Mackintosh— On the Cliffs and Yalleys of 'Wales. 389 



latter. Further to tlie north there are several detached or nearly- 

 detached square pillars of rock, one of which is represented in Plate 

 XV. Fig. 1. At this spot the rocks show no sign of weathering ; but 

 in most places weathering has proceeded to a very considerable ex- 

 tent. Its effect, however, is to rum tlie diffs, and most assuredly not 

 to form them. It is precisely undoing what the sea has accomplished. 

 The line of demarcation between the rough, splintered faces of rock 

 invaded by weathering, and the smoothed and sculptured faces left 

 by the sea, and preserved in sheltered situations, is often clearly de- 

 fined. On the broad level platforms at the bases of the lines of cliff 

 there are here and there a few very large blocks which could no 

 more have fallen from the cliffs than from the moon. One of these 

 blocks, between twelve and fifteen feet long, rests on the outer edge 

 of a wide platform, and might easily be mistaken for a hloe percM. 

 It appears, however, to be simply an unscathed mass of mudstone, 

 nearly in situ, excepting that on one side the enlargement of a joint 

 has allowed it to slip down (See Plate XV. Fig. 3). 



The Abereddw cliffs vary in height from a few feet to twenty. The 

 platforms vary in breadth from a few yards to 160, and are generally 

 covered with silt or loam. The third platform, reckoning upwards, 

 inclines 3° transversely and 5° longitudinally. In several places a 

 line of cliffs turns round at nearly right angles, similar to what may 

 be observed on modern sea-coasts, and beyond the abrupt corners 

 the rocks are frequently the most smoothed and rounded. No agent 

 passing by could possibly have given rise to such phenomena. But 

 they are precisely such as the face-to-face action of the sea produces 

 on lines of cliff, irrespective of their direction. The mossy covering 

 on the above smooth and unweathered faces of rock is often quite 

 one -eighth of an mch thick. 



Marks left by the Sea.- — The most convincing marks left by the sea 

 on many parts of these cliffs consist of smooth curvilinear grooves, 

 and finely-graduated shallow pits, which have been formed independ- 

 ently of any peculiarity of structure in the rock. The most perfect 

 fac-similes of the grooves may be seen on the sea-coast at Aberystwyth. 

 The pits are likewise there represented, but not so much on the ver- 

 tical faces of the cliffs as on the rocks lying under high water. Both 

 are apparently the result of the motion of small stones wielded by 

 the waves as instruments of abrasion (See Plate XV. Figs. 4, 5, the 

 first representing pits and the second grooves on the Abereddw cilia's) • 



Between the Abereddw cliffs and Builth a large terrace extends 

 for some distance along the eastern side of the valley at a consider- 

 able height above the river, and, farther on, the remains of succes- 

 sive terraces may now and then be traced. At Builth the com- 

 paratively narrow gorge, through which the Wye has hitherto flowed, 

 opens into an irregular basin, fringed with cwms, which merges 

 into the great plain of Central Wales. 



Valleys excavated by Streams-. — On travelling along the above plain 

 towards Newbridge Station, I was finally led to admit what I had 

 previously suspected, namely, the probability of streams having 

 mainly excavated the uniformly-continuous V-shaped valleys that 



