 Most of the bedrock of the Vale of Aylesbury comprises clay and chalk that formed at the end of the ice age and at one time was under the Proto-Thames.At this time the vast underground reserves of water formed that make the water table higher than average in the Vale of Aylesbury.The last advance of the Scandinavian ice-sheet to have reached this far south covered much of north-west Middlesex and finally forced the Proto-Thames to take roughly its present course.