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The Ordovician System was named by Lapworth in 1879 from the Ordovices to resolve the problem of the boundary between the Cambrian and Silurian systems which resulted from the controversy between Adam Sedgwick and Roderick Murchison. The primary sub-divisions are as follows: Arenig, Llanvirn, Llandeilo, Caradoc, and Ashgill. There are three main areas of Ordovician rocks in Shropshire: Shelve, Caradoc and Pontesford.

Whittard mapped the "Bohemian faunas" using groups of trilobites, include: Ampyx, Cyclopyge, Calymenid trilobites and Trinucleids. At Hazler Hill, crevices in the Precambrian rocks ‑ so called neptunian dykes ‑ are filled with a sandy development of Harnage Shales with no representative of the usually underlying Hoar Edge Grits.

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Skinner, A. (1981). Geological aspects of Shropshire groundwater investigations, p.6-8

Of concern to hydrogeology are grain size, porosity and permeability. Also of interest are faults and aquifer geometry, changing their thickness and affecting the amount of storage capacity in the ground. The most striking feature of North Shropshire is the range of sandstone hills running east-west, north of Hodnet, and their sudden displacement 8 km down the A49, where the outcrop of Keuper Waterstones (now called the Bromsgrove Series) suddenly appear at Grinshill.

The Wem map has large expanses of superficial deposits of the Ice Age, largely boulder clay (till) interspersed with outwash gravels and sands. Where great thicknesses of clay exist, the ability of rainfall to infiltrate the sandstone and provide a water supply is much reduced. The River Tern area north of Wellington was looked at in detail, showing two main faults: the Hodnet Fault and the Preston Brockhurst Fault, both trending northeast-southwest. At Ellerdine, 8 km north of Wellington, the IGS map shows Keele Beds, Bunter Pebble Beds, and Lower Mottled Sandstone which is basal Bunter Sandstone. The second area was in the catchment of the River Perry.

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Trilobites have two main types of eye: where the individual lenses are in contact and a compound eye, each lens having its own cornea. Each lens is made of a single crystal of calcite and at every moult these would be lost and new ones developed with the next shell. The problem of double refraction was overcome by orienting the lens along the C axis.

The shell of a trilobite is composed of 91% calcite which becomes replaced by pyrite. Clarkson made calcite lenses with cartesian surfaces. Sisney used the technique of stereoscopic X-rays. There is a tendency for some groups of trilobites to grow convex and smooth, e.g. Homelanotus, a smooth Calymene from the Welsh borders, a smooth Olenid from the Cambrian of Spitzbergen, and the smooth Asaphids from the Ordovician of North America. There is also the rather beautiful British fossil trilobite of the genus Bumastis.

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Field examination of Caradocian mudstones at Pant, Carboniferous Limestone on Llanymynech Hill, and Caradocian shales near Llanyblodwel.

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Dolerite sills seen on Titterstone Clee. Carboniferous limestone and mudstones, the latter containing several seams of coal up to 30 cm in thickness with good plant fossils were found in the clays such as the articulate Calamites, some Lycopod stigmaria, and Lepidodendron. One curious feature, in a large joint face or fault plane in the intrusion were a series of spheroids, all about the size of cricket balls, probably formed by weathering or chemical action.

Farlow exposed Lower Carboniferous limestone on top of a conglomerate of red sandstone and quartz pebbles. Although the sandstone at the top was not red, the soil in the fields round about showed the characteristic colour. Soils above the conglomerate in the limestone are alkaline and those below, in the Old Red Sandstone (e.g. Green Dingle), quite acid. Oreton quarry exposed fossiliferous Lower Carboniferous Limestone with corals, brachiopods, crinoids and polyzoans.

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Three principal rock suites have been identified: Longmyndian, Uriconian and various outcrops of metamorphic rock such as the Rushton Schist. The synclinal core is in part worked at Sharpstones, from west to east: the Bayston Group, the Bridges Group and the Oakswood Group. The Haughmond Conglomerate passes upwards into a thick sequence of greywackes which form the main quarry reserve. One strange feature of a number of the faults in the quarry is the presence of bitumen within the fracture zone. The principal uses for the stone are as railway ballast (the quarry has its own rail head) and roadstone.

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Gold in the valley of the Mawddach river, north of Dolgellau. The present mining project at Gwynfynydd is being carried out as a speculative venture.

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