Steve's Herpetological Blog

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#SciFri What I've Been Up To

#SciFri: Norwich Geology Walk

Not long ago, I was in Norwich for a week whilst attending a PhD related summer school at the University of East Anglia. Whilst I was there, I was given the opportunity to go for a guided walk of the city to explore how the local geology has been used to create the iconic buildings within Norwich. As a somewhat enthusiastic geologist, I seized the opportunity.

The Plantation Garden, now it is an idyllic location for reading a book or taking a break from a busy work day

The tour started at the Plantation Gardens which is the site of both a former prison and a chalk quarry. Norwich unfortunately suffers from a lack of local building materials expect for chalk and flint which dominate the older buildings. The quarry helped to provide flint and chalk which could then be later used to make lime mortar – which needed to hold flint nodules together. Flint is a hard cryptocrystalline form of silica that is resistant to wear, making it the perfect material for construction or the weather-proofing of buildings.

This source of chalk had been mined for hundreds of years until the introduction of modern building techniques. Most of the tunnels are unmapped and following the dissolution of the surrounding rock – sinkholes can sometimes open up. One well-known examples occurred on Earlham Road on 3 March 1988 when a sinkhole opened up and caused the number 26 bus be stick out of the road at a 45 degree angle.

Roman Catholic Cathedral of St John the Baptist in all of it’s splender

Next on the tour was the Roman Catholic Cathedral of St John the Baptist, which has quite the name! It is an early Victorian cathedral which like many is built in a medieval style. The main structure is made from a Jurassic oolitic limestone which is relatively weather resistant. The mouldings are carvings are made from Beer stone, a Cretaceous chalk that is less weather resistant than the Clipham stone. As you’d expect with sedimentary rocks, there are plenty of fossils to be seen. The steps to the cathedral are filled with thousands of crinoid fragments and the pillars show hundreds of beautiful rugose corals.

Take a look at them corals! The pillars are made of Frosterly marble but it is in fact a fossiliferous Carboniferous limestone

Later in the tour we visited St Giles Church which largely consists of flint and shows several styles of flint work. These could be for a number of different reasons including stability, weather proofing and the expense of each technique. The main structure itself uses flint nodules that have been split, exposing their smooth surfaces and conchoidal fractures. At one end of the church napped square flints have been used (pictured below), which are pyramidal in shape so that the lime mortar has something to adhere to.

Napped flint blocks at St Giles Church – notice the tight spacing of the flints

There were a couple of smaller stops on the tour from which I failed to get photos but I shall explain quickly here. One of these was St Giles’ House which has the lower parts of the building faced with white granite containing large feldspar phenocrysts. These phenocrysts are sodium-rich and zoned containing inclusions of hornblende and biotite mica. There are groups of pillars that vary in colour, caused by the dark granitic magma still being viscous and not very well mixed as it cooled. Further along our trail we came across the Fired Earth tile shop which is interesting it is dressed in Larkvikite, which shimmers blue in bright sunlight known as labradorescence.

Our final stop on the tour was the Guildhall. Again there are several different styles of flint working on display, one of which is a coarse outer face of split flints that have the spaced filled with small flint shards. This helps to improve the weatherproofing of the surface. The east face of the Guildhall as a chequerboard pattern which consists of squares of flint and limestone, known as ashlar stones. This gives the building quite an attractive appearance and it certainly stands out!

The Guidhall in Norwich city centre

I found the walk highly enjoyable and I learned a lot about the local geology. It’s a shame we couldn’t go on all evening but unfortunately our stomachs beckoned for food and beer. I hope you found this post useful and informative.


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