Notes upon the Dlhivlal and Alluvial Deposits of the Taffe Valley. 2/5 



limestone, makes round pebbles, and flints or flint fossils make pebbles 

 according to tlieir original form or fracture. 



4. The geological character of the pebbles at any given place, will de- 

 pend primarily upon the rocks through which the river has flowed ; and 

 secondarily upon its ])owers of wear and tear. In the Taffe and its tri- 

 butary valleys above Castel Coch, the pebbles are all derived from tlie 

 sandstones of the coal, with an intermixture, not very copious, of quartz 

 conglomerate. They are tlierefore of all textures and colours; but, with 

 the exception of the conglomerate, all more or less fissile. The Rhonddha 

 runs nearly down a synclinal line; and the coal, the basset edges of which 

 are exposed by its waters, affords a plentiful sup|)ly of pebbles ; but these 

 do not travel far from their source, for three hundred yards down the 

 stream they become pulverised, and below that distance scarcely one is 

 seen. The quartz conglomerate occurs, both as rounded pebbles and as 

 semi-rounded erratic blocks. 



5. All pebbles that are not round, or that are more nearly flat than 

 round, will be found, if deposited by a running stream, to dip up that 

 stream. Thus all the great body of the pebbles in the valley of the Taffe, 

 and those subordinate to it, dip up their respective valleys, including also 

 those pebbles in the existing watercourses. Below the ravine, the pebbles 

 in the plain dip in various directions, but for the most part towards the 

 north. Pebbles on the sea beach dip towards the sea. It would be in- 

 teresting to compare the pebbles of a slow with those of a rapid stream, to 

 ascertain whether there existed any difference in the angle of their dip. 



AVheiiever a vertical, or even a horizontal section is exposed, the general 

 dip of the pebbles can be readily observed ; especially when many of them 

 are large and flat. 



With respect to the boundary of the diluvial gravel of this district ; be- 

 low the ravine of Castel Coch, it is bounded on the west by the right bank 

 of the Taffe, as low as Llandaff, wlience it extends nearly to Canton, and 

 terminates a little below Pwll Coch on the Ely, resting chiefly on rocks of 

 the new red sandstone series. From hence it extends just south of the 

 village of Canton, through Cardiff, towards Roath, being covered up on 

 this boundary by alluvial mud. Its eastern limits have not been so well 

 determined. 



II. We now arrive at the second division of our subject ; the debris by 

 which the sides of the mountains are covered up. 



Along the sides, upon the platforms, and in many cases ascending very 

 nearly to the summits of the sandstone mountains of the Taffe, lies a coat- 

 ing or mask of greater or less thickness, composed of sand, clay, and frag- 

 ments of rocks, sometimes with their acutcr angles broken off, but more 

 commonly as sharp as when broken from their parent rock. 



Those who are at all familiar with the connexion between the physical 

 features of a country, and the geological structure of its rocks, are aware 



