SW20 - Polymict channel sandstone
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

SW20 - Polymict channel sandstone

A coarse channel sandstone with a wide variety of clasts, collected from Doulton's Claypit.

This sandstone was deposited in a river channel within a low-lying, deltaic environment when the region lay near the equator. These deltas were thickly forested with trees whose fossil remains can be found in some of the strata.

In thin section, the texture is so coarse that sedimentary bedding cannot be identified. The sandstone is very poorly sorted and is quite immature both texturally and compositionally. A wide variety of grains includes clear quartz, dusty feldspar, chlorite, opaque oxides and many lithic fragments from quartzites, sandstones, siltstones to brown or greenish mafic rock-types; some sedimentary clasts even show discernible sedimentary lamination. Grains are subrounded to angular.

A notable feature of this rock is the abundance of tiny (most <50 microns), very high relief grains with very high birefringence and bright interference colours. These occur singly and also as clusters in the matrix between larger grains. These are 'heavy minerals', so called because of their high density. They tend to become concentrated in patches on the streambeds of river channels, where finer and less dense grains are transported away, leaving the largest and heaviest grains behind. Some of these (perhaps most) are zircon, but other accessory minerals rich in heavy elements likely occur.

Fuzzy, pale brown interstitial material is likely fine grained clays, with occasional patches of pale green chlorite.

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

Additional images
  • Hand specimen of sandstone on black background
52.481351, -2.096982
Boulders in Doulton's Claypit
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
Accessory minerals
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).
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We would like to thank the following for the use of this sample: