Urban Geology of Liverpool - Real Rocks in a City Centre? 19th October 2008
Who would have expected to find proper rock outcrops in a City Centre? 10 members joined Leader Alan Diggles in Liverpool on a grey October day to see if it was true.
We met outside Lime Street Station in front of St George’s Hall and after a quick briefing, set off for our first location: Everton Park about 10 minutes walk away. Liverpool is set on Triassic sandstone which can also be seen at Alderley Edge and Frodsham. At the time of deposition Britain was part of a landmass at a latitude approximately that of the Sahara today. The climate would have been fairly similar.
In Everton Park we clambered over two outcrops of red sandstone showing multidirectional dune bedding and containing pebbles. Although the environment was desert-like, examination of the rocks proved them to be water lain. There were rip up clasts within the bedding and at the third location in the park, some of the best cross cutting bedding I have ever seen.
There were thin mud layers too. This represented a braided river system with the changes in paleocurrent noticable. It had been cut by later faulting.
Alan explains this normal fault in channel fill deposits
We then walked down to the Metropolitan Cathedral, starting near the Crypt. Since Alan had first visited the area, work had been done which had removed quite a large amount of rock. However, it had exposed more of the fine grained, well sorted and horizontally laminated bedding. Alan told us this suggested a lower energy environment than Everton Park.
The Cathedral itself was built initially (from 1933 - 1940) of granite and quartz, feldspar, biotite and muscovite were easily seen without the aid of a hand lens. Magma flow currents could be made out occasionally due to aligned phenocrysts of white feldspar. There also were some very fine aplite veins in evidence. After the war, the building was completed using Portland Limestone in which shelly debris (brachiopods and bivalves) could be seen. The oolitic nature of the rock could be seen with the aid of a hand lens. It was interesting to note some differential weathering of the limestone on some of the building corners. Evidence of volcanic ashfalls could also be seen.
After a tasty lunch in the Cathedral cafe, and with the weather becoming windier and chillier, we moved to the Anglican Cathedral. This huge building, completed only in 1978, dominates St James Mount. There was rock exposure to one side and we were able to look more closely at the well-sorted fine-grained and laminate bedding. There were inclusions and again this all added up to a water-lain environment. This was thought to be a sandy plain with shifting rivers and during very dry periods, windblown sand dunes formed.
Inside the cathedral, which is the largest Anglican Cathedral in the world (it is massive - an awe-inspiring site), some of the flooring is made of limestone slabs: black and white with calcitic brachiopod fossils and some corals. The brown French Marble font sits on a limestone dias containing stromatolites and tabulate corals (rather difficult to photograph). It is probably Ashburton Marble (Devonian from Torquay area). The redness is due to leaching from the Permo-Triassic beds above. It was also deformed during the Variscan Orogeny.
The Cathedral is built next to an old quarry (350m long, 50m wide and 30m deep - a very large hole!) which was turned into a cemetery after all the useful stone had been exhausted in 1825. By the time of closure in 1936 57774 burials had taken place. It has since been ‘tidied’ up so that you can actually walk around and reading some of the gravestones was very interesting. It is now a conservation site. On the east wall, there is an outcrop of red and grey/green mudstones and siltstones which are flat bedded and indicative of a low energy environment. Near the base is Liverpool’s only running natural spring - permeable sandstone and impermeable mudstones come together. The water still flows, tastes ok (according to a couple of people who ricked drinking it!) and was recommended a a cure for rickets, weak eyes and lowness of spirits.
The font with decorative marble plinth
We saw evidence of the three tunnels out of the quarry. One of them, the Quarrymens Tunnel runs under the cathedral and did cause problems when the foundations were laid.
We then walked back to St George’s Plateau where we looked at the paving slabs which showed some evidence of trace fossils. Alan told us that Lime Street is so named because of lime kilns which used to be found there. Walking to the Wellington Column, we passed Cornish Granite plinths although the column itself if of Darley Dale Sandstone with an Aberdeen Granite plinth. Most impressive however were the bollards around it which were nodular limestone with lots of corals, brachipods and crinoids.
It was a wonderful trip, opening our eyes to a side of Liverpool which I don’t think any of us really appreciated. Alan Diggles was an inspirational leader and provided us with an excellent handout which could be used again on private visits to see other aspects which we did not have time to see.
Coral in bollard