The character of Therfield Heath is dictated to a large degree by the properties of the free-draining chalk rock underneath it.
Chalk is formed under warm, shallow seas, so around 65-95 million years ago, when this sediment was deposited, this area was under water. Sea level was much higher then (in the Cretaceous) than it is now.
In the late stages of the Alpine Orogeny (mountain building event when the Alps were formed), the rocks of SE England were folded around the Weald, leading to tilted and uplifted ridges of chalk – the North and South Downs and the Chilterns, for example.
Over the last million years, there have been a series of ice ages. The Anglian ice age reached as far south as Essex. As the ice sheets retreated, the melt water eroded valleys in the chalk that are now dry. These dry valleys are commonly known as coombes. If you are interested in what the area might have been like during the ice ages, the Sedgwick Museum has produced an interesting blog.