Stray Boulders in Wieringen

(text based on the article "KORALEN VAN HET WAD" ("CORALS FROM THE MUDFLATS") - "Een tropische verrassing in Hollandse bodem" ("A tropical surprise in Dutch ground") in Mens & Wetenschap ("People and Science") 5, 1997, by Bert de Vries, curator at "Het Huis van de Aarde")

Stray boulders, in northern North Holland - they're everywhere. In fields, gardens, on farm paths, and even on the Wadden coast of Wieringen. There are even vast fields of them on the bottom of the Wadden Sea and the neighbouring North Sea.
In Wieringen, stray boulders were formerly used in many ways. In fishing, for example, they were used as net weights. A number of grooves were cut into the stone, through which ropes could be run to tie the stone to nets. Larger boulders were also used as hinge stones for large farm doors, or as landmarks, to mark the edges of property. These border stones were called 'dolven' in Wieringen.
Even the folklore of Wieringen is influenced by these boulders. It is said that the large Standing Stone of Westerklief turns around when the clock strikes, and when the dog barks.

"En menigeen vond zijn lief - bij de steen van Westerklief... "
"Many a man found his sweet, by the stone of Westerklief"

Second Last Ice Age

The boulders were transported from Scandinavia to the Netherlands by glaciers during the second last ice age, (called the "Saalian") which occurred about 150 thousand years ago. During the peak of this ice age, there was a layer of ice almost 3 kilometers thick lying over northwestern Europe. The pressure of this enormous mass of ice gave it fluid characteristics, and it began to flow. The ice began moving in a southwestern direction, through the Baltic Sea basin, scouring the bottom as it went. On the underside of the glaciers "boulder clay" collected. This was a mixture of clay, sand, gravel and coarser stone material, which can now be found as compact deposits in much of the northern Netherlands. During an early phase of the glaciation of the Netherlands, the border of the Scandinavian glaciers lay in a belt that reached from Texel and stretched over Wieringen, Gaasterland, Steenwijk and Hoogeveen, right up to Coevorden. The ice periodically melted somewhat, but froze right up again, pushing the underlying sediment along like a bulldozer, including the deposited boulder clay. Along this belt, "push moraines" developed, which are still visible today in the dutch landscape, including Wieringen. Later, these moraines were covered by ice, and the leading edges of the glaciers ended up in the central Netherlands, where they formed additional moraines. These latter moraines did not consist of pushed boulder clay, but of sandy sediment from the deltas of nearby large rivers. The moraine of the Utrecht Ridge, for example, probably arose due to the movement of the ice, but it does not consist of deposits carried here in the glaciers themselves.

Boulder fossils

The stray boulders, brought here by the glaciers, partly consist of igneous rock, such as granite. Igneous rock develops from hardened magma. Some of the rock is sedimentary. Sedimentary rock develops from pressurized clay, sand, gravel, or a mixture of these. The pressure of rocks lying on top causes the sediment to harden into rock. Fossils can appear in sedimentary rock. In areas where lime boulder clay appears, we can find limestone containing the remains of long dead sea organisms.
Here and there, the Wadden coast of Wieringen has lime boulder clay, with limestone rocks. The sea rinses the clay and sand out of the boulder clay, leaving the coarser material behind. Most limestone rocks originated in the Baltic Sea region. During the Ordovician and Silurian geologic periods, about 510 to 420 million years ago, there was a shallow sea at the height of the current Baltic and neighbouring republics. All kinds of tropical organisms lived there, of which the corals were the most striking representatives.

Coral

The coral reefs of Gotland

Corals are cavity dwellers. They require temperatures of at least 18 C, and sunlight (which can only penetrate about 50 meters into the sea). Corals typically build reefs - a collection of hard "houses" made from calcium. On the island of Gotland, in the Baltic Sea, some cliffs consist entirely of beautiful, fossilized coral reefs, originating from the periods discussed above. In the stray boulders of Wieringen, we can also find fossils of different corals, such as Halysites ('chain coral'), Heliosites ('sun coral') and Syringopora ('organpipe coral'). All three corals date to the Silurian and come from the Baltic Sea area. Another fossil life form that is frequently found in this area is Favosites ('honeycomb coral'). It is not truly a coral, but rather a sponge (Porifera). Favosites also built reefs during the Silurian and Devonian periods. Besides coral and sponges, stromatoporen (now extinct) were also reef builders. Fossils of Favosites and various sorts of stromatoporen are frequently found in the stray boulders on the Wieringer mudflats.

Left: Favosites, right: Heliosites

Continental Drift

The fossils on the Dutch mudflats are related to the fossils in Scandinavia. It is an intriguing story that has another chapter, for we must ask ourselves: how is it possible that fossils of tropical orgnisms can be found in the Baltic Sea region? This question has only been answered in recent decades, and the answer has to do with the movement of the earth's crust.
The distribution of land and sea hasn't always been the same. Hundreds of thousands of years ago, there was just one large continent, named Pangaea, of which our current northwestern Europe was part. For a long time, it was situated in the tropical zone. Later, volcanic activity caused this protocontinent to be broken up into various pieces, called plates, which slowly dispersed over the globe. These plates are still moving in relation to each other. The movement of the continents over time explains why tropical fossils can now be found in the Baltic Sea regions. During the Silurian, the "Baltic Shield" - the part of the earth's crust that now makes up Scandinavia and Finland - was located near the equator, and was, therefore, exposed to ideal circumstances for coral growth. Over the 400 million years that followed, the Baltic Shield moved slowly, in a northerly direction. During the second last ice age, 150 thousand years ago, the Shield (which now lay in the northern hemisphere) was covered by gigantic glaciers, which also left their mark on the northern Netherlands. The tropical fossils found in the stray boulders are small windows into the distant past, an impressive record of Earth's history.

Last updated: 19-3-2002