Cretaceous walk Sint-Pietersberg (Mount Saint peter)

Due to marl mining, the continuation of the former Luikerweg has disappeared. The viewing platform symbolises this vanished road in the present. From the platform you look out over the quarry and the lakes with their deep azure colour, caused by the reflection of the sky in the clear water. The deepest point of the quarry is 5 metres below NAP, while the Maas is 45 metres above NAP here.

The quarry was created by the marl extraction of the ENCI (First Dutch Cement Industry). Since 1926, lime-rich marl was mined for the cement industry in the heart of Sint-Pietersberg (Mount Saint peter). 900,000 tonnes of cement were produced here annually, and the ENCI plant's striking chimney still towers high above the landscape. In 2018, ENCI stopped extracting marl, after which Natuurmonumenten took control of the area in 2020. Thanks to this takeover, nature was once again given room to develop here, and the area was transformed into a nature reserve.

In the quarry you can see islands, these mark the sites of fossilised Mosasaur remains. Over the years, four have been found; Bèr, Carlo, Kristine and Lars, all named after those who found them. The rock layer in which these mosasaurs were found is called the Maastrichtian. The naming of this geological layer after the city of Maastricht was done in 1849 by Belgian geologist Dumont.

Marlstone formed at the end of the Cretaceous period, some 65 to 75 million years ago. At that time, the area was covered by a warm, shallow sea. The skeletons of many small animals that lived in this sea sank to the bottom and over time were buried under silt and sand. This eventually created the type of stone we now call marl.

Marl has a high chalk content and the white marl consists of almost 100% lime. The more orange pieces contain iron and other minerals in addition to chalk. You can still find plenty of fossils in the marl, such as shells, mussels, shark teeth, sea urchins and squid arrowheads.

Flint is the hardest type of stone found in the Netherlands. Along the footpath, you can clearly see different layers of flint. A fault line, where the layers no longer run straight, indicates an ancient landslide. The exact origin of flint is still not fully understood. It is likely that the Cretaceous Sea occasionally fell dry, leading to the growth of plants and trees. Crustaceans also dug corridors through the soil.

After the Cretaceous, the mountains in the Ardennes formed, causing the Upper Maas to start flowing faster. This led to more erosion and the carrying away of material such as gravel and silt. The acidic gravel slowly seeped through the marl layer and filled the cavities and passages left there. You can often recognise a root or branch structure in the flint layer.

You are now on the Zandhoek, the upper part of the dry valley, also called the Poppelmondedal. At the top, the soil is sparse, with mostly herbs and little grass. In the calcareous part below, (very) rare plants such as limetrip grow. You will also find orchids, groundsel, wild marjoram and stone thyme. These herbs attract various butterfly species, such as the Spanish Flag, Queen's swallowtail and cockchafer Aurelia.

From this western slope, you can once again enjoy stunning views over the Jekerdal. Across the road, to the left on the slope, is the Apostelhoeve, the Netherlands' first commercial vineyard (1970). The vineyards belong partly to the Apostelhoeve and partly to the Nekummerhoeve. There were vineyards here from Roman times until the 17th century. The limestone soil provides excellent conditions for growing grapes. Therefore, historically, many vineyards in Europe have been planted on limestone or lime-rich soils.

Also read Explore Maastricht's blog

From the Kiekoet, you have a view over the Oehoe Valley, home to Europe's largest owl. This imposing bird can grow up to 75 centimetres and has a wingspan of as much as 1.80 metres. An eagle owl pair has been successfully nesting here since 1998. The eagle owl needs a quiet nesting site, with as little human disturbance as possible, enough food and a steep wall to nest in. These owls have a large territory and hunt pigeons and rats at night in the city of Maastricht.

In 1673, Maastricht was besieged by Louis XIV, the Sun King. After the Netherlands regained the city, stadholder William III had Fort Sint Pieter built in 1701 to protect the city from attacks from the south. The fort was last used during the Belgian secession. After 1867, it lost its military function and fell into disrepair, being used as a warehouse, stable and 'quarry', among other things. During World War II, the fort was a lookout post for air alert, whose concrete building is still visible. Between 2007 and 2014, the fort was restored. Inside the fort is a subterranean spiral staircase along the well, which provides access to a chamber connecting the Northern Corridor System.