Dear Dave, Or, what I have know as Calc Tufa? - from certain books. If this be the case, I know of an interesting quite sizable surface deposit that I discovered by chance in the Alston area in the latter part of the 1970's. It is a place that can only be found by chance. The large fossilised worm channels were interesting, some being 3/4 of an inch dia., with miniature 'cave systems' and 'caverns' amongst it all. After I found it, I spent three days looking around the area, and, to my knowledge it is unique since I found nothing anywhere else like it - indeed, in fact, anywhere. Regards, Bernard