Vol. 6^.^ OKIGIN OF CEETAIN CANON-LIKE VALLEYS. 487 



up to the western face of the Jurassic ridge covers the low ground 

 below the latter, extending northwards from the Lincoln gap for 

 nearly 20 miles (see map, PI. XXXIII). There seems to be no 

 difficulty, therefore, in believing that in pre-Glacial times the Trent 

 may have followed its present course towards the Humber, the point 

 to which the rivers of the district naturally converge. 1 



The existence of beds of gravel, principally composed of Triassic 

 pebbles, between Newark and Lincoln, has led JMr. Jukes-Browne 

 to the conclusion that the Trent formerly crossed the Jurassic 

 escarpment, flowing through the gap to the Wash, as the Witham 

 does now, the excavation of the gorge commencing, however, at an 

 earlier period before the erosion of the Triassic plain, when the 

 Trent ran eastwards at a higher level across a plain of marine 

 denudation. 2 Similar gravels, indicated on the map (PI. XXXIII) 

 by small dots, occur, however, north of Lincoln, also pointing 

 towards the gap but from an opposite direction. 



The gravels in question are at a much lower level than the crest 

 of the ridge through which the gorge has been cut. They afford no 

 evidence, therefore, as to the origin of the gap, although they 

 may indicate the direction of drainage which at a later stage flowed 

 through it. 



It may be pointed out that the Lincoln gap does not lead from 

 Newark to the Wash, the present course of the Witham through it 

 being an unnatural one. If the Trent had flowed eastwards to 

 the Wash at some remote period, before the Jurassic escarpment 

 had come into existence, it would have chosen, we may suppose, a 

 more direct route, and one farther to the south. 



It does not seem to me probable that, after following for so long a 

 distance the strike of the soft Triassic strata, the Trent should have 

 suddenly left them to cut into the harder Jurassic rocks, if it could 

 have continued its natural course towards the Humber. Moreover, 

 it is difficult to understand that this straight-cut, short, and 

 recent-looking notch in the escarpment can represent an isolated 

 portion of an ancient river-valley, older than the great plain itself, 

 which near Lincoln is more than 10 miles wide. 



The exposures of Boulder-Clay at Barnsley, and at Balby near 

 Doncaster, in which Lake-District erratics occur, seem to show 

 that an ice-stream from the Tees reached at one time to the south 

 of the Humber. 3 The crescentic mounds of Drift near York and 

 Escrick, formerly believed by Carvill Lewis and others to mark the 

 farthest limit of the Teesdale ice, are still regarded as its terminal 

 moraine, but accumulated at some stage of its final retreat. 



Forty years ago, moreover, S. Y. Wood, Jun., discovered a mass 



1 The fall from Lincoln to the Humber in pre-Glacial times was greater than 

 it is at present, borings in the neighbourhood of Hull showing Glacial deposits 

 to descend to a depth below sea-level considerably lower than the sub-Glacial 

 levels of the Lincoln gorge. 



2 Quart. Journ. Geol. Soc. vol. xxxix (1883) p. 606. 



3 See Eev. W. L. Carter, Proc. Yorks. Geol. Soc. vol. xv (1905) p. 411. 



