Humberside Geologist no.11

Geology in the City

by Mike Horne

Traditionally the study of geology has concentrated on examining hand specimens in the laboratory or visiting quarries and cliff exposures. These days the large numbers of people visiting some of our classic sites poses a major conservation problem. Also it is ironic that we geologists think that geology is in quarries, it is as if once the quarried products leave the site they are no longer geological! Yet the quarried products abound in our towns and cities and are mostly ignored by geologists. Is this because we don't think it is proper fieldwork to study city geology or are we just embarrassed to be seen gazing at building stones when surrounded by Saturday shoppers?

With increased pressure on classic sites, the introduction of the National Curriculum and the general decline in the number of quarries due to their value for landfill, more and more people are starting to take notice of the rocks used in our cities. In fact city geology walks are ideal for introducing beginners to the subject and require less organisation than traditional 'fieldwork'.

We can find lots of 'exposures' of rock in the city. Try looking at civic buildings and churches, cemeteries and churchyards, statues, shop fronts, roads (especially the old cobbled variety) and pavements, roofs, garden walls and even rockery gardens!

We can find all the major rock types in our towns and cities.

Igneous rocks - basalts, diorites, gabbros, and several varieties of granite. Igneous rocks are used for gravestones, building stone, facing stones for shop fronts, statues, kerbstones and cobblestones.

Sedimentary rocks - limestones, sandstones and 'marbles' (pretty limestones). Limestones are used for building stone (especially good quality 'freestones'), cottage and garden walls (usually lower quality, local stones), windowsills and steps on brick buildings, and gravestones (Portland Stone is often used for military graves). Some fossiliferous limestones and tufas are erroneously called marbles and are used as decorative stones inside churches and civic buildings or used for facing shop fronts. Sandstones are used as building stones, gravestones and kerbstones, thinly bedded flagstones are used as paving stones and, in some areas, as roofing stones.

Volcanoclastics - tuffs. Tuffs are sometimes used for facing buildings.

Metamorphic rocks - slate, true marbles and serpentinite. Slates are used for roofing, but were also used at one time for gravestones and damp proof layers in buildings. True marbles are used for statues, decoration in churches and civic buildings, gravestones and facing shop fronts. Green serpentinite makes an attractive facing stone.

Man made metamorphic rocks - bricks, tiles, glass, ceramics and metals. Man made materials are the most common building materials these days. Bricks and roofing tiles are now standardised but were once hand made, often from local clays and silts. Ceramic tiles and castings were popular at one time. Don't forget that glass is made from sand and that metals are refined from naturally occurring ores.

We use these rocks to study and teach geology in much the same way as we would if they were still in the quarry. We can study the mineral composition and grain size of the rocks. We may find xenoliths in the igneous rocks and we can study their crystallisation history. In sedimentary and volcanoclastic rocks we may find bedding structures such as graded bedding, cross bedding and slumping, and use these to determine if the rock is the right 'way up'. Some limestones and 'marbles' contain fossils, so we can study them and their assemblages. We may find small scale folding and faulting, or intrusive veins in metamorphic rocks. And in man made rocks, we can study the processes used in their manufacture or investigate the origin of the clay in old bricks which may contain unaltered grains or even fossils.

We can also study other related topics such as weathering and pollution. Gravestones and some buildings have a date on them telling us when they were erected, we can use this information to study the weathering of the rock over a known time span. We can study gravestones to see which type of rock gives best value for money and which are most susceptible to weathering. We can also study the effect of acid rain on carbonate rocks: white Cararra Marble gravestones usually have lead lettering which is flush with the stone when new, but stands proud as the rock is dissolved. We can compare sites to see if weathering and pollution are worse in cities or in the countryside.

We can use building materials in historical studies. When did a certain rock first appear in a city and why? The introduction of new building stones are a good indicator of the opening of new trade routes. They also give information of the economic status of an area. We can study social trends in architectural styles and cemeteries can tell us a lot about social attitudes to bereavement.

And we can also involve ourselves in conservation matters. We can encourage the owners of buildings to look after them and where necessary make appropriate repairs. Shop fronts and facing stones look nicer if they are cleaned regularly, just like the windows. Bad repairs using the wrong materials can speed up the deterioration of building stones, as well as making eyesores. Perhaps one day a local shop front may be listed as an SSSI!

We do not need any special equipment to carry out urban fieldwork, just: waterproof clothes or a brollie, a notebook and a pen or pencil (a large clear plastic bag is useful so that you can write inside it if it is raining). A magnifying glass or hand lens and a grain size scale or ruler would be useful.

Hammers, chisels, trowels etc. are not necessary, and are really not encouraged! Other possible items you might find useful are an acid bottle (but please use it with discretion) and a compass (to help determine if weathering is worse on a particular side of buildings).

Published in Humberside Geologist no. 11, 1995.

Copyright Hull Geological Society.