In reply to Alastair:
There are some great basalt crags not too far from there I think in a national park area called The Eifel. I've entered a few routes on UKC's database for the crag 'Ettringen', have a look. There is a guide book for the bouldering and one for the routes (there is a good mix of sport and trad) .'Schwarze Saulen' is the guide for the routes in the basalt quarries and has sections in English and German.
I'm pretty sure there's more around that area, ranging from limestone to sandstone, some of the crags you can't climb on and some you can. I personally haven't climbed at any of the others apart from Ettringen and a wicked bouldering venue called 'Glees', but I've heard of other spots.