Ardennes slate & Belgian bluestone

These are two different stones from two different parts of Belgium, and it is worth separating them before anything else.

Belgian bluestone, properly Petit Granit or Pierre Bleue de Belgique, is not granite. It is a limestone, quarried around Soignies in the south, and its texture is made of animals. About 350 million years ago the southern half of Belgium lay under a shallow sea, and at what is now Soignies the water was low enough and the bed muddy enough with lime for crinoids to grow in enormous colonies. Their fragments settled into the calcareous mud, the sediment stacked up in horizontal layers and compacted, and the result is a dark limestone flecked throughout with fossil crinoids and corals. That fossil content is exactly what people buy it for. Later tectonic movement made the rock brittle in places, and the pale calcite veins that sometimes run through a polished slab are the record of it.

The slate is a different country entirely. It comes from the Stavelot-Venn Massif in the east, a thick sedimentary pile deformed and fractured by two separate mountain-building events, the Caledonian and then the Hercynian, metamorphosed to greenschist along its southern edge, cut by the collapse structure known as the Graben of Malmedy, and still moving vertically today. Its geology is quite distinct from the Devonian and Carboniferous rocks around it, and the landscape that resulted is protected as the High Fens Natural Park.

The Massif gave up more than roofing. It holds manganese and molybdenum deposits, mineralised and iron-rich waters, and coticule, the rock cut here into sharpening stones. Slates and quartzites from the Massif, arkoses and sandstones from its borders, are what give the local buildings their character. Some workings have been turned to visitors, among them the slate quarry at Recht and the Museum of the Coticule; others were left alone and have become refuges for rare plants and animals.