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William Smith's 1815 geological map of England & Wales. Source: Wikimedia commons
Geological maps usually consist of an Ordnance Survey base map 'coloured in' according to the underlying geology. By consulting a geological map you can determine the age and type of rocks in a certain location. This information can be very helpful if you're trying to identify a fossil or just work out where to look in the first place!
The process of mapping involves locating and identifying rocks where they are exposed at the surface, then – based on their angle of burial and degree of deformation – interpolating what path each unit takes as it passes buried under farmland or cities. Each age or type of rock is assigned a colour, and a map shaded in according to the underlying geology.
A geological map will come with a key to the colour scheme particular to that map. There are some general rules: for instance, rocks deposited during the Cretaceous period tend to be coloured green, whereas igneous (volcanic) rocks tend to be a bold, heavy colour such as red or deep blue.
Explore Britain's geology from your computer (for free!) with OpenGeoScience
Once you're equipped with a map, it's simple enough to use the key to work out where you can find rocks of a specific age, or what rocks are in your back yard! It might also illuminate features in the landscape: I was excited to discover that the Lincoln Edge, a prominent ridge near my home town, coincided with the outcrop of hard Jurassic rock; and on my holidays to Scotland I like to trace out gullies that follow the paths of fault lines or volcanic dykes.
Geological mapping was pioneered by William Smith, a 19th century canal digger who singlehandedly mapped the geology of England. (An unfathomable task! You can read this fascinating story in 'The Map that Changed the World', listed among our book recommendations.) Higher resolution mapping has since been carried out by the British Geological Survey who today produce high-resolution maps of the entire British Isles. You can view this mapping data, for free, as a Google Earth overlay via the OpenGeoScience service (follow the 'Maps' link).
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