Vol. 6$.~] ORIGIN OF CERTAIN CANON-LIKE VALLEYS. 485 



this is what the facts to be observed in the neighbourhood of Oxford 

 may lead us to expect. 1 



Tracing the course of the Avon towards the sea, we find condi- 

 tions of lake-atm-gorge recurring on a smaller scale near Bristol, 

 as they do to the west of Shrewsbury. Avoiding the Flax-Bourton 

 valley, the river leaves the Bristol plain abruptly, going out of its 

 way to cut laterally a narrow gorge across higher land (that of 

 Clifton Downs), as it did in an earlier part of its course near 

 Bradford-on-Avon. 



The Flax-Bourton valley follows a belt of Triassic marl, bounded 

 both on the north and on the south by Carboniferous rocks. Its ex- 

 cavation and that of the Clifton canon could not have been simul- 

 taneous, nor could the more recent of the two have been formed 

 while the river had an unobstructed passage through the older. The 

 tendency of a drainage-system, once established, is to become per- 

 manent unless interfered with by some external influence. 2 The 

 original course of the Avon may have been through the Flax- 

 Bourton valley 3 ; it is the more direct, and erosion would more 

 readily have taken place through Triassic strata than across the 

 harder rocks of Clifton. 



If the Flax-Bourton valley had been blocked by drifted snow or 

 the ice which, as before suggested, might have also existed in parts 

 of the Severn estuary at that stage, a lake would have been formed 

 with an overflow across the higher land, which subsequently found 

 its way to the Bristol Channel through or over the ice : having 

 once commenced to erode the Clifton gorge, the river must have 

 kept in it, as, indeed, it seems to have done — since, if it had 

 diverged to flow over the Triassic strata of Flax Bourton, it would 

 have cut a deeper channel in the latter and have there continued. 

 It seems improbable that the Avon could have first flowed through 

 the Clifton canon, forsaking it to excavate the Flax-Bourton valley, 

 and then returning to occupy for a second time its original bed. 



The Clifton gorge presents a distinct affinity to the Ironbridge 

 type (fig. 2, p. 473). Like the latter, it has no obvious connexion 

 with the natural drainage of the district, having been excavated 

 transversely through a narrow isthmus of Palaeozoic rock and cut 

 down to the level of the Triassic plains on each side. 



Prof. C. Lloyd Morgan, 4 however, following Jukes 5 and Ramsay, s 

 believes that it originated at a remote period when the Clifton 

 region was covered by Jurassic deposits ; but, on such a view, the 

 conditions under which the erosion of the Clifton gorge and the 

 low-lying Triassic areas close at hand took place should be discussed 

 together. 



1 The level of this ridge, however, may have been lowered at some 

 subsequent period. 



2 See also, as to this, Rev. E. Hill, Geol. Mag. 1903, p. 70. 



3 This view was suggested to me by Mr. E. H. Rastall. 



4 Proc. Bristol Nat. Soc. n. s. vol. iv (1884) p. 171. 



5 Geol. Mag. vol. iv (1867) p. 444. 



6 'Physical Geol. & Geogr. of Gt. Britain' 5th ed. (1878) pp. 511 et seqq. 



