Tlie Age of tlie Hirnant Beds. 



411 



The relation of these beds to those above and below is shown in 

 the accompanying section (Fig. 2) ; the pale grey Moelfryn Sand- 

 stones occupy the greater part of the Pass, which owes its steep-sided 

 character to them ; at its south-east end are seen the Foel y Ddinas 

 or Phacops Mudstones, a sandy mudstone group clearly related 

 lithologically to the Moelfryn Beds ; the development of the Hirnant 

 Beds which succeed, ushers in an important lithological change, 

 for these are soft dark-blue mudstones, in which slaty shale bands are 

 intercalated with increasing frequency until the whole is a purely 

 slate groiip with more and more definite banding the higher we go 

 in the series. The higher beds have yielded graptolites characteristic 

 of two definite horizons, those of the Upper Birkhill and Lower 

 Gala respectively. So that so far as this section is concerned, the 

 Hirnant beds are either referable to the highest portion of the 

 Ordovician and the whole Valentian, extremely thin, or else they 

 represent beds of Lower Valentian age. 



N.W. 



S.E. 



I o e a f-t. 



Fig. 2. — Section S.W. side of Bwlch yr Hwcli. Hor. scale 6 inches = 1 mile. 1. Moelfryn 

 Sandstones. 2. Foel-y-Ddinas Mudstones with Phacops. 3. Hirnant Beds. 4. 

 Valentian. 



Traced northwards along their strike these beds gradually lose all 

 their calcareous and argillaceous matter and become sandstones and 

 then grits of fairly coarse texture, but they still contain a " Hirnant 

 Fauna " ; traced southwards, on the other hand, they seem to pass 

 definitely into sediments of a deeper-water nature, who.se age can 

 be determined without any difficulty. Unfortunately, owing to the 

 nature of the ground, the outcrop cannot be followed continuously, 

 as it disappears for the most part under a wide stretch of boggy, 

 peat-covered moor ; the Moelfryn Sandstones can, however, be 

 traced easily, as they form a more or less definite feature, and out- 

 crops are fairly numerous, and the trend of the higher Banded Slates 

 is also fairly obvious, the intervening softer beds being concealed. 

 When, however, all the beds are once more seen about 2| miles south 

 of the original outcrop, their relationships seem perfectly clear. 

 Here at Bwlch y groes, the Pass over from Llanuwchllyn to 

 Dinasmawddwy, they are very well exposed, and can be traced for 

 some distance on either side ; the light grey Moelfryn Sandstones 



