LINES OF SUBMARINE DRIFT DEPENDENT ON ELEVATIONS 



521 



singularly void of drifted matter. The lines of most powerful dislocation are, however, 

 often obscured by heaps of rubbish, already alluded to in describing the remarkable 

 elbow of Silurian rocks and Old Red Sandstone near Cynic 1 . 



The facts recorded of ancient drifts, in positions not within the range of existing drain- 

 age, must have great weight in leading us to conclude that the coarse materials thus 

 strewed over the surface cannot have been accumulated by fmviatile action, but by bodies 

 of water which had certain directions given to them, and which have fixed relations to 

 the lines upon which the strata have been elevated. The Teme and the Wye, with 

 their tributaries, flow in the same direction with the marine drift, and hence the higher 

 sides of their valleys are naturally loaded with detritus derived from the slopes of the 

 elevated north-western ridges. But wherever the rivers quit the main course of this 

 ancient drift, we no longer find that they have upon their banks similar coarse materials. 



The Severn affords an excellent illustration of this rule. That river, rising in the 

 eastern slopes of Plinlimmon, first follows a devious course to the north-east, and 

 passes through Montgomeryshire in a longitudinal valley parallel to the strike of the 

 chain. Beyond Welch Pool it is increased by the united waters of the Ffyrnwy and the 

 Tannat, and then glides from the Welsh territory into the plain of Shrewsbury. Now, if 

 existing fluviatile action had been concerned in transporting the coarse drift and bowl- 

 ders, some portion of the materials would accompany the Severn in its course to the 

 north ; but not so : all such materials are left behind, in the tract whence that river 

 escapes, and without reference to any line of existing drainage, are strewed over hill 

 and dale in a south-easterly direction, having thus obeyed the great movement impressed 

 upon them by the elevation of the adjacent chains. 



1 Besides the detritus above described, Pembrokeshire contains a small singular deposit, to which my atten- 

 tion was directed by the Earl of Cawdor, namely, an accumulation of pipe-clay, chiefly white, but sometimes 

 striped red, or mottled like Castile soap. It alternates, and is in part mixed up with gravel, composed princi- 

 pally of pebbles of white quartz. This mass occurs about three miles west of Stackpole Court, and covers 

 about five or six acres of the carboniferous limestone of that promontory. It has been cut into to a depth 

 of 45 feet, 35 of which have, in some parts, proved to be of pure pipe-clay without a pebble. In other 

 parts the clay becomes nearly black towards the bottom. It makes excellent earthenware, especially when 

 mixed with about one third of the coarser clay. The whitest quartz pebbles, being selected and sifted, are 

 ground into powder, which being united with the clay forms excellent fire bricks. As the tract of Stackpole, 

 the southernmost promontory of Pembroke, is entirely free from other detritus, it seems difficult to account 

 satisfactorily for the origin of this patch of clay and gravel. On the whole, however, it so strongly reminded 

 me of the older tertiary deposits on the coasts of Hampshire and Dorsetshire (particularly in the composition 

 and aspect of the pipe-clay), that when on the spot I could hardly resist the persuasion that this accumulation 

 might be a relic of the same period*. The quartz pebbles, indeed, were evidently derived from the adjacent 

 conglomerates of Freshwater West, on the north; and, therefore, to whatever epoch we assign it, this matter 

 has been drifted in the same direction as all the other detritus of Pembrokeshire. 



* M. Elie de Beaumont wiU probably remark that this deposit, like those of Hants and Dorset, is on a line 

 of fissure proceeding from east to west. 



