﻿150 MR. W. G. FEARNSLDES ON THE [May I9IO, 



IY. The Ffestiniog or Grey Flag and Grattwacke Series. 



The Ffestiniog Series of the district is a thick and massive shallow- 

 water deposit, which, by reason of its monotony and the paucity of 

 its fossils, I have not been able to subdivide. The outcrop varies 

 from about half a mile to one and a half in width, and is continuous. 

 The lowest beds are not well marked off from the highest Maentwrog 

 Beds, but the line of dolerite sills which comes in from the sea at 

 Craig-ddu and goes out at Gareg-goch, near Borth, serves as a con- 

 venient line for mapping. This line of intrusion can be followed 

 from Craig-ddu along Careg-yr-Eryr ; it appears in the marsh of Llyn 

 Ystumllyn, a quarter of a mile south-east of St. Cynhaiarn's church 

 and along the side of the field south of the barn of Coed-y-llyn, 

 again in the crags of Capel Siloam, at Ynys Cyngar, Careg-cnwc, 

 and finally at Gareg-goch, near the bathing-place of Borth. 



Like the Maentwrog Series, the Ffestiniog Beds show innumerable 

 alternations in the coarseness and fineness of the material. Usually 

 of a flaggy nature, they include at many horizons bands of massive 

 and very compact grauwacke grits, rich in felspars more or less 

 decayed, and set with quartz-grains, in a tough chloritic, or mica- 

 ceous, muddy base. These, although sometimes massive through 

 a thickness of 3 feet or more, are never coarse enough for real 

 grits, and a new term to describe them is badly needed. 



Their lithology and association with more slaty beds is strongly 

 reminiscent of the ringers in the Maentwrog Beds below, but the 

 marking-off of the beds is rarely so well defined. A notable feature 

 of the coarser of the grauwackes is the occurrence of false bedding, 

 and with it a more complicated arrangement of bedding-planes 

 which I would term ' curled.' The minute unconformities and over- 

 laps which constitute false bedding are well known, and are common 

 to all sediments deposited by drift -bedding in shallow water. They 

 are associated with true ripple-drifting and with worm-tracking, 

 ScolitJies, Cruziana, etc., occurring upon the upper surface of the 

 coarser beds. The ' curled ' bedding and general contortions of the 

 planes of deposition within massive strata is less easy either to 

 describe or to explain. 



Curled bedding is generally seen only in section. As so observed, 

 it bears a strong resemblance to the figures and description of the 

 contortions of the Langdale slates given by Sorby. 1 It is always 

 associated with considerable variations in the thickness of the bed 

 which it affects, and often where the curvature is strongest cracks 

 have formed transverse to the bedding and are now filled with 

 quartz. 



When seen in plan or upon an exposed dip-surface the curled- 

 bedded grauwackes are seen to be broken into nodular pavements. 

 These, in extreme cases, become discontinuous ; and in their general 

 behaviour the curled-bedded masses even strongly resemble the 

 nodular cement-stones of the Middle and Lower Lias. The curling 



1 Q.uart. Journ. Geol. Soc. vol. lxiv (1908) pi. xiv. 



