Chilmark Main Building Stone
A sandy limestone in the Tisbury Member of the Late Jurassic Portland Stone Formation of the Vale of Wardour. The area of rock shown in this XPL photomicrograph (right) is 0.55mm x 0.35mm.
Technical Note: Sellotape-thin rock sections on glass slides have been examined under a petrological, polarising, microscope with plain polarised light (PPL) using one polariser, or two polarisers crossed at right angles (XPL). These methods allow the component minerals, cementing, strength and porosity of building stones to be determined. These thin sections, photographs and descriptions have been made by Pete Bath.
.ABOVE – UPPER GREENSAND
Images A-C (XPL) show the main components: green pellets of glauconite & quartz sand grains variably coloured white to black (the PPL image D reveals more).
1.Glauconite - Many pellets of this green-yellow mineral, which gives the rock its name.
2.Quartz - Depending on their sawn-through angle, grains of quartz can be white, grey or black under XPL, but the black can be confused with porosity cavities and iron-rich cement.
The bridge over the River Stour at Durweston was built in 1795 for Henry Portman in order to divert the main road from the west away from his Bryanston estate. Most of the bridge is Chilmark Stone, but the western end is Upper Greensand from Shaftesbury. The stone may have come from the demolished Eastbury House.
ABOVE (XPL) – CHILMARK PORTLAND STONE
1.Pastel shades of sparitic calcite – cementing this glauconitic quartzose limestone.
2.Quartz grains vary between white and dark grey
3.A grain of plagioclase feldspar from older weathered rock, probably volcanic.
4.Dissolved shell cavities which, if greatly interlinked, create porosity.
5.Brown micrite envelope enclosing minerals replacing a fragment of shell.
6.Originally a fragment of shell, replaced here by large bright calcite crystals.
7.Pellet of green/yellow glauconite – an alteration product of North Sea volcanic ash.
8.Patches of muddy micritic calcite in stronger sparite (sparry calcite) cement.
..LEFT Image D (PPL)
3. Quartz grains are seen more dimly and in the same faint colour.
4.Cement - in this iron-rich rock the natural cement between the grains is revealed between the quartz and glauconite by the speckled black iron grains.
5.Glauconite - Rusted (oxidised) iron in the glauconite is browner or grains brown-rimmed