Mol. 58. | THE RIVER-SYSTEM OF SOUTH WALES. 209 
the form of a wedge faulted down to a depth estimated by Mr. 
Gibson at about 900 feet, and unite to form the Taff near Merthyr 
Tydfil. The Taff passes without deflection both the steep central 
anticline of the Coalfield at Pontypridd, and the Caerphilly syncline 
to the south of it, escaping from the Coalfield by a narrow gorge 
at Walnut Tree. Thence its course lies on Keuper Marl. 
Some tributaries of the Taff, known as the Cynon and the Rhondda 
Fawr and Rhondda Fach, rise within the northern margin of the 
Carboniferous area, and join the main stream at Abereynon and 
Pontypridd respectively. While flowing almost in the direction 
of some north-north-westerly faults, they do moe actually follow 
any one of them. 
Some small streams west of the Taff deserve a brief mention. 
The Ely rises on the north side of the central anticline*of the Coal- 
field, and crosses it, steep though it is. It leaves the Coalfield 
by a break in the escarpment at Llantrissant, which is probably 
of pre-Triassic age, and then flows eastward to Cardiff over the 
Secondary rocks which occupy the hollow of the deeply-denuded 
Cardiff anticline. The Ogwr, Garw, and Llynfi rise in the Coal- 
field north of the central anticline, the southern limb of which is 
here replaced by a great east-and-west fracture known as the 
Moel Gilau Fault. They unite to form the Ogmore, and pass the 
‘Carboniferous escarpment by a broad gap, probably of pre-Triassic 
age. Other smaller streams follow a similar course, the high 
Pennant land to the south of the fractured anticline offering no 
-obstacle to their assumption of a southerly direction. 
It may be remarked of the rivers described in the foregoing 
paragraphs, that they show parallelism in a south-south-easterly 
-direction, and that their courses are maintained regardless of the 
structure of the rocks over which they flow. That they are not 
diverted by the great east-and-west fiexures, such as the anticlines 
and synclines of the Coalfield, is too obvious to need further proof, 
but their independence of the north- north-westerly system of faulting 
is not immediately apparent. 
The faults of the north-north-westerly system form a conspicuous 
feature in the structure of the district, and recur at frequent and 
fairly regular intervals throughout the Coalfield, where their 
-courses and effects are known with considerable certainty. Out of 
so many faults and valleys, agreeing approximately in direction, 
it might have been expected that several would coincide. Examina- 
tion of the maps’ will show, on the contrary, that coincidence of 
valley with fault is rare. Of the nine or more long ridges of 
Pennant which divide the valleys enumerated above, more than 
half are cut longitudinally by north-north-westerly faults; but 
not one of the valleys follows a fault, though they sometimes 
cross the faults at an oblique angle. The want of coincidence is 
equally marked in the Lower Carboniferous escarpments: neither 
of the Taff rivers has utilized a fault in passing the northern 
+ The area under consideration is included in the Ordnance Survey (New 
Series) l-inch Maps 230, 231, 232, 247, 248, 249, 262, & 263. 
