The chalk hills, known as the Chiltern Hills, which run diagonally across the north of Hertfordshire from Tring in the west to Royston in the east, have two important gaps in them, one at Tring the other to the north west of Stevenage, known as, the “Stevenage–Hitchin gap”. The Stevenage-Hitchin gap was created at the end of the last Ice Age when a glacial overflow channel now buried, ran from Biggleswade to cut through the chalk and continue towards Hertford. These gaps are important for migrating birds, particularly sea birds and waders. Gulls for example, that pass through the County use these gaps as they travel to and from the Thames Valley and the Wash. 

The gap at Tring is of ornithological importance because of the four reservoirs constructed there in the nineteenth century to supply water for the Grand Union Canal. These reservoirs attract a variety of waterfowl and waders. It is interesting to compare, although to a much lesser degree than Tring, how the construction of Fairlands Valley Lakes has at times had a similar effect.