Spotted dolerite - Preseli Hills
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

Spotted dolerite - Preseli Hills

This metamorphosed dolerite comes from the Preseli Mountains, Pembrokeshire, Wales. It is particularly notable for its spotted appearance in hand specimen, an effect caused by low grade regional metamorphism during the Caledonian orogeny. This rock and its location are particularly important to the history of the United Kingdom because the same rock was used to build Stonehenge, over 200 km to the east. Although most of Stonehenge is built of sandstone, this spotted dolerite was used for some of the smaller upright stones, known as bluestones because of the slight blue colouring they have and to distinguish them from the other stones at Stonehenge. A debate has raged among academics over many years concerning the transport of the rock from Wales to Salisbury Plain. Was the rock transported by ice in the last glaciation, or was it quarried and transported by humankind during the building of Stonehenge?

In thin section the rock contains large pyroxene (augite) and altered plagioclase grains. The original igneous minerals have been partially altered to chlorite and epidote during greenschist grade metamorphism, and although large pyroxene grains remain, almost all the plagioclase has been altered. The remainder of the fine-grained matrix was also altered by metamorphism, although many igneous mineral shapes are evident.

Additional images
  • dolerite (wet) - width 2.5 cm
  • dolerite - width 7.5 cm
  • dolerite - width 7.5 cm
51.9596, -4.7029
Preseli Mountains, Pembrokeshire
About this collection

The United Kingdom Virtual Microscope (UKVM) collection consists of igneous, sedimentary and metamorphic rocks from around the UK.

It is intended as a teaching resource, helping to tell the story of the common rock types and how they form, and reflecting the history of the UK at the margins of the continent of Europe. The collection is a series of teaching sets, for example igneous rocks from the North Atlantic Igneous Province and SW England; high-temperature metamorphic rocks from Scotland and low-temperature metamorphic rocks from Wales; and sedimentary rocks, including English limestones and sandstones.

Sample details

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).
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: