Imagine that you’re filling in a large hole with domestic rubbish. The year is 1895, so you throw away much less than we do today. Indeed, it takes several weeks to gather enough bottles, tins and broken crockery to fill the bucket standing by the kitchen door. Once it is full, you lift it by the handle and carry it over to the pit; you do this a few times over the following months, and the bottom of the hole begins to fill. Then you decide to put some clay over the rubbish and stamp it down, partly to deter vermin. The year is now 1896. Since the pit is only about a quarter full, you continue dumping fresh rubbish on top of the clay. This time, you also dump ash from the grate, because the ash pit, which was over by the hedge, is now full. After a few bucket loads, you cap this second layer with soil. It is now 1897, and you decide to build a greenhouse and demolish the old one. You salvage any useful building materials but dump all the rubble and broken window glass in the hole, filling it almost to the brim. A couple more bucket loads of domestic rubbish fill it completely. Then you lay some clay and soil over it and plant a pear tree on top.
A hundred years later, the pear tree is gone. (Sadly, so are you.) An archaeologist decides to excavate where it stood. First she discovers a thin spread of rubbish on top of a layer of rubble. In it is a penny dated 1897, which had slipped out of your pocket.She labels this layer ‘Context 1’. Digging through the soil below it, she finds another layer sealed beneath and therefore obviously earlier than Context 1. This deeper layer contains domestic rubbish mixed with ash. She labels it ‘Context 2’. At the bottom of this context she finds more clay, but she realises that it isn’t simply the natural subsoil (which is London Clay laid down in the Lower Eocene Epoch) because it’s crumbly and contains tiny bits of pottery and brick. Beneath this she finds the layer of rubbish you dumped in 1895, which she labels ‘Context 3’. In her report, she concludes that the pit was sealed no earlier than 1897.
A context, in short, is a sealed deposit that can be dated by the latest datable object it contains. It can just as well be a layer of soil that has built up over a flat surface, or a layer of demolition rubble lying over the floor of a vanished building.