DIRECTION OF SILURIAN DRIFT FROM NORTH-WEST TO SOUTH-EAST. 511 



the level of the sea. The nature of the excavation indicates also the action of water dif- 

 ferently propelled at different times, perhaps by tidal currents, the directions of which 

 were determined by local causes, as the deposition of banks of sand or partial elevations 

 of the bottom of the sea. 



The tract thus exempted from foreign drift, is, in great measure, circumscribed by the 

 course of the Severn 1 . This most magnificent of the British rivers, rising in the moun- 

 tain of Plinlimmon, escapes from Wales into the plains of Shrewsbury by a northern 

 course, and traversing a ridge of Silurian Rocks and coal measures at Coal Brook 

 Dale, is deflected southwards to the sea, passing within a few miles of the Abberley 

 and Malvern Hills, the eastern limit of the tract under review. The drainage of the 

 country bounded by the Severn, is effected by a number of its tributaries, which 

 springing from the mountains of Shropshire, Radnorshire, and Brecknockshire, de- 

 scend into the lower districts of England by south-easterly courses, at right angles to 

 the direction of the chains which they intersect. It has been shown, indeed, that these 

 streams traverse the mountain ridges in channels produced by cross fractures of the 

 strata. The three principal rivers are the Teme, with its chief feeders the Onny and 

 the Clun ; the Wye, with its tributaries the Lug, the Ithon, and the Irvon j and the 

 Usk. All these streams flow from north-west to south-east, through ridges of Silurian 

 rocks and Old Red Sandstone, and empty themselves into the Severn. 



The Towy or great Caermarthenshire river, also the Taaf and the Cleddau streams 

 to the west of it, run southwards and south-westwards into the Bristol Channel through 

 lateral depressions in the Silurian and other rocks of South Wales ; and though flowing 

 in divergent directions from the Teme, the Wye and the Usk, they conform to the same 

 rule and have been regulated by the same cause. In short they all pass through fissures, 

 transverse to the direction of the strata ; for it has been shown, that in Caermarthenshire 

 and Pembrokeshire, an east and west strike being predominant, the great transverse 

 breaks or lines of drainage trend from north to south, (p. 406.) 



Within the area watered by these streams, not only the valleys but various elevated 

 combs, and basin-shaped cavities as well as the slopes and escarpments of hills, are 



1 In the annexed geological map no attempt has been made to distinguish by any peculiar colour the fine 

 alluvium from the coarse drift, it having been found impossible so to do without conveying erroneous impres- 

 sions. It was, in fact, impracticable to mark every spot in which gravel has been lodged so as to distinguish 

 marine drift from fluviatile and lacustrine gravel. The region covered by Welsh and local detritus only is, how- 

 ever, pretty nearly defined on the map, by the outline of the Cambrian, Silurian, and Old Red Systems. On the 

 other hand, the country coloured as New Red Sandstone, though also partially occupied by local detritus, is 

 that over which the northern drift has been spread. If the reader will convey his eye over the whole of the 

 region coloured in the map as New Red Sandstone and Lias,, and also over those coal-fields which are sur- 

 rounded by New Red Sandstone, he at once sees the region which has been the recipient of the northern 

 or foreign drift. On the contrary all the country to the west of the New Red Sandstone is exclusively covered 

 by local drift. The river courses mark the chief channels in which ancient deposits of lakes and rivers have 

 been accumulated. 



3 s 2 



