Have you ever heard the term urban geology? If you are an engineer you probably have… or if you’re a geology student or geek you may have invented the term and thought ooh that’s awesome… to everyone else it may conjure one of two images:
- either geologists lying on the floor of the local shopping centre… wondering at the fossiliferous rock used – most likely for aesthetics – to tile the floor. (Prof. John Marshall i’m looking at you!)
- or the image of a load of hip-hop merchants rapping about geology.
Thankfully it’s more likely to be the former (though also I think the latter could be fun) and that’s what i’m blogging about tonight, so if you came here looking for tensile strengths of different lithotypes, you’re in the wrong place and we geologists aren’t the sort for such exacting standards… apparantly.
I began thinking about this on friday when I was out filming a mini-documentary for one of my friends’ film and media modules on his university course. For reasons i’m still not wholly aware of he decided to use me for the documentary and cover some geological topics, including me taking him on a tour around southampton showing him some fossiliferous building stone, of which two got filmed.
I stuck to fossils mainly because that’s the area of geology with which I am most comfortable and familiar, but on the way between sites one could observe some fantastic – almost perfect – phenocrysts of plagioclase in the pavement curbing on the high streets. Furthermore on the floor of the WestQuay shopping centre one could see what I think are schistose pavings on the floor of the above bar street entrance hall*… I’m told there are yet more fossils somewhere inside the shopping centre, but I didn’t have the time to look.
Also found amongst the building stones of southampton was a useful juxtaposition of oolitic limestone and a dense biomicrite. These were filmed as part of the mini-documentary with me trying to give a very poor geological appraisal of their origin and environment of deposition.
The oolitic limestone (upper, yellow-grey stone in image on left) is made up almost entirely of tiny “balls” of calcium carbonate (CaCO3), or more rarely silica (SiO2) which have been precipitated around a “seed” which can be anything from a quartz grain, to a bacterium or a shell fragment.
These ooids form in very shallow, high energy shelf seas supersaturated in the forming mineral, such as tropical coastlines seen around the world today in areas like the bahamas and some parts of Turkey.
The dense biomicrite (lower, grey-white stone in image above) is made up predominantly from shell fragments – mostly from small bivalves and brachiopods, with one or two larger oysters present – and is cemented together by lime mud (or micrite) in between the shell fragments. This rock happenned to be less coherent than the oolitic limestone which was evident from the greater weathering of the micrite from the rock, leaving many shell fragments proud of the modern surface. Depositionally this rock would have been deposited on a shallow carbonate platform, probably in the lagoon of an island atoll, of which many modern examples can be found in the pacific and indian oceans.
I will not bore you all with more information but will encourage you to go and have a look around your local village, town or city for any hidden geological gems… I know of some excellent solitary corals in Lyme Regis and red-bed sandstone here in southampton… go see what you can find! All it takes is a magnifying glass and an eye for interesting patterns… even if you don’t know what you’re seeing, take a photo and send it to a local museum or a geological blogger and they’ll try and identify your finds.
Honestly… it’s great fun!!
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*EDIT: From Dave Gardiner:
The West Quay floor is a rare type of rock that is linked between fluid injection into young sediments associated with volcanics…..John Marshall made us have a look at it for our tutorial in 1st year. He also said that they are worth a ridiculous amount of money each. I think he said sourcing them somewhere in Brittany.