VOL. XIX. (2) 
THE SILURIAN INLIER OF USK 
131 
of Wenlock beds, with perhaps a very thin layer of Ludlow 
rocks above them. Hence considerably greater thickness 
of Ludlow and Wenlock beds is seen at Usk than in either of 
the neighbouring Silurian districts. 
The Old Red Sandstone between the Coalfields of S. Wales 
and of the Forest of Dean is bent into a great arch, which has 
been fractured in parts and has had its crest denuded away 
between Panteg and Usk so that the Silurian beds have been 
exposed. 
There are to be seen three inkers of Silurian rocks, a large 
one, called the Usk Inlier, which measures 8 ^ miles from 
S.S.WC to N.N.E and 4 miles from W. to E. at its broadest part, 
and two quite small ones to the South-East in the neighbour- 
hood of Llanfrechfa. 
The Usk inlier is roughly oval in shape, and is cut through 
by the broad valley of the Usk river from the North and by 
the Berthin Brook from the East. 
Near these streams there is alluvium covering the Silurian 
beds, and glacial drift completely hides the underlying rocks in 
much of the northern part of the district, and extends south- 
wards as far as the Berthin Brook. 
II. GENERAL SUCCESSION and STRUCTURE. 
The Silurian rocks seen in the Usk Inlier are as follows : — 
Ludlow Beds. 2. Sandstones. 
(1300 feet thick) i. Impure sandy shales containing thin calcareous 
layers, especially towards their base. 
^\'E^■LocK Limestone in bands with sandy partings, with two thick beds 
(35-40 lect thick) of hard massive limestone near the base. 
Wexi.ock Shales 2. Upper shales, brown in colour, of a sandy nature 
(about 850 feet seen) with sandstone bands near their summit and 
with calcareous bands and nodules near their 
base. 
I. Grey mudstones. 
Detailed mapping on the six-inch scale shows that towards 
the South the Inlier is formed of two anticlines separated l)y 
a fault (see kg. i). The western fold, which is the wider of the 
two, will be called the Coed-y-paen Anticline, and the narrower 
eastern one the Llangibby Anticline. If one starts in the extreme 
South at Llandegvcth and goes nortliwards along the crest of 
