part 4] UPPER ORDOVICIAN OP THE BERWYN HILLS. oO'.i 



of conspicuously large size. Then came a marked change in the 

 physical conditions, which seems to have affected the type of 

 deposit and fauna ; for, instead of limestones and mudstones, with 

 large well-grown forms, black limestones and shales occur con- 

 taining dwarfed graptolites, inarticulate brachiopods, and small 

 thin-shelled mollusca. 



This change is thought to be due to the formation of an 

 enclosed lagoon between the land on the east and the open shallow 

 sea on the west. Whatever these conditions may have been, they 

 were of comparatively short duration, and soon gave place to a 

 more normal type of sedimentation with the incoming of the 

 Ashgillian fauna. This fauna was composed of numerous small 

 (but highly, or even excessively) developed forms, which flourished 

 in the Lower Ashgillian muds. This fauna seems to be exotic, 

 rather than descended from the former indigenous Caradocian 

 fauna. 



During the whole of Ashgillian time, as represented in this 

 district, the sea appears to have remained fairly shallow, and 

 supported a large fauna — trilobites, polyzoa, and brachiopods being 

 the commonest forms, with a few cystidea and gastropods. 



The basal sandstone of the Silurian indicates the first break in 

 the succession, and it is probable that, after the deposition of the 

 muditones with Trinucleus bit \elcl 'audi and perhaps other beds 

 corresponding to the Phacops-mucronatus Beds, the area was 

 subjected to tilting, the uplift being greatest in the Welshpool 

 district, slight in the Vyrnw T y district, and at Glyn Ceiriog and 

 Corvven possibly not raising the sea-floor enough for erosion, but 

 only bringing that region within reach of the sandy sedimenta- 

 tion which gave rise to the Glyn-Corwen Grit. When submersion 

 and deposition again began in the Vyrnwy area the shallow-water 

 massive Meristina-crassa Sandstone was laid down, and with 

 deepening water came the overlying Valentian shales and mud- 

 stones ; while, in the neighbourhood of Welshpool, the greater 

 elevation and more prolonged .period of denudation removed the 

 whole of the Ashgillian, the sea not entering that area until 

 Upper Valentian times. 



It appears, therefore, that the South-Western Berwjui area 

 throughout Upper Ordovician times was one of shallow water, 

 but also one in which the conditions of deposition and supply 

 and type of sediment varied considerably from time to time ; 

 throughout, however, it was one well able to support a large and 

 varied fauna, the remains of which can be seen in the rocks as 

 now exposed. 



VII. PALiEOXTOLOGUCAL NOTES. 



General Remarks. 



The Caradocian fauna, with the exception of that of the highest 

 beds, is the typical North Welsh fauna of the age. The pecu- 

 liarities of the fauna of the highest Caradocian (black-shale group) 

 have already been noted. 



Q. J. C.S. No. 310. 2 M 



