Fact sheet
Medium grey sandstone, from the same unit as SW5, but collected approximately 20 cm from the contact with the microgabbro intrusion (SW4 and SW13), near Highbridge Road Steps. There is little sign in the field of contact metamorphism ('baking') of the sandstone by heat from the microgabbro intrusion.
The sandstones were deposited in a low-lying, deltaic environment when the region lay near the equator. Flattened fossilised tree-trunks have been found in Carboniferous rocks close by in Brewin's Cutting, indicating that the area was thickly forested.
In thin section, the rock has a typical fragmentary, detrital texture; there are no clear signs of contact metamorphism. Weak colour variation defines clear bedding layers that are oriented vertically, across the short axis of the thin section. Colourless quartz grains are mainly subrounded to subangular, but a few highly rounded grains are also visible. Feldspars are also present, generally with dusty alteration. There is much brown material: some appears to be biotite mica, variably altered, as it shows high birefringence. Much of the fuzzy dark brown matter along grain boundaries is probably a mixture of clays and iron oxide or similar insoluble material. Uncommon, small clear laths are of muscovite mica. Rare grains under crossed polars are finely polycrystalline, made up of numerous much smaller grains.
A different type of grain occurs sporadically throughout the section. These are pale brown to almost clear, elongate, typically gently undulating laths, or shorter fragments. Many appear laminated or fibrous. Elongate grains tend to align subparallel to the bedding. Like the grain highlighted in sample SW1, these grains are isotropic. Might they be fragments of shells from the brachiopod Lingula?
This sample was collected as part of the 'Macro to Micro' project.
This Collection showcases the geodiversity of a classic geological site: the Saltwells National Nature Reserve in the West Midlands.
As well as displaying thin section and hand specimen views along with information setting them in the context of their landscapes, we also include perspectives and creative responses to the geological heritage of the sites from the local community.
| Explore the stories of the rock layers at Saltwells and Wren's Nest NNRs, designed by students at King Edward VI School, Stourbridge: |
This Collection was made possible by funding awarded to the 'Macro to Micro' project by the Natural Environment Research Council (NERC) under their 'Growing Roots' scheme.
