﻿Vol. 6 I.] ARENIG FAWR AND MOEL LLYENANT. 635 



The Upper Series belongs to the Ordovician (Lower Silurian of 

 some authors), and may be correlated division by division with the 

 series described by Prof. Lapworth and Prof. Watts in the Shelve 

 country. This series conveniently divides into a lower member of 

 calcareous, gritty, or graptolite-bearing sediments, a middle member 

 chiefly of direct volcanic origin, and an upper shaly member which 

 is not described. The lower member is the equivalent of the 

 Upper Skiddaw Slates of the Lake District, and is the Arenig of 

 most British authors ; whereas it was to the Middle Series that the 

 name of Arenig was first applied by Sedgwick. The Volcanic Series 

 is immediately overlain by beds belonging to the Caradoc or Bala 

 Series, which are essentially Upper Ordovician in their fauna. 



Having thus stated the facts, I leave it to others to suggest a way 

 out of the tangle in which our nomenclature has become involved, 

 and shall hope to hear the matter fully thrashed out anon. 



IV. Glaciation. 



Forming, as the Arenig Mountains do, the highest ground in the 

 neighbourhood of the main watershed of Wales, their glacial geology 

 is perhaps worthy of passing notice. Of the period of maximum 

 glaciation there is little record, and it is uncertain whether or not 

 the mountains were ever entirely overridden by ice. Certain it is, 

 however, that the highest summits are so much frost-split, that they 

 would not be likely to retain such striae as they might receive. No 

 Drift has been found at higher levels thau 2000 feet. 



The stage when the ice stood at about 2000 feet is more 

 interesting. Then and onwards, until it had retreated to about 

 1800 feet, all the important hilltops protruded as nunatakkr, and 

 withstanding the powerful ice-stream which came along from Snow- 

 donia and other districts to the west, retained numerous big boulders 

 of a black-looking felspathic agglomerate like the Glan-y-pwll 

 trap-rocks of Blaenau Ffestiniog or Manod. Perched blocks of 

 this rock frequently occur quite high up on Moel Llyfnant, and 

 are particularly abundant on the Lower Agglomerate slopes below 

 Craig-yr-Hyrddod. They are quite distinct from any rock in the 

 district. At this stage, the ice was able to supplement the local 

 glaciation of the Blaen-Lliw Valley, and to send a weak'lobe bearing 

 a few Snowdonian boulders into the Erwent Valley. At this 

 stage, too, the ice dragging round the Daerfawr-Shale ground under 

 Daerfawr, towards the open low ground of Nant Hir east of 

 Arenig Fawr, began the formation of the lateral moraine, which 

 ultimately formed the barrier that holds up Llyn Arenig Fawr, 

 and similarly passing round Arenig Fach to the Gelyn Valley 

 also formed the barrier for Llyn Arenig Fach. 



At a later stage, the ice-lobe crossing the col between Moel Llyf- 

 nant and Arenig Fawr became broken, and the part in the hollow 

 to the north was left stagnant. In melting, this gave rise to a 

 fine series of parallel ridges of morainic material, which, running 



