486 ME. E. W. HARMER ON THE [Nov. I907, 



We may assume, I think, that the suggested Jurassic covering 

 would have been removed more or less contemporaneously from 

 the Trias and from the Carboniferous Limestone, but that the 

 subsequent denudation would have gone on more rapidly in the 

 former case. It seems to me, therefore, more probable that the 

 excavation of the Triassic depression which extends southwards 

 from Tytherington was effected continuously with the Elax-Bourton 

 valley (apparently its natural outlet towards the Bristol Channel), 

 and at a comparatively early stage (being in that case older than 

 the Clifton cafion), than that these closely-connected and similar 

 areas should have originated as subsequent valleys during the 

 gradual deepening of the latter. 1 



The narrow gorge cut through a low ridge of Carboniferous rocks 

 near Henbury, mentioned by Prof. Lloyd Morgan, may also be an 

 instance, on a small scale, of Glacially-diverted drainage. Unless it 

 had been obstructed in some abnormal manner, the Henbury rivulet 

 should rather have taken a short and easy course to the Severn 

 through the low-lying Triassic district to the west of that place. 



Although the question cannot be regarded as free from doubt, 

 it seems to me that the origin of the Clifton canon may be best 

 explained on the view here taken. The latter may at least offer an 

 alternative hypothesis for further consideration. 



VI. The Gaps in the Jurassic Escarpment at Lincoln and 

 Ancastee. (See map, PI. XXXIII.) 



It may be interesting to enquire whether the origin of these two 

 well-known gaps, which connect, in an apparently artificial manner, 

 the basins of the Trent and the Lower Witham, may be explained 

 on the theory adopted in the cases before dealt with. 



All the drainage of the great plain stretching from Staffordshire 

 to the Tees, with the single and anomalous exception of the River 

 Witham, directs itself towards, and eventually falls into, theHumber. 

 The Pennine rivers south of the Tees, among which the Don and 

 its affluents should be especially mentioned, trend towards the same 

 point, and may probably have done so since the valley-system of 

 that region was first sketched out. 



This plain, which extends northwards and continuously to the 

 mouth of the Tees, follows the strike of the Trias and the Lower 

 Lias, formations which in other parts of England have especially lent 

 themselves to erosion. It forms an important and, I think, a com- 

 paratively ancient feature of the physiography of this region. 2 



During the Glacial Period its condition seems to have been more 

 or less similar to that of the present ; a sheet of Boulder-Clay bedded 



1 The deepening of the area nearest to the entrance of the gorge, a feature 

 common to all the cases here discussed, was due, I suggest, to lacustrine 

 denudation. 



2 The present paper deals with the physiographical conditions of Glacial or 

 pre-Glacial times only, and not with those which may have obtained at a more 

 remote period. 



