After the Mertens company stopped the sand excavation between roughly Brunssum and Heerlen Noord , the area was left with some gaps filled with water . And like many gaps , nature took it over . Off course there was some work done to avoid complete wilderness , now it looks like a parc . Straight after the “shutdown” the Heidserparc in wintertime looked like the image below , trees are grown since then and now it is a nice parc with a big fishing pond and a kind of open air theatre visible at the end . Date image : 1985 .