652 . MISS E. M. E. WOOD ON THE [Nov. 1906,. 



The rocks are arranged, broadly speaking, in a long syncline, the 

 axis of which runs almost due north and south (see fig. 7, p. 688). 

 The central part of the Tarannon tableland is everywhere occupied 

 by the Denbighshire strata, except along the valley of the Afon 

 Cwm Calch, where they have been removed by denudation, and the 

 underlying Tarannon Shales are exposed. The outcrops of the older 

 beds follow that of the Denbighshire Group in successive and fairly- 

 regular curves, the lowest beds of all being exposed in the valley 

 of the Twymyn, which lies to the extreme west. The dip of all 

 the beds older than the Denbighshire Group is high, varying from 

 40° to 60°, and their outcrops, especially on the west side, often 

 make an almost straight line across both hills and valleys. 



The large syncline has been subjected to considerable minor 

 folding, due to lateral pressure coming apparently in the main from 

 the north-west. The result of this movement has been to fold the 

 rocks into repeated subsidiary anticlines and synclines, the axes of 

 which cross that of the main axis obliquely and run in a north- 

 north-east and south-south-west direction. Where the arenaceous 

 beds predominate over the argillaceous, the strata have yielded in 

 regular and symmetrical folds. Where, on the other hand, the 

 strata are chiefly shaly, the rocks have suffered more violent de- 

 formation and have been bent into repeated sharp folds, which are 

 almost isoclinal in their character, and are frequently broken and 

 faulted to a considerable extent. In addition there is a north-and- 

 south movement which has bent the rocks into broader waves,, 

 causing the folds to pitch in these two directions, but not producing 

 any acute cross-folding. In the western and southern parts of the 

 district the rocks are much cleaved. 



Some idea of the number of the more important folds may be 

 obtained by an examination of the 1-inch Geological-Survey map 

 of the area lying to the north and east of the present district. On 

 this the outcrops of the ' Tarannon ' and so-called Llandovery Beds 

 are shown extending north-eastward, in long narrow V's into the 

 Denbighshire Series, each V marking the position of an anticlinal 

 fold. 



Practically the only fossils that I have obtained in this district 

 are graptolites. In the upper and more gritty half of the suc- 

 cession these appear to be restricted to occasional bands, which are 

 rarely more than a fraction of an inch in thickness. They are 

 difficult of detection among the large mass of unfossiliferous strata,, 

 as the lithological character of the beds gives but little guidance. 

 On the whole, however, it may be said that graptolites are most 

 commonly met with in brilliant orange-weathering shales which 

 immediately underlie small flaggy beds, and they are frequently 

 found plastered on to the under side of the flags themselves. The 

 graptolites found in some of the upper beds are generally pre- 

 served in low relief ; but they are seldom seen in the normal profile 

 view, and are therefore often difficult of identification. 



In the lower part of the succession, where the beds are more 

 generally shaly, graptolites are more abundant, and occur at frequent 



