128 STRATIGRAPHICAL GEOLOGY 



2. Shropshire 



As the Ordovician Series of South Shropshire is a continuation 

 of the Welsh facies of the formation, it is unnecessary to give a 

 detailed account of the local subdivisions. For a description of 

 these the student is referred to that given by Professors Lapworth 

 and Watts in Proc. Geol. Assoc. vol. xiii. p. 312. Only a brief 

 summary will here be given, with a separation of the Llanvirnian 

 by means of the graptolite zones which Miss Elles has more 

 recently been able to establish. 



The Ordovician Series is brought up to the surface in Shrop- 

 shire by the broad compound geanticline, of which the Longmynd 

 may be regarded as the central axis (see map, Fig. 21, and section, 

 Fig. 7). The principal area of Ordovician rocks is on the western 

 side of the Longmynd, around Shelve and Corndon, where the 

 general dip is westward, and the whole succession is found from 

 Arenig to Upper Bala. On the eastern side (Caradoc area) the out- 

 crop is narrow and only equivalents of the Bala Series occur, resting 

 directly either on Archaean or on Cambrian rocks. From this 

 disposition it is evident that all the lower members of the 

 Ordovician System thinned out against the shores of a land-area 

 consisting of the older rocks, and that this land did not sink below 

 the Ordovician Sea until the beginning of Caradocian time. 



Arenig Series. The base of this division consists of a hard 

 siliceous grit or quartzite forming the ridge called the Stiper stones ; 

 above this are grey and green flagstones with interbedded shales, 

 in which Professor Lapworth has found Ogygia Sehvyni, Tetragrap- 

 tus quadribrachiatus with species of Phyllograpttis and Trigono- 

 yraptus which mark them as Lower Arenig. In the higher part of 

 the series both Didymograptiis extensus and D. hirundo have been 

 found, so that probably the whole of the Welsh Arenigjs represented 

 and the total thickness may be 1500 feet. 



Llarrvirn Series. The Hope shales, which have yielded 

 Did. bijid-us, form the lower part of this series and are succeeded by 

 hard andesitic tuft's with intercalations of shale which prove the 

 whole to have been deposited in water. These beds are overlain 

 by flags and shales with few fossils, but at the top are the Betton 

 shales with Didymograptus Murchisoni. The thickness of this 

 series is probably about 2000 feet. 



Llandilo Series. This is represented by the Meadowtowii 

 Beds, a series of flags and flaggy limestones containing Asaphus 

 tyrannus and Ogygia Buchi, overlain by black mudstones and 

 shales with Leptograptus flaccidns and Nemograptus gracilis. Their 

 thickness has not been estimated, but is probably about 1000 feet. 



