Sample Rocks

BASALT

What does it look like?

Basalt is a fine grained (1 mm) crystalline rock that looks black or dark grey, sometimes with a green or brown tinge. It can have flecks of white crystals and sometimes bubbles trapped within the rock. The minerals that make the ground closer to the surface (as a dyke or sill), or over the ground as a volcanic lava. It commonly forms across the globe at spreading ridges where volcanoes are erupting, or where this has happened in the past.

Common minerals are olivine, pyroxene and plagioclase feldspar.

Where is this rock found in Shropshire?
The area around Limekiln Woods and Little Wenlock.

Basalt
DOLERITE

Dolerite is a coarse black or dark grey crystalline rock. Crystals within dolerite can be up to 5 mm in length. The minerals in dolerite are rich in iron so it weathers a dark rusty brown. Dolerite is a very strong, heavy rock that makes an excellent road stone. Dolerite is an intrusive igneous rock, cooled beneath the ground, more slowly than the basalt found in lava flows. It can also be injected into the crust in sheet-like structures called dykes (vertical) or sills (parallel to bedding). It is commonly found at spreading plate tectonic ridges where crust is being pulled apart, infilling the gaps as new molten material flows in to fill the space.

Common minerals are pyroxene, olivine, feldspar and hornblende.

Where is this rock found in Shropshire?
Dolerite Dykes are found at the foot of the Wrekin in Forest Glen and the summit of Brown Clee and Titterstone Clee where it is quarried for road stone.

Dolerite
GNEISS (Pronounced ‘nice’ )

What does it look like?

Gneiss is a coarse crystalline rock with distinctive coloured bands usually separated into dark and light coloured minerals. Gneiss is a metamorphic rock, which means that it was once a different kind of rock altogether. It is made during mountain building and continental collision events where temperature and pressure reach high levels and this causes the minerals within the rock to change. They reform themselves into a structure that is better able to withstand the high temperature and pressure conditions. The bands in the rock form because the minerals have different weights and when they are squeezed, this forces them to line up at right angles to the direction of the stress.

Common minerals are quartz, feldspar and mica.

Where is this rock found in Shropshire?
Gneiss is only found in a couple places within Shropshire: at Primrose (or Little) Hill at the south west end of the Wrekin and around the village of Rushton. Both outcrops are very small and the rocks are badly eroded and weathered so can be very difficult to identify.

Gneiss

 

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