THE NORTHERN DRIFT ENTERS INTO NO PART OF SILURIA. 



529 



taining a variety of granite pebbles mixed up with a large proportion of the adjacent 

 Welsh rocks, including the coal measures ; and the same distribution of materials pre- 

 vails to Oswestry, where the southern course of the drift seems to have been checked by 

 promontories of the carboniferous rocks. 



Let the reader bear in mind, that from Oswestry to Shrewsbury there are no con- 

 tinuous ridges, ranging from west to east, which could oppose the southward course of 

 this drift. On the contrary, the ridges of Silurian rock trending from the south-west, 

 expose their north-eastern extremities, leaving between them a succession of longi- 

 tudinal valleys which are open to the tract which is strewed over with the granite 

 bowlders. The most striking of these valleys is that of the Severn, which river, issuing 

 from the Welsh and Silurian regions in a northern direction, enters the country covered 

 by the northern drift, and winding through it to the Iron Bridge, then traverses a 

 gorge at the north-eastern extremity of Wenlock Edge. (See Map.) Now, if the pre- 

 sent relative positions of the Silurian and northern drifts had existed during their ac- 

 cumulation, or if Siluria had not been then at a higher level than the submerged 

 district, the northern drift must have been propelled far into the low depression from 

 which the Severn issues into the plain of Shrewsbury. But though this drift lies on 

 contiguous heights, several hundred feet above the Welsh Pool valley, not a pebble of 

 it can be traced southwards into the low grounds beyond the Breidden Hills, all the 

 detritus there being exclusively of Welsh origin. Again, the longitudinal 1 valleys on 

 the sides of the Long Mountain, between the Longmynd and the Caradoc, or between 

 the latter and the Wenlock Edge, all exhibit the same phenomena. Their northern 

 and north-eastern extremities being low, are consequently open to this line of drift, yet 

 they are wholly exempted from any portion of its debris, which is lodged in such great 

 quantities upon the opposite slopes of the Wrekin, Haughmond Hill, &c. The fair 

 inference, therefore, is, that great changes have taken place in the relative levels, and 

 that the Silurian region was above, while the tract occupied by the northern drift lay 

 beneath the sea. In other words, while the country about Shrewsbury was sea, the 

 Severn of that epoch must have terminated at the Breidden Hills, by emptying itself 

 into a bay, in which the northern drift was accumulating. 



This hypothesis is further sustained, by tracing the present line of the Severn to the 

 south, after it has escaped from the gorge of Bridgenorth. Throughout this portion of 

 its course, the river flows in the same direction as the great northern drift ; and thus 



gravel around Wrexham ; stating, that from its structure, composition and method of deposit, it must have 

 been formed by very long -continued subaqueous action, and is wholly unlike the result of a transient inun- 

 dation. His views are, therefore, quite in unison with my own. 



1 By longitudinal valleys is meant those which range parallel to the ridges or general strike of the mountains. 

 The country is now drained by rivers flowing through valleys or gorges transverse to these ridges. (See Map,) 

 Example the Severn at Coal Brook Dale j the Teme at Downton, Ludlow, and Knightwick Bridge ; the Lugg 

 at Aymestry ; the Wye between Ross and Chepstow, &c. 



