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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