530 PROF. G. A. LEBOUR AND DR. J. A. SMYTHE ON [Allg. I906, 



25. On a Case of Unconformity and Thrust in the Coal- Measures 

 of Northumberland. By George Alexander Lebour, M.A., 

 M.Sc, F.G.S., Professor of Geology in Armstrong College ; and 

 J. A. Smythe, M.Sc., Ph.D., Lecturer in Chemistry in Armstrong 

 College, Newcastle-upon-Tyne. (Read April 4th, 1906.) 



[Plate XLII — Section.] 



I. Introduction. 



The object of this paper is to describe a case of unconformity, 

 accompanied by a certain amount of horizontal thrusting, which 

 occurs in the Coal-Measures exposed — chiefly at low water — on the 

 coast of Northumberland at the foot of the Whitley cliffs, and is 

 traceable for upwards of a mile. 



The section studied stretches from the bold headland formed by 

 the Table Rocks, 1 northwards almost as far as the Brier- Dene Burn. 

 At the southernmost end — that is, at the Table Rocks— the plane of 

 erosion in question dips and disappears beneath the sea. Near the 

 mouth of Brier Dene it is permanently covered and concealed by 

 Drift and beach -deposits. 



The Table Rocks consist of a series of ledges of sandstone, bedded 

 almost horizontally, and presenting vertical cliff-faces to the sea. 

 X little north of this the cliff becomes overhanging, owing to the 

 undercutting by the waves of the soft beds which come to light at 

 its base. 



Near the spot marked M on the sketch-map (fig. 1, p. 532), the 

 sandstone- cliff, which so far varies from 30 to 40 feet, gradually 

 lessens in height, and is overlain by a considerable thickness of 

 Boulder-Clay. Up to this point also the foreshore is free from cover, 

 though strewn with boulders, and in places obscured by talus-heaps 

 burying the cliff-foot, and the strata comprised in it can be well seen 

 at Ioav tide. Farther north, however, the boulder-strewn outcrops 

 give place to a broad sandy beach (Whitley Sands proper), which is 

 broken, near low-water-mark only, by a reef of sandstone. But, 

 after specially-heavy storms, the shifting sands, at rare intervals of 

 time, bring into view sometimes shingle, and sometimes solid rock. 

 It was after such a storm that an unusually-fine exposure of the 

 unconformity was observed and recorded by one of us more than 

 twenty years ago. 2 It is only by watching the coast, and seizing 

 favourable opportunities during many years, that most of the details 

 here given have been obtained. 



The uppermost deposit along this portion of the coast is the 



1 This name, although always used in the locality, has not, unfortunately, 

 been adopted by the Ordnance Survey. 



2 See 'Nature' vol. xxv (1881) p. 79 (November 24th). 



