Vol. 63.] ORIGIN OF CERTAIN CANON-LIKE VALLEYS. 513 



Plate XXXII. 



Contour-map of the Trowbridge Plain, showing the course of the Avon 

 through the gorges at Bradford -on- Avon and Clifton to the estuary of the 

 Severn, on the scale of 6 miles to the inch. 



Plate XXXIII. 



Contour-map of the Eastern Plain and the surrounding region, showing the 

 general distribution of the Pleistocene deposits, on the scale of 6 miles to the 

 inch. 



Plate XXXIV. 



Contour-map of the Oxford Plain, with the Jurassic and Cretaceous escarp- 

 ments and the surrounding region, on the scale of 6 miles to the inch. 



Plate XXXV. 



Sections from the west of the Jurassic escarpment, through the Vale of 

 Aylesbury to the Chilterns and the Thames Valley ; from Edge Hill across the 

 Oxford Plain to the Chilterns ; and from the Valley of the Avon to that of the 

 Thames, all on the (horizontal) scale of 5 miles to the inch. 



Discussion. 



Prof. W. W. Watts referred to the fact that the Severn system 

 consisted of two distinct parts, each practically a complete river- 

 basin, with mountain-, valley-, and plain-tracts, connected by the 

 Ironbridge Gorge. This phenomenon had been explained by 

 Prof. Lapworth (a short account having been published by the 

 Geologists' Association in 1898) by the theory that the Upper 

 Severn and Vyrnwy had drained towards the Dee before Glacial 

 timl$, but had been, during the Glacial Period, diverted into a new 

 track across the continuation of Wenlock Edge at Ironbridge. 

 Prof. Lapworth had used this explanation to account for the re- 

 juvenation of the tributaries which flow into the Severn below the 

 Ironbridge Gap. The diversion of this great body of water into 

 the small valleys originating on the dip-slope of Wenlock Edge, 

 cut down one of them into an abrupt gorge, the steepness of which 

 accentuated the cutting-power of the tributary streams in that part 

 of Shropshire, Staffordshire, and Worcestershire. 



The Eev. E. C. Spicer said that the paper raised many points 

 for discussion which could not be briefly dealt with. It was quite 

 possible that the gravels of the Oxford region did not come through 

 the Evenlode at all, but from another direction. If the Chiltern 

 gaps were considered in relation to the valleys, they might both be 

 found to have a common cause, and not to be necessarily due to 

 overflow from an Oxford lake, for which there was not yet sufficient 

 evidence. He considered the Witham to be simply a beheaded 

 stream, and not the trail of an old lake-outlet, since the upper 

 Trent made straight for the Lincoln Gap. 



The Author said that he was much gratified to learn that, in 

 Prof. Lap worth's opinion, the erosion of the gorge at Ironbridge 

 took place in Glacial times. The features of the isolated areas of 

 depression and of their outlets, discussed in the present paper, so 



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