The whole Westerwald region lay under a tropically warm arm of the sea in the Palaeozoic (600 to 270 million years ago). This sea deposited layers of sediments many kilometres thick into the Variscan geosyncline, which were heavily folded in the orogeny that followed. The towns of Siegen and Koblenz on the Westerwald's north and southwest edges even gave two Lower Devonian layers, with their colourful slates, their names. The upper mountain layers are formed of volcanic strata made of basalt containing tuffs.