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T. M'KENNY HUGHES OST THE GEOLOGY OE ANGLESEY.
21
The beds from Pwllgoch to Caemawr are probably Arenig (lower-
most Bala of some). At Pwllgoch we have the ironstone so charac-
teristic of Arenig, and at Caemawr we find Arenig Graptolites (see
p. 26).
Where the dykes cross the series at Llyseinion (fig. 3), sandy beds
begin to appear, and near Yr efail newydd predominate over the
shales, as seen in several large quarries near the road. The low
ground S.W. of this, by Plas Llandyfrydog, may be partly due to
faulted broken rock

for it is probable that for every fault I have
detected in this turf-covered ground I have missed 10 or may be 100.
The lowest sandstones seen pass down into the conglomerates of
y Eoel, E.N.E. of Llanerchymedd

and south of y Eoel, apparently
cropping out from below the conglomerate, there are green chloritic
schistose rocks, weathering olive and brown, which are probably
part of the Bangor beds (Pebidian). There is no rock exposed in
the valley immediately south of this, where the word Llanerchymedd
is engraved on the survey map. There is probably a fault through
here, with a downthrow on the south repeating the Cambrian con-
glomerates, which can be traced from nearly west of Pen cefn, through
the cottage near the junction of the roads from Plas Coedana and
Capel Penial, and sweeping round north of the windmill, and crossing
the road to Llanfihangeltre'rbeirdd obliquely, are seen, east of the
farm road leading to Tyddynbach, resting with a sharp line of junc-
tion upon schists similar to those mentioned above as cropping out
from below the conglomerate on y Eoel (see fig. 3). Near Pen cefn
the conglomerates pass up into fossiliferous flaggy grits and sand-
stones (see p. 25) .
Pig. 4. — Section from Mynyddmawr to Llanerchymedd and Pen cefn.
Llanerchymedd.
N.E
Pen cefn.
a. Sandstone and subordinate shale.
b. Ashy -looking brecciated con-
glomerate.
c. Sandstone with subordinate shale.
d. Sandstone.
e. Grit with annelid-tubes.
/. Quartz-jasper conglomerate.
g. Archaean.
If we now make a traverse from near Pen cefn (mentioned above)
S.W., about 1 mile to Mynyddmawr (see fig. 4), we leave the con-
glomerate and grit dipping westerly, and crossing the valley that
runs south from Llanerchymedd, find the brown sandstones in a
quarry near the n of Cerygydrydion dipping at about 75°, still
westerly. Near Cilgwyn, however, we find ourselves on the other
side of what is probably a broken synclinal, and in a quarry