Vol. 58.] THE ORIGIN OF THE RIVER-SYSTEM OF SOUTH WALES. 207 
13. On the Ortein of the River-Sysrem of Sour Waues, and its 
Connection with that of the Severn and the Tuames, By 
AuvsBrey Srranan, Esq., M.A., F.G.S.* (Read February 26th, 
1902.) 
[Prare V—Map. | 
ConTENTSs. 
Page 
Pe bROGUCEON 2.25 Bice ee oN. Se eee ok al Meetefelsis toned ca eeamemedt clas 207 
II. Relations of the Valleys to the Geological Structures ......... 208 
HH CheAce and Character of the Structures) 5.3.2 5-42..¢6s-2...2955 212 
IV. Determination of the River-System by a Caledonian Move- 
DIVA Gi se eg crac ware Seg ICE oe ste ain 8 seinhclo rote va tC Raat nae eon 216 
V Connection with the Course of the Severn, and of the Thames 
EARG| EUW OUTLET) ie At RP ae ey APRS cee par RRP Ar Se 219 
VI. Date of the Initiation of the River-Systems ..................... 220 
I. Iyrropvuction. 
Tur Welsh rivers, of which I shall endeavour to trace the history 
in the following pages, flow through a hilly region composed almost 
wholly of Paleozoic rocks. The greatest height is attained in 
the Old Red Sandstone of the Brecknock Beacons, which fall but 
little short of 3000 feet; but the Coal-Measures also rise into 
escarpments exceeding 2000 feet, and form large areas of mountain- 
land with an elevation of 1000 feet or more. This high ground is 
trenched by a series of valleys, of no great length, but important 
from their depth and from the fact that they penetrate a busy 
region otherwise difficult of access. 
A watershed map of Wales presents at first sight the appearance of 
a meaningless mosaic, and it will be convenient to commence by dis- 
entangling its leading features. A main water-parting, separating the 
westward- from the eastward-flowing rivers, runs from the northern 
coast of Pembrokeshire near Fishguard, through the counties of 
Cardigan and Montgomery. It will be noticed that it commences 
with an east-and-west trend, but curves gradually northward in 
Central Wales, keeping near and almost parallel to the west coast. 
Westward from this water-parting there runs a series of short 
streams into the Irish Sea; eastward from it flow the Severn and 
the Wye, both of which traverse Wales in a south-easterly direc- 
tion, but then turn south-westward to the Bristol Channel. South of 
the Wye we come to a group of rivers rising on subsidiary water- 
partings in the counties of Brecknock, Monmouth, and Glamorgan, 
and three which traverse the counties of Carmarthen and Pem- 
broke from the Central Wales water-parting southward. It is 
these South Welsh rivers which I propose to discuss in detail. 
Taken in order from east to west, they include the Usk, with its 
1 Communicated by permission of the Director of H.M. Geological Survey. 
