WN2 - Altered biomicrite
Click the microscope button to view a thin section for this sample.
Click the microscope button to view a thin section for this sample.

Fact sheet

WN2 - Altered biomicrite

This sample is a muddy limestone of Silurian (Wenlock) age, packed with rather indistinct fossil fragments (bioclasts) embedded in fine-grained, brown micrite (fine particles of recrystallized lime mud). This sample was weakly altered by hot fluids percolating through the rock from a nearby fault zone, which may partly account for the very patchy colour variation - although this probably also reflects mixing of different material during deposition. The sinuous, dark brown seams wriggling through this section represent deposits of insoluble iron oxy-hydroxides, which may have been deposited as the fluids passed through. Pale, sinuous veins of calcite may represent the re-precipitation of dissolved calcium carbonate from those same fluids

The limestone was laid down in a shallow tropical sea south of the equator around 428 million years ago. The area of Wren's Nest featured small coral reefs teeming with life, separated by shallow banks and troughs of lime mud.

The limestone reefs were home to compound and solitary corals, bryozoans, crinoids, brachiopods, gastropods, cephalopods and trilobites. Fossils are harder to identify in this thin section than others in this collection; they tend to be smaller fragments which may indicate they were more roughly treated by the sea before deposition. Some examples include fragments of crinoid stem, echinoderm plates and curved brachiopod shells. The indistinct appearance of many of the fossil fragments may indicate that they were partially dissolved during the fluid percolation.

Rare pale patches in the thin section are voids infilled with crystalline (sparry) calcite, which may have replaced dissolved bioclasts or infilled voids.

This sample was collected as part of the 'Macro to Micro' project.

Additional images
  • Hand specimen of limestone on black background
52.520756, -2.093354
About this collection

This Collection showcases the geodiversity of a classic geological site: the Wren's Nest 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.


Sample details

Collection: Wren's Nest
Rock-forming mineral
Category guide  
Category Guide
Refers to any word or phrase that appears in the individual rock names. Names are generally descriptive; they allow users to search for broad terms like ‘granite’ as well as more specific names such as ‘breccia’. However, the adjacent descriptions of the specimens captures a wider range of general words and phrases and is a more powerful search tool.
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Accessory minerals
Minerals that occur in very low abundance in a rock. They are usually not visible with the naked eye and contribute perhapssver, they often dominate the rare elements such as platinum group metals.
Rock-forming minerals
Minerals that make up the bulk of all rock samples and are also the ones used in rock classi?cation.
Selecting one or more period, for example 'Jurassic'.
A term used to group together related samples that are not already gathered into a single Collection. For instance, there is a ‘SW England granites’ theme that includes such rock types as granite, hydrothermal breccia, skarn and vein samples.
A general term used to label a rock sample. It is a useful way of grouping similar samples throughout a collection. Category names are often, but not exclusively, common rock names (e.g. granite, basalt, dolerite, gabbro, greisen, skarn, gneiss, amphibolite, limestone, sandstone).
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We would like to thank the following for the use of this sample: