Recording a built environment

A significant modern form of geological 'deposition' that does stand some chance of being preserved in the future is the result of our human habit, at least since around 8000 BP in Mesopotamia, of constructing very large structures out of reasonably durable materials. We call them towns and cities. The fact that archaeological remains of early permanent human settlements in regions such as Mesopotamia, made out of brick and stone, have lasted for such a time suggests that our much more massive modern structures are potentially of some geological significance. The Great Barrier Reef off the northeastern coast of Australia is normally quoted as the largest rock structure created by living organisms. But major cities such as London and New York are very big, with areas of around 2000 sq km (600 sq miles). Like the Barrier Reef, they can be seen from space and have similar preservational potential.

All modern cities can be seen as object lessons in the continuing economic need for rock materials and the importance of understanding geological processes that have generated these natural resources. These days, most of us, and city dwellers in particular, are far removed from any direct dependence on the natural world. Unlike our early ancestors, we do not need to know what stone makes a good hand axe, spear point, spark or even a mill stone or roofing material. However, there are plenty of people in the so-called third world who still are very close to such raw materials and have a good knowledge of rocks and minerals that are useful or valuable. Today rural markets in China have stalls selling lumps of minerals such as sulphur, rock salt, cinnabar (a mercury sulphide) and haematite (iron oxide), which have been used in medicine for millennia.

Just look around a city and reflect for a moment on its material basis. Virtually the entire external fabric and infrastructure is made of concrete, steel, glass, stone and perhaps brick - all derived from rock. And yet, very little of this huge mass is derived locally, most has been imported for construction over the decades and centuries, sometimes from considerable distance. Old cities like London grew in a fairly organic way and were founded on small settlements that were originally built using local materials. In London these were essentially organic - wood and rushes plus mud, as there is no local rock that can be used as a building stone.

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