Sea clay

One of the most common sceneries on the Dutch coast is the sea clay. It is a rather young and dynamic scenery, which continues to change shape today.

The creation of the sea clay scenery

Nederlands landschap
About 6,000 years before Christ, the Atlantic period begun. In this period, the sea greatly influenced the Dutch landscape. Because of global warming, which had started during the beginning of the Holocene (about 8,000 years before Christ), the sea level and the groundwater raised. Together with the drop of the seabed, the coast line moved about five kilometres east each century. Eventually, the coast line reached the height of the present Dutch coast.

Because of the rising sea level, the ground water level rised also. The seabed became saturated and peat was deposited on the Pleistocene layer. During each high tide, small layers of mud remained behind with the peat. A thick layer of mud was soon created, as a result of which small sand dunes developed. These sand dunes formed seaside shores - long, narrow sand ridges which ran parallel to the coast line.

The sea level continued to rise, and the seaside shores were not able to stop the water. The sea would regularly flood the land behind the shores. Gradually, a shallow sea was created, seperated from the main sea only by the long ridges of sand.

During high tide the sea water would flow in via large mud canals. The sea water brought in new sand and seaclay. During ebb tide the water would gradually flow back to the open sea. Because of this, the water level dropped and the force of the current decreased. The sand would deposit and form sand bars (muds or mud flats), which would be uncovered during ebb tide.

A little further inland, the clay is deposited. The mud flats, which are higher at the coastline, provide an excellent environment for salt-loving plants. These plants restrained the flow of the water. The small canals between the plants become increasingly deeper and eventually formed creeks. During flood tides the water in the creeks would rise and flood the salt marshes. On the shores of the creek, sandy clay is deposited, and a little further in, very heavy clay settled. Because of this, the salt marshes became higher and higher.

At one point, the salt marshes were so high, they would only flood during extra high flood levels. Moreover, about 5,000 years ago, the rate of the sea level rise actually decreased. Because the amount of water flowing from the creeks was reduced, sand was left behind in the bedding of the creek. Gradually, the creeks silted up, originating in creek ridges. Creek ridges lie a little higher than the clay. This is because clay settles further than sand during dehydration and reclamation.

The sea started rising more slowly and at one point, the sea even started to retreat. This is called regression. During this period of regression, the seaside shores became larger and peat formed high up on the salt marshes. 500 years before Christ, the sea began to rise again (transgression). This was because of a very strong westerly wind, which created storm surges. The sea could break through seaside shores and once again flooded the mud canals. Erosion occured from the sea, but new sea clay was quickly deposited in return. Since sea breakthroughs primarily took place in Southwest and North Holland, the new sea clay was primarily deposited there.