228 STEATIGRAPHICAL GEOLOGY 



sandy marls with layers of hard calcareous nodules which some- 

 times coalesce so as to form beds of compact red or green limestone ; 

 while in other cases the embedding marl seems to have been 

 washed away by contemporaneous erosion, leaving the nodules to 

 form a " cornstone conglomerate." The nodules themselves consist 

 of amorphous carbonate of lime, and nothing but obscure traces of 

 plants have been found in them. They have probably been formed 

 by the agency of lime-secreting algas. 



The Senni Beds only occur along the northern outcrop, and 

 gradually thin out westward, disappearing a little before the 

 Brownstones. Their maximum thickness is 1200 feet, and they 

 have been included in the lower series because the remains of 

 Pteraspis have been found in them. Both Pteraspis and 

 Cephalaspis have been found in the Eed Marls, and consequently 

 it may be inferred that this Lower Old Eed is the equivalent of 

 the Lower Devonian and that there was some water communication, 

 by river or inlet, between the two areas of deposition. 



During the resurvey of the Gower district, in the south-west 

 corner of Glamorgan, it was discovered that the underlying 

 Silurian rocks rose to the surface within the Old Red Sandstone 

 area and formed two small inlying tracts ; further that the 

 Cornstone Series, though present, thinned out within the area 

 against the slope of the Silurian rocks. Thus some 300 feet of 

 red marls are visible below Rhossili Down, and are overlain by 

 Brownstones which are succeeded, without any sign of uncon- 

 formity, by 300 feet of quartz-pebble conglomerates (see Fig. 77). 

 On Cefn-y-Bryn, however, the red marls are absent, and Brown- 

 stones (only 100 feet thick) rest on the Silurian and are covered 

 as before by the conglomerates. 16 



Still farther west, however, in Pembrokeshire the Eed Marl 

 Series regains its normal thickness, being over 3000 feet along its 

 southern outcrop and nearly as much on the northern side. If, 

 therefore, the Gower ridge was continued westward it must have 

 passed some distance to the south of Milford Haven. 



Upper Old Bed Series. The Brownstone Group consists 

 mainly of sandstone, bright red or reddish brown, with subordinate 

 beds of marl and occasional cornstones. In the area round 

 Brecknock, where they form the lower slopes of the Brecon Vans 

 and the Black Mountains (see Fig. 76), it may have a thickness 

 of about 2000 feet, but this decreases in all directions. As already 

 stated, they thin out entirely to the westward and do not occur in 

 Pembrokeshire. No fossils of any kind have been found in them. 



The highest group of beds is never more than 500 feet thick ; 

 it commences with a conglomerate composed in most places of 



