairs to the years when the Radyr quarry was open. The original building material is Pennant Sandstone with grey granite coping at the top edge where it would take more wear.
Next stop (though we did slightly take the long way round) was to Scott Harbour where we admired the dark red rapakivi textured granite cladding, and examined the variety of granites, gneisses and migmatites in the Compass Rose. Rapakivi granites with their signature orbicular feldspar crystals, are A-type (anorogenic) granites formed within in Proterozoic plates and fault zones and found today within metamorphic belts in these plates, particularly in Scandinavia, Greenland and Canada. This may be a variety called "Baltic Red granite" from Finland and is likely to be around 1500 Ma old. Rapakivi is Finnish for "crumbly" and refers to the poor weathering qualities. The round K-feldspars often have plagioclase rims, which we saw, and also circular zones with small flecks of more mafic minerals.
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| The Compass Rose at Scott Harbour was created by Sebastien Boyesen in 1997 and commemorates Scott's Antarctic expedition which sailed from here in 1910 |
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| Detail of the Compass Rose with Rapakivi Granite in the centre as well as dark red/black migmatite, pale grey and pink granites and a darker grey rock, possibly granodiorite. |
Walking back towards our planned lunch stop near the Norwegian Church we saw our first "Beastie Bench", carved brick animal benches by Gwen
Heeney - 1994. This series of nine benches are inspired by the mythical creatures described in the Dylan
Thomas poem ‘Ballard of the long legged bait’.
The Atradius building at the end of this road has two types of spectacular "Brazilian granite" cladding.The light one “Samba white
granite” is a leucocratic garnet granulite and the dark one “Kinawa granite” is a wonderful migmatitic
biotite granodiorite gneiss.
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| Leucocratic garnet granulite (the GA leaflet notes that the garnets have "no commercial value"!) |
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| Above and below, migmatitic biotite granodiorite gneiss with areas of pegmatite and boudinage. |
Just across from the Atradius Building is the Waterguard Pub, This was built in 1870 as the HM Customs and Excise House and was moved a short way from its original location. The old part is constructed of grey Pennant
Sandstone and yellow buff Bath Stone. The new
part, added in 2002, is built of fine grained cream Magnesian
Limestone, supplied by High Moor Quarry, Tadcaster, Yorkshire.
The Norwegian church (where Raold Dahl was christened) has some comfy benches and a "seatwall" outside as well as serving good coffee and enjoying a lovely view across the Bay.
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| Cardiff Bay, with the St David's Hotel in the middle distance |
We then abandoned building stones for a while and walked on, past the "Dr Who Experience" to the Barrage and crossed it, stopping to look at the coal display, to Penarth.
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| Above and below: information boards about coal mining and the coal trade |
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| One of the enormous lumps of anthracite which are on display. No pyrite and no fossils, but it was easy to see the original bedding as well as the high reflectance of this high grade coal. |
Arriving in Penarth we headed down the path to the left of the old Docks Building onto the beach below Penarth Head where we had a go at matching up what we could see with an annotated photo.
The main exposure of Jurassic Rock begins with Westbury Mudstone, then up through the Lower and Upper Cotham Members into the White Lias and then the Blue Lias. With almost horizontal rocks, and scree at the bottom of the cliff, there was no temptation to get up close but a potter around the beach turned up some slabs with ripples (which looked as though they came from the White Lias or Cotham Member), some with fossil bivalves, and numerous chunks of alabaster (gypsum) from a bed midway up the cliff.
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| Cliff section north of Penarth Head |
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| Bivalve (? Pecten) |
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| Alabaster |
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| Wave ripples on a slab of White Lias |
At 3pm we caught the Water Bus back to the Pierhead and as a last stop took a look at the Pierhead Building.
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| The Pierhead Building dates from 1896, designed by William Frame. |
This striking building is built of glazed terracotta bricks. Unglazed terracotta was used between 1860 – 1890 but became very grimy in urban environments; glazed
terracotta (faience) dates from 1890 and stayed much cleaner. These are handmade, thin, decorative bricks or tiles,
filled with concrete during building; an inexpensive material. The clay used was
Etruria Marl from Ruabon/Wrexham area. Etruria Formation was Westphalian, 311 – 303
Ma. and deposited directly above the Coal Measures. The colour varies and so therefore do to final bricks produced. There's an interesting display inside.
What really made the day was that the sun shone much of the time, and it stayed dry - fortunate for the end of October! The less sunny photos here were the ones I took during the reccy three weeks earlier!
