Urban Rock Hunting with Dr Brooke Johnson

The story of the Earth is written in the rocks, and as a geologist, I read those rocks to tell that story. This often leads me to remote and exotic places. Like the heart of desolate Australian deserts or to the tops of mountains in the Canadian Rockies. But that story is all around us, too, in the towns and cities we live in. Humans have been using natural rock as building stone for thousands of years, the Great Pyramids of Giza, for example, are made from 50-million-year-old limestone littered with shells from the ancient seas.  

Despite the advent of modern building materials like concrete, many of the buildings in the centre of our towns and cities are still made from natural stone. Either because, like the pyramids, these buildings are old, and the natural stone has stood the test of time. Or because the architects wanted to enhance their buildings with the beautiful textures and colours only found in natural rock. Either way, it means there are whole chapters of the Earths deep history sat on our doorsteps, waiting to be read. In this blog and the linked video, I will give you a few basic tools, and show you examples, to help you start deciphering the ancient story of the rocks. You don’t need any special equipment for urban geology, but if you want a notebook and pencils/pens would be good for drawing sketches and making notes. A magnifying glass or hand lens can also be useful but are not essential.  

There are only really two types of rock; granular rocks made from sediments grains like sand and mud, and crystalline rocks made from crystals that grew from a cooling liquid rock, or due to the effects of heat and pressure.   

In granular rocks, grains can be made of little bits of rock or minerals eroded from pre-existing rock. The grains can also be fossils – the preserved remains of living things. Grains can also grow from water on the sea bed – this is common in modern tropical environments where the warmth of the sun causes white calcium carbonate to grow from the sea water and helps to produce the soft white sand we associate with tropical beaches.  

The size of the grains tells us how much energy the ancient environment had; if the rock has broken shells and large (bigger than 2 mm) grains, then it was probably a high energy environment like a beach. Imagine all those waves breaking onto the sand and mixing up all the grains. If a rock has small grains, perhaps too small to see, and the fossils are mostly intact, then this was a low energy environment like the deep sea or a lake. Imagine all those tiny flakes of mud settling slowly on the seabed, never disturbed by the waves far above. If you have a granular rock that combines large grains surrounded by a mass of small grains, this usually indicates a special event. Examples of this include things like sudden floods, tsunamis, volcanic eruptions or the actions of glaciers and icebergs.  

The same is true for crystalline rocks, large grains indicate high energy levels – though in this case the energy is heat from deep within the Earth. If a crystalline rock has large chunky crystals, this means that the rock was exposed to heat and pressure deep in the Earth giving the crystals plenty of time to grow. Small crystals show the rock formed closer to the surface and the crystals had less time to grow. Finally, if the crystalline rock formed on the surface, such as lava pouring from a volcanic vent into cool water, then the crystals will be too small to see. Such rocks might not form any crystals at all and instead cool so quickly they form natural glass like obsidian.  

The building stones where you live might all be the same type, or many different types. You will need to go look at them and decide for yourself if they are granular or crystalline, and if they formed in high or low energy environments. It takes a bit of practice to get the hang of reading rocks so don’t give up, and there are lots of resources online and in your local library to help you out. Who knows what might find hidden in plain sight when you start learning to read the rocks?  

Check out this video which will get you looking at the building stones where you live – together the blog and video give common examples of things to look for and what they mean!


Want to find out more?

https://www.rockwatch.org.uk/ – Young Geologist club of the Geology Society

https://www.virtualmicroscope.org/ – Open university virtual microscope