SW12 - Basal polymict conglomerate
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

SW12 - Basal polymict conglomerate

A polymict conglomerate of Carboniferous age, collected in Brewin's Cutting from immediately above the unconformity with underlying Silurian strata including samples SW2 and SW11. This sedimentary rock is of Carboniferous age, around 420-417 million years old.

The conglomerate was deposited directly on top of the weathered surface of the eroded Silurian sandstones and siltstones below, in a low-lying, deltaic environment when the region lay near the equator. The pebbly nature of this rock tells us it represents one of the river channels. These deltas were thickly forested with trees - in fact, flattened fossil tree trunks have been found in these strata.

The irregular surface between the top of the Silurian strata and the base of the Carboniferous conglomerate is an example of an angular unconformity: a time gap in the sedimentary column where strata are missing. In this case, the time gap amounts to around 100 million years. These gaps generally mark episodes of uplift and mountain building.

In thin section, the coarse-grained, fragmental texture is very clear. Clast shapes vary from rounded to subangular, and the rock is poorly sorted, with a wide range of grain sizes. Although many clasts are of quartz or feldspar, several are polycrystalline and represent grains or pebbles of other rock-types, including sandstone, granite and quartzite. One large grain is of a fine-grained rock cut across with quartz veins, now recrystallised. Several other grains are darker brown or olive-green in colour, indicating altered mafic rock-types; there are also some pale green grains of chlorite. In between the grains there is yellow to brown, fine-grained material that probably represents a mixture of insoluble iron oxides and clay minerals. However, there are also numerous colourless, isotropic patches - these are empty pore spaces.

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

Additional images
  • Hand specimen of conglomerate on black background
52.486818, -2.094579
Canalside exposure just east of bridge over canal at Highbridge Steps, in Brewin's Cutting.
About this collection

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.

Sample details

Collection: Saltwells
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.
Refers to any word or phrase that appears anywhere in the descriptions of the specimens
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).
The owner of the sample that appears in the collection. For example, NASA owns all the samples that appear in the Moon Rocks collection
We would like to thank the following for the use of this sample: