A. Thin section, under crossed polars, of Lower Calcareous Grit taken from a loose block at High Redcliff, TA076840, south east end of Cayton Bay, North Yorkshire. The section illustrates grains of quartz, showing grey and pale straw yellow interference colours set within a matrix of calcium carbonate. Occasional bioclastic material is visible as near the SW quadrant as well as towards the centre right. Although given the name of a grit, the average size of the sand grains is only 0.3mm, i.e. it is a very fine grained calcareous sandstone.

B. Thin section, under plane polarized light from an old quarry, SE563890 on Newgate Bank, 4.5 miles NW of Helmsley, North Yorkshire. Some horizons in the Lower Calcareous Grit are packed with spicules of a sponge, Rhaxella sp. (?perforata), now in the form of chalcedonic or opaline silica.

C. About the same section as B. but under crossed polars. The spicules show pale to very light grey interference colours, some of them showing a hint of radial structure as can be seen in two of them towards the edge of the SW quadrant. Occasional quartz is visible, pale straw yellow interference colours, e.g. in the NE quadrant. The pale pink fragments are calcite and the black holes are where sponge spicules have escaped during the section making process. The spicules measure approximately 0.1mm in diameter. 

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