Geology and Genesis

All sediments in the coastal plain and peri-marine area of the Netherlands form the Westland Formation. Within this Formation four depositional environments can be distinguished:

• coastal barrier with beach and dune sediments
• coastal plain with lagoons and tidal marshes
• peri-marine area with river deposits and peat, deposited under the influence of sea level rise
• Almere Lagoon, situated at the current IJsselmeer- area

The Amsterdam Water Supply Dunes are part of the Younger Dune sediments of the coastal barrier. Before 5000 BP sea level rose relatively fast and coastal erosion took place and the coastal barrier moved land inwards. Between 5000 BP and 2500 BP a coastal barrier formed along the coast of western Netherlands, in a period with a relatively low sea level rise. A vast and broad series of sandbanks lying parallel to the coast originated, only broken by beach plains and river mouths. The sandbanks were partly covered with small dunes (Old Dunes). After 5000 BP there was a large sand supply towards the coast and therefore the coastal barrier was not eroded. Instead, the coastal barrier moved westwards. The coastline was located west of the current coastline. Because the coastal barrier was largely closed, sand loss to the inland tidal systems stopped and in front of the tidal outlets large outer deltas originated. When the tidal outlets silted up with sand the growth of the deltas ceased and a large amount of sand became available again for coastal progradation.

Between 1000 AD and 1600 AD a new period of dune formation took place, the formation of the Younger Dunes. Part of the older coastal barrier was eroded and a more steep coastal profile had developed. The formation of the Younger Dunes started with a mobile sand phase in which mobile dune complexes formed. These dunes are located on top of the coastal barrier and Old Dunes. When more vegetation started to grow, parabolic dune forms originated. About 1600 the formation of the Young Dunes was completed. The Amsterdam Water Supply Dunes are part of these Younger Dunes, composed of sand with fragments of shell.

There are several theories about why the period of strong coastal erosion started in the Middle Ages. Possible explanations are the effect of fluctuations in sea level, deforestation, higher storm frequency and/or erosion through a long shore current. The erosion steepened the undersea slope and a large supply of sand to the beach allowed the Younger Dunes to become very high.
 

References:
 

• Berendsen H.J.A. (1996) De Vorming van het land, inleiding in de geologie en de geomorfologie; Fysische geografie van Nederland; Van Gorcum & Comp, Assen.
• EUCC (Doody P. et al) (1991) Sand dune inventory of Europe. (BCD1991a#)