A bit of geology of Holland
During the last ice age most of the North Sea was dry.
When the water returned wind and waves formed a stretch of dunes
in the area of the Rhine delta. The water level gradually rose further
and the area behind the dunes became flooded.
The area filled with fresh water from the Rhine, and a swamp was formed.
Smaller rivers in this swamp carried excess water to the Rhine or other rivers.
Along these rivers the banks were clayish, firmer and somewhat higher
than the swamp itself. When the first human beings migrated into the area
they settled on the banks. They lived on fishing and hunting.
These people came from the dune area which was inhabited already long before.
The outer part of the dunes facing the sea was not attractive because of the
harsh conditions. The inner part was covered with trees; there was a large
forest from which Holland derives its name (Hout-land meaning Wood-land).
This still is a sought-after region where richer people have their houses
and where rich estates are found.
During extreme conditions the swamp was flooded from the sea;
in order to keep their feet dry the houses were built on mounds consisting
of ground and their own rubbish.
The photograph shows a farm on the river Alblas; it lies about 1 m above the
surrounding terrain.
The frequency of flooding varied due to changes of
climate which occurred on the time scale of many centuries.
In adverse climatic conditions people had to give up their settlements
on the river banks. Whether they returned to dryer land or perished is unknown.
In the 11th century and before the river banks had become inhabited again
and people had started farming on the swamp. When the flooding frequency
increased in the 12th century their living conditions worsened.
This time they were better organized and they took more drastic measures to
defend themselves. They started surrounding their living area with dikes.
Read more about this subject, e.g. in: