208 MR. A. STRAHAN ON THE ORIGIN OF [May 1902, 
tributary the Afon Lwyd, the Ebbw Fawr and Ebbw Fach, the 
Sirhowy, the Rhymney, the Taff, the Nédd or Neath, the Tawe, 
the Loughor, the Towy, the Taf, and the Cleddau. 
II. Revartons oF THE VALLEYS TO THE GEOLOGICAL STRUCTURES. 
The Usk takes its rise in the Old Red Sandstone and Silurian 
rocks near Brecon, and flows eastward at the foot of the great 
Carboniferous escarpment of the north side of the Coalfield. At 
Abergavenny it turns southward, and, meandering across the Usk 
inher of Silurian rocks, enters the Bristol Channel near Newport. 
It is to be noticed that it does not make for the synclinal region of 
Carboniferous rocks, but, on the contrary, cuts right across the area 
of great upheaval which has brought the Silurian rocks to the 
surface. 
The Afon Lwyd rises on the southern slopes of the northern 
escarpment of the Coalfield. It then maintains a south-easterly 
and southerly course, crossing the Blaenavon Fault without being 
deflected along it, and skirting the margin of the Coalfield in 
preference to following the dip-slopes towards the central syncline. 
Finally it escapes from the Carboniferous area by a gorge at 
Pontypool, aud passes close by a Silurian inlier on its way to join 
the Usk at Caerleon. 
The Ebbw Fawr rises in the northern margin of the Carboniferous 
area, and traverses the Coalfield in a direction slightly east of south. 
The Ebbw Fach originally followed a similar course, but having 
suffered the loss of its head-waters by the cutting back of the 
gorge of the Clydach, as described by Mr. Walcot Gibson,’ it now 
commences existence some 3 miles farther south. The two rivers 
unite at Aberbyg, traverse the central syncline, and, escaping 
through the Carboniferous escarpment by a gorge at Risca, join 
the Usk near Newport. 
The Sirhowy follows a parallel and similar course. It joins the 
Ebbw a mile above Risca, and to effect the junction makes a sharp 
turn eastward, instead of continuing a course which would have 
led it into the deep Caerphilly syncline. 
The Rhymney in its upper part resembles the Sirhowy and Ebbw, 
but it enters the Caerphilly basin, a deep trough both structurally 
and physically, and, turning eastward along it, traverses the Car- 
boniferous escarpment at Machen. At first sight the turn might 
be attributed to the syncline, but it corresponds to the turn made 
by the Sirhowy, which was away from the syncline, neither to it nor 
along it. The Rhymney then makes direct for a Silurian inlier near 
Cardiff, despite the fact that the strata have there been raised 7000 
or 8000 feet higher than in the Caerphilly syncline. 
The Taff Fawr and Taff Fach differ only from the rivers already 
described, in the fact that they rise on the dip-slopes of Old Red 
Sandstone to the north of the Coalfield. Flowing slightly east of 
south, they cross the Vale-of-Neath disturbance, which here takes 
1 Mem. Geol. Surv. 1900 ‘ Geology of Abergavenny’ p. 93. 
