The above link gives info on the Chilterns. Formed 65 million years ago. Heres an extract...The chalk forms a major aquifer (water bearing rock sufficient to provide a water supply to wells).
In London it has been possible in the past to have artesian wells, because the surface of the land
was lower than the water table in the hills to the North and South. This caused the water pressure
to force water up the well to try to restore the water level to that in the chalk. (This means that it is
particularly important not to pollute water in Chilterns since the chalk in this area is a point of input
to the aquifer underlying the whole Thames basin.) In Victorian times, the water table fell because
of the increase in industrial activity until there was no artesian effect. The reduction in industry
over the last 30 years has caused the water table to rise by about 45’. This is having an impact on
the deeper sections of the London Underground. It also partly explains the increase in winter
flooding around Chesham and other towns in the Thames basin. ... See MoreSee Less