560 E. Greenly — Arenig Shales at Menai Straits. 



VIL — On the Occurrence of Arenig Shales beneath the 



Carboniferous Rocks at the Menai Bridge.^ 



By Edward Greenly, F.G.S. 



THE sliore of the Menai Straits on the Carnarvonshire side, near 

 the Suspension Bridge, is composed, as is well known, of 

 sandstones and shales belonging to the Carboniferous Series. About 

 a quarter of a mile west of the bridge, however, some fissile, reddish 

 and greenish shales appear on the foreshore, dipping at higher 

 angles than usuaL This might easily be ascribed to local dis- 

 turbance, as a large fault is not far oif, though the shale differs 

 somewhat from that which is common in the Carboniferous rocks of 

 the district, a fact which was noticed by Mr. G. H. Morton,^ who 

 went with me along this part of the section. The difference, 

 however, might easily escape the eye, especially as the exposures are 

 small. The dip is about 25°-30° S.E. or S.S.E., but sometimes as 

 high as 50°. The shales soon rise into a low cliff', overhung 

 by woods which mask their relation to the Carboniferous rocks to 

 the east, and extend along the shoi'e for about 80 yards as far 

 as a little glade in the woods, beyond which drift comes down to 

 high-water mark for about another 80 yards. The shales here rise 

 again, and form a little rocky point some 20 to 30 feet high, where 

 the red and green varieties can be seen to be merely the upper and 

 stained portion of a mass of black shales, which form the lower 

 part of the rocky point and the foreshore. These are dark, evenly 

 bedded, platy shales, very fissile, and quite unlike anything in the 

 Carboniferous system in this district, but strongly resembling the 

 usual Ordovician type of S.E. Anglesey and the Straits. They are 

 quite uncleaved, and dip E.S.E. at about 25°. Drift then again 

 conceals the rock for about another 80 yards, beyond which the 

 annexed section is seen. 



Fig. 1. — Sketcli of section on sliore below Treborth. 



At the top are four or fi.ve feet of red and green, soft, flaggy 

 sandstones, very micaceous, and of a tj'pe common in the Carboni- 

 ferous rocks ; passing down into about two feet of pebbly sandstone 

 or conglomerate, which rests upon the same reddish fissile shales as 

 ■"hose above described. The junction is only a foot or two above 

 'o;h-water mark, to which the sandy beds descend in a few yards, 

 " which they form the whole cliff and foreshore, the shales not 

 ^earing so far as I have been able to see. In this section the 

 beds are practically horizontal, while the shales dip E.S.E. at 



■iri^ated to the British Association, Bristol, 1898. 



'^erous rocks here seen will shortly, I hope, be described by him. 



