As some living things get older they grow upwards and outwards in layers. A different layer is added every summer and winter and in some things you can see these very easily. By counting these layers we can work out the ages of trees, shells and even glaciers.

The climate affects the size and appearance of each layer. So each season’s layer is different from the one before. This means that groups of layers can never be repeated. Individual creatures or sites that overlap in time will partly display the same pattern. This overlap allows scientists to build up a very long sequence of layers. Comparing a single pattern from a wooden object, a shell found in an excavation or an ice-core with this long sequence can give an accurate date when they were growing.