Building Stones of Manchester University and its Environs
10th October 2009
Leader - Norma Rothwell
A group of MGA and OUG members, plus visitors, gathered with Norma Rothwell outside The Manchester Museum, Oxford Road, for this, the last MGA outdoor event for 2009.
In the boot of her car, Norma had an extensive and fascinating collection of building stones with matching postcards for reference, including exotic granites, local sandstones and limestones, and marbles. Norma made the point that the older the building, the more likely it was that the building stone was local - with obvious exceptions, such as Canterbury Cathedral, built with limestone from Caen, France. However, over time, as transport links improved, more “exotic” building stones reflecting the economic prosperity of towns and cities were used, such as Imperial Mahogany, which is a dark igneous rock from South Dakota, the decorative limestone from Portland and Larvikite from Norway. Many fine examples of these stones and more besides can be seen locally in the buildings of Manchester and Liverpool.
Moving on we made our way to the plaza and foyer of the Museum both of which are paved with microgranite, a pale grey, medium-grained crystalline stone with easily seen crystals of mica, feldspars and quartz. This was imported from China in 2004. The polished foyer stones contain numerous autoliths, the result of later pulses of magma moving up through the original magma on thermal currents. The Egyptian obelisk in the foyer was carved from a coarse- grained dark pink Syenite quarried in Aswan and twinning was seen. Opposite the Museum entrance, the Carboniferous Darley Dale sandstone walls of the geology gallery have fine examples of cross-bedded sandstone.
On Oxford Road we observed that the Museum had been built from thick and thin blocks of Darley Dale sandstone; the thinner blocks had been used as ties between the thicker blocks to secure the building, making an attractive design feature. In some of the blocks, more cross and graded bedding could be seen and higher up one block had a fine example of Liesegang Rings – a form of iron staining that looks like fine lamination, except where they are clearly seen to cross-cut bedding planes.
The flags under the archway leading into the University quadrangle are Haslingdon Flags from the Carboniferous period with parting lineation, giving them an uneven appearance. The archway has intricate carvings and Norma pointed out that this was constructed from a freestone, which is a competent stone, easy to carve and withstand weathering.
Passing under the archway and into the university quadrangle, paved with ripple marked Haslingdon flags, we looked back at the archway. There was a clear colour difference in the stone used for the archway and the adjacent buildings. The archway stone was pinker, and is a Binney Sandstone of Carboniferous age and was older than the more buff coloured Darley Dale Sandstone used in the other buildings.
At the far end of the quadrangle is an imposing glacial erratic discovered in 1888 during excavations for a new sewer in Oxford Street. The weathered erratic is an andesite from the Lake District Borrowdale Volcanic Series. The striations on its north and south faces are indicative of transportation by ice.
Surrounding the erratic is a decorative collection of smooth, rounded, iron- stained metaquartzite cobbles. During the Triassic period, the cobbles were carried by large, fast flowing rivers from a mountain range in what is now Brittany and deposited in desert basins in what is now southern Britain. The pebbles become smaller towards the north of the country indicating that the rivers flowed from the south. The small bruise like marks on the surface of the cobbles is caused by pressure solution at the points of contact between the cobbles as they were buried and compacted during lithification.
Back in Oxford Road, we made our way to the Students’ Union Building to search for Jurassic fossils in the Portland Stone. Weathering has caused many fossils to stand proud of the matrix, so with a little patience, oyster, Turitella and Solenopora could be found. We were asked not to collect!
Across the road from the Students’ Union Building is the Holy Name Catholic Church. The church was built in 1871 from Warwick Bridge Stone, a competent sandstone from the Upper Carboniferous age and younger than the Collyhurst sandstone.
Inside the church use had been made of a variety of aesthetically pleasing material. The pulpit is constructed from an amber and cream streaked alabaster (a form of gypsum) and has serpentinite columns. Serpentinite (a green, coarse-grained crystalline ultra basic metamorphic rock) is also used for the altar rail; underneath this are inlaid flower designs made from jasper and serpentinite. The high altar is a beautiful Caen limestone and the statues of St. Teresa and Our Lady of Fatima have been carved from white, medium-grained Italian Carrara marble, a metamorphosed limestone that is much valued by sculptures both past and present. The font at the rear of the church is alabaster, similar to the pulpit. The floor is flagstones with parting lineations and the piscinas near the door are clamshells (Tridacna).
Passing the Williamson Building with its new granite seats, we crossed the road to the Museum’s geological garden in Bridgeford Street. This collection of fourteen specimens demonstrates the rich diversity of natural stone found in the wider region of Manchester and ranges in age from the Ordovician Green Slate (~500 Ma), a metamorphosed volcanic ash from Cumbria, to the brick red, sedimentary Triassic St. Bees Sandstone (~225 Ma). There is a splendid specimen of Shap Granite (~400 Ma) with its pink orthoclase feldspar crystals and autoliths of darker material. The Carboniferous period is represented by specimens of limestone from Swaledale and Derbyshire and a basaltic lava from Millers Dale, alongside sandstones from Darney, Blaxter and Doddington. Local sandstones, found in the base of the Coal Measures, are represented by specimens from Appley Bridge, near Wigan and include Harrock Hill Grit, Crutchman Sandstone and Old Lawrence Rock.
After lunch, we met up at the entrance to the Whitworth Art Gallery and a discussion took place as to whether the small fragments of darker material within the grey granite portico columns were xenoliths or autoliths. It was mooted that, as a rule of thumb, xenoliths tend to have angular edges and autoliths tend to be rounder. As there seems to be both angular and rounded examples within this granite, the jury is still out on this. Interestingly, though, some of the more angular fragments showed signs of weathering.
Beyond the steps of Carboniferous sandstone, the foyer floor is constructed from white marble and black limestones tiles; the limestone showing indistinct fossil traces.
Inside the art gallery proper, more tiles of marble and limestone can be seen alongside beautiful examples of green ophicalcite (a marble inundated by serpentinite, similar to Connemara marble). The crowning glory of this area is the columns of Blue Pearl Larvikite.
This coarse-grained crystalline igneous rock is a variety of syenite from Norway. The iridescent blue ‘Schiller’ effect is caused by the predominant large anorthoclase feldspar crystals.
The dark Otta Slate used for the floor of the art gallery was of note. It is not a slate but a schist and lineation is obvious when the rock was examined by the window; the acicular (needle-like) porphyroblasts of amphilbole (hornblende) can be picked out standing proud where few people had walked. These acicular porphyroblasts are the result of secondary mineral growth after metamorphism.
Many thanks to Norma from the MGA, OUGS and guests for bringing to our attention the wide diversity of accessible building stones within the University locality and giving us such an informative and enjoyable day.