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.

A woman with glasses, wearing a colorful scarf, a blue patterned coat, and brown boots, is walking on a sidewalk in front of a building with a black door and a small tree. She is holding a device in her left hand and appears to be engaged in a conversation.

Podcast Illustrations

Images courtesy of Ruth Siddall unless otherwise noted.

Urban Geology Walk in the City of London

A city scene with modern glass and stone buildings, a tall historic column monument, and pedestrians walking on the street during overcast weather in 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 (top right), 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.

Close-up of a marble wall with a fossil.
A close-up of a light-colored, speckled, textured surface with a small, dark, sharp object embedded in it. Fossil in a London building.

Santa Maria del Fiore Cathedral and Baptistry. Brunelleschi kept good records of the sourcing of the stone.

Photo: Sailko

Close-up of weathered cement blocks with visible cracks and moss.
Close-up of a syringe on a white speckled surface.
Close-up of a fossilized fish embedded in a sandy sedimentary rock.

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.

A modern multi-story building with a unique architectural design featuring a mix of glass windows and stone facades. The building has rounded arch structures on the lower level and is situated on a city street with parked cars, street signs, and surrounding buildings.
Ancient petroglyph of a humanoid figure etched into a pink and gray granite rock surface.

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, 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.

View of St. Paul’s Cathedral in London, with its prominent dome and spire, against a partly cloudy sky, surrounded by trees and urban surroundings.

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.

View of Florence Cathedral with its distinctive marble facade, dome, and bell tower against a clear blue sky, with a few people walking nearby.
Close-up of a textured beige and cream surface with visible cracks and dark lines, including a small, faint heart shape drawn within the cracks.

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.

A large stone monument with a plaque and a symbol mounted on it, situated on a paved sidewalk near a busy city street with cars, pedestrians, and buildings in the background. The plaque contains a map and text about the history of the Mediterranean Fleet.