Ely is built on an clay which is one of the highest points in the Fens. This is very different to the West of Cambridgeshire which is made up of limestone. The river Great Ouse runs through the city, and is one of the longest rivers in the UK with many others flowing into it. The drainage area of the rivers that form the Great Ouse is around 6000 square miles (16,000 square kilometres). This is five times larger than the Fens itself The landscape was a large marsh until the 18th century, when much of the Fens was drained, making Ely no longer an island. Because of this, the ground around the city is very fertile and most of the region is used for farming. The marshes also had many different animals in them such as the Eel, which the city is named after. 