If you start your walk from Brunssum then you will come, after approximately 30 minutes, to a small stream called the Red Brook. The high concentration of iron causes it to colour this way.
You might think that a little stream through a sandy landscape isn't such a big deal, but the Netherlands actually doesn't have many brooks left. During the last iceage a large portion of the country was covered in pack ice, which not only flattened the land but also destroyed small creeks, brooks and streams. Because the ice never got as far as Limburg, all the watering systems are still intact here.