2S4 S. S. BUCKMAN [april 



less time for the work. The probable explanation is that the date of 

 capture was not very different, and that the Usk-[Avon] valley was 

 originally the deeper, having received the larger drainage ; and so it 

 gave the obsequent stream a better chance. 



The Bristol Avon has become the longest obsequent branch of the 

 Severn north of the Mendips. In working back it takes off the waters 

 of the Kennet, and prevents much westward growth of the west sub- 

 sequents of the Salisbury Avon. 



The long eastward course of several Avon tributaries, especially on 

 the south side, points to former eastward flow of the Avon's predecessor. 

 The tributaries join the present Avon in more or less opposition to its 

 stream. 



After the Monnow lost its head to the Middle Wye it is possible 

 that it was itself captured by a subsequent branch developed north- 

 wards from its then stronger companion the Usk-[Avon]. And when 

 the Usk group was captured by the Severn, this subsequent, which could 

 give a much quicker fall, continued to grow northwards to Hereford, 

 and captured the Wye group, thus beheading the Stroud valley stream. 

 This subsequent w r ould have been developed in Mesozoic rocks from 

 about Chepstow, somewhat along the line of the present Lower Wye ; 

 but, probably, the difficulty of cutting through Carboniferous Limestone 

 prevented the Severn working back in that direction, and it found it 

 easier to break through the Palaeozoic barrier at Tortworth, 1 whereby 

 it gained the Forest of Dean drainage. 



The remarkable horse-shoe curves of the Wye can scarcely be 

 accounted for by this history. They had perhaps been previously 

 developed in an earlier river flowing over a low-lying area of Carboni- 

 ferous Limestone during Jurassic times. The valley of this river 

 was subsequently buried beneath accumulating Mesozoic deposits. 

 After the capture of the later stream this infilling was easily 

 re-excavatecl. 



It was pointed out to me by the late Edward Wilson that the 

 Clifton gorge of the Bristol Avon was first excavated in Triassic times, 

 because it contains Dolomitic Conglomerate ; therefore the lower part 

 of the Bristol Avon occupies an old pre-Jurassic channel. A similar 

 history may attach to the present Wye. 



As the Severn captured the Forest of Dean drainage it started 

 obsequent streams towards Dursley and Stroud. The Stroud obsequent 

 — the Frome — has gradually abstracted the head-waters of the true 

 Thames. Very noticeable is the elbow of capture, where the eastward 

 working- back Frome has broken into small south-eastward flowing 

 Thames consequents, and directed them to itself. 



According to the theory the Frome should not be so important an 

 obsequent as the Bristol Avon, because it has not had so much time to 

 work back since its predecessor was captured,— and it agrees. 



1 That it thereby followed the strike of weak strata may have been an important factor. 



