DEVONIAN AND OLD RED SANDSTONE SYSTEM l"J7 



B. STRATIGRAPHY OP THE OLD RED SANDSTONE 



I. IVales and the IVekh Borders 



In Mon mouth, Hereford, and Brecknock a great thickness of red 

 sandstones and marls intervenes between the Silurian and the 

 Carboniferous, and from these counties extensions of Old Red 

 Sandstone stretch westward through Carmarthen and South 

 Pembroke, northward into Radnor and Shropshire, and eastward 

 into Worcester and Gloucester. It is evident, therefore, that in 

 all these directions the formation originally extended considerably 

 beyond its present limits. According to recent estimates, its 

 maximum thickness in Brecknock may be 7000 feet, but from this 

 central area it thins in every direction, being only 4200 feet at 

 Abergavenny in Monmouth, and 3500 at Newport ; while north- 

 ward in Shropshire its thickness is believed to be 3700. 



The area over which the sandstones and marls of this system 

 still extend measures over 100 miles from west to east, and about 

 80 miles from Cardiff in the south to beyond Bridgenorth in 

 Shropshire. The system is naturally divisible into three parts or 

 series, which were long ago recognised by W. S. Symonds, under the 

 names of (1) Lower or Cornstone Series, (2) Middle or Brownstone 

 Series, (3) Upper Old Red. These divisions have been adopted by 

 the Geological Survey in their recent revision of South Wales, but 

 there remains much doubt whether the Brownstone Series repre- 

 sents any part of the true Middle Old Red. In the eastern parts 

 of the area the whole seems to form a conformable succession, 

 but when followed westward into Carmarthen the Brownstones 

 thin out near Kidwelly, and the conglomerate of the Upper Group 

 overlaps on to the lower series. They recur in Gower to the south, 

 but are only 100 feet thick, and seem to belong rather to the 

 Upper Old Red than to be an independent group. Hence the 

 arrangement of the Geological Survey is as follows : 



/"Yellow and red sandstones, grey grits, and quartz-pebble 

 Upper | conglomerates. 

 Series \ Brownstones. Red and brown sandstones, with occasional 



V. beds of marl and cornstone. 



T ^,, >,. /Green and dull-red sandstones with some beds of marl, corn- 

 uo\vcr or i j 



Cornstone-J 8 tone, and conglomerate. 



Series ! ^ e( * niar ^ s w i* n beds f nodular limestone (cornstone) and 

 {. occasional beds of micaceous sandstone. 



Cornstone Series. In Brecknock and the east of Carmarthen 

 this red marl and cornstoue series has a great thickness, possibly as 

 much as 4500 feet. The greater part of it consists of hard red 



