Fig. 2

Chertified limestone (a) and screwstones from the bedload of the River Maas, the Netherlands; all presumed Tournaisian/Visean in age (see text). a NHMM 2014 061b, a crinoidal limestone (‘ftaniet/silexiet’) completely replaced with silica apart from rare vugs. b, c NHMM 724. b One surface of the clast with numerous mouldic crinoid columnals and pluricolumnals. c Detail of the most prominent ‘screw’. Scale bar represents 10 mm. d NHMM MD 8154 (compare with Fig. 3). The prominent external mould has lost its axial canal (‘screw’) due to breakage during transport. Specimens uncoated. Scale bars represent 20 mm, except where stated otherwise