Ruth Siddall on Urban Geology
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In the podcast, Ruth Siddall explains the kinds of geology on display in the building stone of cities and takes us on one of her favorite urban geology walks in London. She has developed nearly 50 urban geology-themed walks and built up a database of over 4,300 urban localities of geological interest.
Siddall is a postdoctoral researcher at Trinity College, Dublin, studying the social history and geological provenance of stone in 18th century buildings in Britain and Ireland.
Podcast Illustrations
Images courtesy of Ruth Siddall unless otherwise noted.
St. Paul’s cathedral in London, completed in 1710, was built with Portland Stone. There is a rich archive documenting architect Christopher Wren’s ordering of Portland Stone from the quarries.
Santa Maria del Fiore Cathedral and Baptistry. Brunelleschi kept good accounts of the sourcing of the Pietra Serena sandstone used to build the dome. Decorative white marble was sourced from Carrara and Pisa, pink limestone came from Verona and other locations in Tuscany, and green serpentinite came from Prato. A closeup of the polychromy decoration on the Cathedral is shown at right.
Left photo: Sailko
Urban Geology Walk in the City of London
Monument commemorating the Great Fire of London of 1666 which started close to this site and raged across the City for the next three days. The main building material is Portland Stone, the stone chosen by Christopher Wren and his fellow architects to rebuild London in a monumental style.
Portland Stone has a long history of quarrying, but only came into mainstream use following the Great Fire. It is quarried from the Isle of Portland on the Dorset Coast from the uppermost strata of the Jurassic succession. There are a number of varieties of Portland Stone but this is the most frequently used, called Whitbed. It is a pale grey limestone with scattered fossils, mainly the oyster shell species Liostrea.
Fossils in the pale-brown stone walls of Plantation Place. This is an Upper Jurassic limestone sourced from the Southern Frankonian Alb of Bavaria. It comes from the Treuchtlingen Formation, representing a marine platform limestone with sporadic sponge reefs and bioherms (reef knoll comprising a pile of calcareous material that had previously accumulated on an ancient sea floor). Top left: nautiloid; top right: belemnite; bottom left: ammonite and belemnite; bottom right: nautiloid.
Minster Court, with its pinnacles and spires, it has been described as ‘postmodern gothic.’ It was completed in 1991 when exotic granites were at the height of fashion in architecture. The building is clad with Brazilian Rosa Torcicoda (right), a pink granite migmatite. This is a rock that has been frozen in the process of melting at depth and this, along with deformation, accounts for its streaky texture. It has a granitic composition and is composed of pink potassic feldspar, white plagioclase and grey quartz. The black mineral is the mica, biotite. This is a very old rock, quarried from the 2.75 billion year old Campo Belo Metamorphic Complex located in the São Francisco Craton of the state of Minas Gerais, Brazil. Although metamorphosed at 2.75 Ga, the protoliths of these rocks are over 3 billion years old.
The igneous plutonic rocks at 24 Great Tower Street.
Left: Dark Shap Granite 9 (field of view 12 cm), with its distinctive, brick-shaped, pink, K-feldspar phenocrysts is from Shap Fell in Cumbria and is just under 400 million years old. The groundmass is made up of orange K-feldspar, grey quartz, plagioclase and biotite.
Center: Bon Accord Red (field of view 12 cm ), quarried near Uthammar on the Kalmar Coast of Sweden, is composed of large crystals of brick-red K-feldspar with grey quartz and black biotite. This is a variety of rapakivi granite, intruded during the same magmatic phase as the Baltic Brown Wiborgite seen at our first location. This red granite lacks the ovoids and is the variety of rapakivi granite called pyterlite. It was intruded as part of a suite of granitoids in the Baltic region, known generically as the ‘Coastal Reds’ by stone workers. This variety comes from the Figeholm Granite, intruded at ~ 1.4 Ga. The name ‘Bon Accord’ is an allusion to the fact that these granites were shipped direct from the quarry to Aberdeen where they were cut, shaped and polished. ‘Bon Accord’ is Aberdeen’s motto.
Right: Labrador Antique (field of view 8cm) anorthsite composed of 90% calcium-rich plagioclase with crystals that flash shades of peacock blues. This is phenomenon, called ‘schillerescence,’ can appear in many plagioclase feldspars, but is well-developed here. Texturally the stone here most resembles a variety from Norway which is (confusingly) called Labrador Antique. It is 930 million years old and is quarried from the Rogaland Igneous Province at Sirevag near Stavanger. In addition to the schillerescence, a distinctive texture is the bimodal grain size with larger grains up to a centimeter or more across
Malta Memorial commemorating the siege of Malta during the Second World War between 1940 and 1942. It is constructed from stone brought from the Oligocene Lower Coralline Limestone of the Isle of Gozo in the Maltese archipelago and is packed with fossil sand dollars, flattened sea urchins of the species Scutella subrotunda.
In the podcast, Siddall mentions several different facies of Portland Stone. This less common reefal facies seen at Caxton House in Westminster is hard to view in the field but well displayed here. It consists of carbonate rocks that are formed from the skeletons of reef-building organisms and lime mud.
A garnet porphyroblast showing top-to-the-right sense of shear is visible at the Hilton Hotel in Park Lane.
The Tower of London along with some churches are the only buildings that survived the Great Fire of London in 1666. The stone used in medieval construction was mainly Kentish Ragstone, a Cretaceous hard grey limestone from Kent. When weathered, it has a characteristic rubbly texture.