| Stages. | Old Classification. | New Classification. | Palæontological Classification. | |||
| 1 Upper Ludlow |  | Ludlow |  | Downtonian |  | Fauna with Encrinurus |
| 2 Aymestry Limestone | ||||||
| 3 Lower Ludlow |  | Salopian | ||||
| 4 Wenlock Limestone | | Wenlock | ||||
| 5 Wenlock Shale | ||||||
| 6 Woolhope Limestone | ||||||
| 7 Tarannon Shales | | Valentian | | Fauna with Harpes | ||
| 8 Upper Llandovery | | Llandovery | ||||
| 9 Lower Llandovery | ||||||
Fig. 18.
L = Ludlow. W = Wenlock. Ll-T = Llandovery-Tarannon.
Description of the strata. Lithologically the Silurian deposits of Britain form a continuation of those of the Ordovician period, with a local interruption due to the elevation of portions of Wales and the Welsh borders at the close of Ordovician times. Elsewhere we find a predominance of shales passing into grits at the top of the system, the change indicating the incoming of the shallow-water phase before the commencement of the second continental period. Particular stress is laid upon the predominant shaley character of the beds, for, on account of the richness and variety of the faunas of the calcareous rocks, greater attention is naturally paid to them in geological works, and the student may get a false idea of their relative importance. An attempt is made below ([Fig. 18]) to give a general idea of the variations in lithological characters of the Silurian rocks in different parts of Britain.
The Silurian strata are mostly found in the same localities as those which furnish exposures of the rocks of Ordovician age.
The development in the typical Silurian region of the Welsh borders is characterised by the abundance of calcareous matter which is found there as compared with that which exists in the other British localities.
The Llandovery strata are sandy, often conglomeratic, with a fair amount of calcareous matter in places. The arenaceous nature is undoubtedly due to the proximity of land caused by local upheaval at the end of Ordovician times, and the Upper Llandovery rocks sometimes rest unconformably on the Lower ones, at other times on Ordovician, Cambrian, or even Precambrian rocks. The Tarannon shales are light green shales with intercalated grits. The Wenlock series consists of a group of shales separating a lower, very inconstant, earthy limestone from an upper, more constant, thicker and purer limestone. The latter, the Wenlock limestone, is composed of fragments and perfect specimens of various fossils, and the fragmentary nature of many of the shells indicates the occurrence of wave-action and probable formation in shallow water, in some places against coral-reefs.
The Lower Ludlow beds consist of sandy shales; they are separated from the Upper Ludlow beds by an impure limestone, the Aymestry limestone. The Upper Ludlow beds consist mainly of grits and flags, often coloured red towards the summit.