The landscape in Thy is the result of movements in the subsurface, glacial movements during the last Ice Age and the subsequent isostatic uplift.
The western part of Thy was a cluster of islands until 4,000 years ago. Since then the land has risen and in many places the coast has moved some kilometres to the west. The coastline of the Stone Age is now visible as distinctive inland slopes, for example along the eastern side of Hanstholm Nature Reserve as well as at Nors Lake and Vandet Lake.
Here it is evident that the subsurface of Thy consists of white chalk and Danian chalk.
The isostatic uplift brought up huge amounts of sand, and this formed the basis for the sand drift which strongly marks the present landscape.
Both prehistoric and historic times have seen periods with sand drift, interrupted by long stable periods. Studies show that strong sand drift has taken place in periods with a colder climate. The most recently known sand-drift periods took place around year 400 B.C. as well as between years 400 and 600 A.D. and from 1450 to 1750.
The dune landscapes we see in Thy today are a result of all these sand drift periods.