This method is not as confusing as it might at first sound. Spheroidal Carbonaceous Particles, or SCPs, are simply ash particles that are round and made of carbon! They are produced by the incomplete burning of fossil fuels (oil or coal). Because the widespread burning of fossil fuels only began with the industrial revolution, SCPs are used to date the uppermost layers of bogs that have grown up over about the past 150 years.
By counting the number of SCPs in consecutive layers of peat, down to a depth of around 20 – 30 cm, we can develop a historical record of regional industrial activity. For example, SCPs might first appear in a bog around 25 cm. Then at around 10 cm there is a rapid expansion in their number, as industry really boomed. Finally, numbers reduce again from 10 cm to the bog surface. We can then combine our objective counts of SCPs with historical records of industry in the area surrounding the bog we are working on. For example, history may tell us that industry in an area began in the 1850s, rapidly expanded in the 1960s and peaked in the 1970s – so we apply these ages to the depths we found these features in our counts. The recent drop off in numbers is the results of legislation designed to clean up our air and reduce the numbers of particles such as SCPs floating around for people to breath in.