We hope these explanations help you in your understanding and enjoyment of this subject!
Dolerite: An igneous (once molten) rock. It has a medium grained texture meaning its constituent crystals can be seen with a hand lens. It is quite dark often being slightly green. Large expanses of dolerite are being quarried for roadstone aggregate on Clee Hill.
Erratic: When glaciers travel they can pick up very large blocks of rock or other material which flow along with the glacier. However, when the glacier is melting it can no longer support the large material and boulders are dumped. These are known as erratics as they are often in strange places and are made of rock types otherwise found locally. A famous erratic stands outside the Public Library in central Shrewsbury.
Genera (Genus): Part of the scientific name given to organisms under the system known as the 'Binomial Classification'. The same Genus name is given to a set of species that share particular features.
Igneous: rock that was molten at some point in its cycle. This could be lava or molten rock that never reached the surface. Caer Caradoc is largely made from igneous rock (but it is NOT a volcano; nor is the Wrekin!).
Limestone: A calcareous rock with a high proportion of the mineral calcite (calcium carbonate). It is formed either by precipitation of calcium carbonate from the water; by the fossilization of a large number of fossils with calcareous skeletons; or by a mixture of the two. Wenlock Edge has a good variety of limestones.
Magma: Rock in its liquid, or molten, state whilst it is underground. When it breaks through to and flows over the surface it is termed lava.
Marl: Rock made up of very fine particles (clay and silt) mixed with a similar proportion of calcium carbonate.
Sandstone: Rock made up of sand grains. There are different grades depending on the size of the grains and their shape. Such grains may be clearly visible with the naked eye (coarse) or with a hand lens (fine). The Grinshill quarries have produced the best quality building stone in the County.
Sedimentary: Rocks formed from sediment that settled over time either in water or on land and then compressed and/or cemented to form rock by burial beneath further layers of sediment.
Sill: A body of igneous rock that has been injected (whilst molten) along the bedding of other, usually sedimentary, rocks (cf. a Dyke, which has been injected across the bedding).
Tectonic: Stresses, usually very large, related to earth movements caused by the constant 'drifting' of the Earth's crust. The crust comprises continental and oceanic plates whose movement can result in crumpling the ground (folding) and breaking it up (faulting). Injection of molten rock can lead to Volcanic eruptions, the heat and stress can cause Metamorphism, and active fault movement can generate Earthquakes.
Tors: Crags of rock which have often been weathered by the action of freeze-thaw resulting in break-up and the production of debris as slopes of loose material below them. This is well seen on the Stiperstones Ridge and below the summit of Titterstone Clee.