London,
Building Stones Walk lead by Dr Clive Bishop.
Essex Rock & Mineral Society Field Trip, 26 October 2002
Clive started the talk just inside the city boundary, outside Tower Hill Underground Station, where he discussed the construction of the Roman wall.
The horizontal tiled bands that appear up to head height in the picture, mark the original Roman construction. The tiled layers are continuous through the width of the wall and help 'tie' it together. The structure above head height is devoid of this feature, as it is a medieval reconstruction using the original (in most cases) stone and tile in random fashion.
The stone used was Kentish Rag.
A
few steps away and in sight of the station is Trinity House. Describe
as faced with 'Portland Limestone'. A later extension at the far end has
been sympathetically built to blend. Here Portland limestone is used at
eye level with brick for the upper floors, a less expensive option! Portland
limestone seemed to feature heavily in the facing of buildings on route.
The
corner of a more modern building faced with slabs of a 'rough' finished
"Granite Gneiss". Notice the joins, these are sealed with a
mastic compound. The solvent or oil from this has bled into the surrounding
rock leaving an unsightly stain.
Not a pretty sight and very common. In fact this was almost universal for all mastic sealing of modern stone cladding to one degree or another, except for one example where the facing material was a 'diorite'.
How can the building industry get away with it, especially on something
that is of ornamental appearance?
This is the same Granite Gneiss but polished. The rock was described by Clive as being a high grade metamorphosed granite from Italy, it's name being FANTASTICO! There are numerous examples around, some more fantastic than others.
St Margaret's Paternoster, our first encounter with some architecture of Wren! Clive talked about planning regulations within the city paying particular attention to a Georgian Town House (just to the right of picture and out of shot) which used the church as one of it's flank walls, (that's handy!).
Hard to believe, but this building was once a Victorian Warehouse, a very prosperous business. The rest of the street was littered with individual buildings from that period and a treat to the eye.
More facades of once prosperous Victorian businesses.
All this must look lovely on fine day, it was cold and drizzling, London not at it's best!
An example of Serpentinite with veins of white Calcite & Talc. Clive described this as from Switzerland and the result of the alteration of Olivine as it's risen through the Earth's Crust.
The picture does not do it justice. There are beautiful tonal ranges of green to black with white/grey veins. The serpentinite surface polishes well but the calcite and talc remain matt and in places eroded. A great contrast of textures!
The bottom section of the Monument. Clive used this as a typical example
of how the use of one stone can be quite suitable for some parts of a
building and almost a disaster in other areas. The flat vertical surfaces
and column show little erosion whilst the cornice and other mouldings
are decaying away. Many buildings suffer similar damage. We viewed many
examples of repair and rebuilding with cement compounds. The practise
is called "dentistry" for obvious reasons.
Another oldish building but with little of no damage to the intricate mouldings which are terracotta. The profiles have lasted extremely well in comparison to some of the natural stone used in a similar way with other buildings.
An example of 'Portland Roach' near Canon Street Station. Some of the fossils sections are intact whilst others have dissolved away. Clive described this as a result of the initial fossilization. Some were preferentially mineralized by Calcite whilst others by Aragonite. Both chemically the same carbonate, but the Aragonite is less stable, and in this instance has dissolved away (during the formation of the rock) to reveal fossil "moulds". An interesting texture!
Here we had a look at some "Black Limestone" steps on the south side of the St. Paul's. These were said to have come from the Isle of Man, but as there is no exposures matching the rock on Manx, it is thought they were processed there. The original rock probably came from the Red Cow quarry (so named after the adjoining pub) near Dublin.
Initially black limestone steps were used at the front (West end) of the Cathedral. These have since been replaced by a much harder wearing stone, a black diorite.
Clive's walk from East to West across the City of London (proper), and a little bit further, encompassed a blitz of rock types, architectural styles, civil engineering techniques and some intriguing historical tales. We encountered: Peterhead Granite, Aberdeen Granite, Shap Granite, Limestone, Sandstone, Larvikite (Laurvigite) from Norway, Baltic Brown, Blau Brun (?)from Scandinavia and a few others that have slipped my mind. There were tales about London Stone, and how the Central Criminal Courts got their name. Explanations as to: why most of our walk was on the level, (one of the Thames Terraces, the Taplow Terrace), why we went up and down twice, (because we crossed the locations of the rivers Waldbrook and Fleet), why they changed the type of building stone in St Katherine's House, (a tale of double carbonates, epsom salts, and acid rain pollution from all those coal fires of the time) and why the Dean of St Paul's used to telephone Bank Side Power Station when the wind was in the wrong direction.. Clive has been doing these walks/talks for students from the mid-sixties He sets a purposeful pace to the route for a little over two hours, a short journey of a mile and a bit. Some 18 members attended this last trip of the season, it was a cold and damp day finished (for me) by a nap on the District Line train to Upminster (where they woke me up and chucked off the drunks, as usual.) If you have never considered a field trip of this type, do so, you will never have cause to be bored when out shopping with the wife and kids.
Many thanks to Bob Blackburn for all the pictures!
Roger Coleman 20 November 2002
(Printing: Document optimized for A4 format
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