OF RHYMNEY AND PEN-Y-LAN, CARDIFF. 477 
23 miles), and from a point north of Llwyn-y-grant-uchaf to 
Ty-y-cyw, near Roath, on the south (a distance a little over a 
mile and a quarter). Besides this area there is another, which | 
have not yet mentioned, lying a little to the north of it, and which 
extends as a narrow band (one sixth of a mile wide) from Cefn- 
coed-isaf on the west to near Pen-twyn on the east, a distance of a 
mile and a quarter. 
General Structure. 
Pen-y-lan (fig. 2).—The southern side of Pen-y-lan hill commences 
rising from the bordering alluvial plain with avery gentle slope, owing 
to the soft and easily decomposable nature of the red Silurian shales 
and sandstones of which it is composed. ‘These are exposed in a 
ditch on the right-hand side of the road going up the hill, and are 
there seen to dip from the N.E. towards the 8.W. After a while 
the slope becomes a little steeper and harder grits begin to appear, 
Fig. 2.—Diagrammatic Section from Ty-y-cyw to Coed-y-gores. 
(Scale, 2 inches to a mile.) 
8.8.W, N.N.E, 
Cefn- Coed- 
Pen-y-lan Llwyn-y-grant- coed-  y- 
House. uchaf, isaf. gores, 
6 d @ ad c 
a. Gravel. e. Old Red Sandstone. 
d, Silurian. é. Rhymney grit? 
still keeping, however, a dip to the south, till just within twenty- 
five yards south of Pen-y-lan House the dip changes and turns over 
towards the north. Accurate measurements made in a ditch on the 
left-hand side of the road give a strike of 60° to 70° W. of N., with 
a dip first of 30° to the south and then of 25° to the north. 
Thus the hill is traversed by an anticlinal axis running from 
W.N.W. to E.S.E., the beds to the south of it dipping southwards, 
and to the north northwards. The northward dip continues as we 
proceed to the north-east, higher and higher beds of the Silurian 
being successively introduced till the Old Red Sandstone beds are 
reached. The dip then rapidly changes; and at the point marked 
on the Ordnance map by an arrow, near Coed-y-gores, Silurian beds 
are brought up by a low anticlinal, the axis of which follows the 
stream descending from Cefn-coed-isaf. At Coed-y-gores itself the 
Old Red Sandstone is typically present, as may be seen in the river- 
cliffs washed there by the Rhymney ; it dips towards the N.E., and 
continues to do so for some distance further N. 
Rhymney.—On the eastern side of the Rhymney the same arrange- 
ment as that just described is to be clearly made out; but the 
southern half of the Pen-y-lan anticlinal, which on the other side of 
the river has suffered extensive denudation, has on this wholly dis- 
