The history of establishment of the Silurian System (and hence of the Silurian Period) and its principal divisions has been reviewed by Cocks et al. (1971) and Holland (1984, 1989a). The System itself, three of the four Series into which it is divided, and the Stages within these three, are all based upon localities in Britain. The names of all the divisions shown in Fig. I have been ratified by the International Union of Geological Sciences. The two major divisions, Lower Silurian and Upper Silurian, remain informal, but they are useful when degree of precision in correlation does not allow reference at the level of Series (Holland 1989a). The boundary stratotypes are all described in Holland & Bassett (1989). Graptolite Biozones have proved especially useful for international correlation in the Silurian and the sequence of Biozones used in Britain is shown in Fig. 1. Correlation of the Biozones wth the global Standard Series and Stages is shown approximately, but is not known with exactitude in all cases.Geodetically the position of the British Isles in the Silurian remained astride the 25°S latitude, a position it had reached in the later Ordovician (Briden et al. 1973); movementwas confined to rotation. During this time the south magnetic pole wandered, in terms of present geography, from near to southwest Africa, to the Horn of Africa in early Silurian times, then back along the equator to west Africa/Brazil at the end of the Silurian.The Llandovery Epoch was chracterized by marine transgressions. Thus in

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The oldest formation exposed in the Gorsley area is a hard limestone considered by previous authors to be equivalent to the Aymestry Limestone. All the available evidence, however, supports a correlation with the Wenlock Limestone. The succeeding strata comprise 4½ feet of Dayia Beds and 7 feet of Upper Ludlow Beds, so that the total thickness of Ludlovian strata is only 11½ feet. AH the Lower Ludlow Shales and the Aymestry Limestone are cut out by the unconformity below the Dayia Beds. This remarkably thin development is attributed to movement at various times during the Ludlovian epoch along an axis passing through the Gorsley region. The main movements probably took place in middle Ludlow times.