﻿Vol. 6 I. ) OP THE NORTHUMBERLAND AND DURHAM COALFIELD. 87 



that the borings proving the rock at only 4 feet below sea-level did 

 not give a true maximum ; but it may be safely concluded that no 

 important pre-Glacial valley exists at this point, there being no 

 such indication of its higher reaches in the more elevated ground 

 to the west, as there is in the case of all the others. Although the 

 Biver Tees lies somewhat outside the area under discussion, it 

 should be noted that the rock-surface there has been proved to lie 

 90 feet below the present sea-level (8). Consequently, the depth 

 of its pre-Glacial course approaches that of the Tyne. This lends 

 considerable confirmatory evidence to the view of the origin of the 

 pre-Glacial valleys taken in this paper. 



The borings here discussed and the evidence obtained in the 

 field appear to make it certain, that in the age immediately 

 preceding the Glacial Period the whole surface of the great Northern 

 Coalfield had been denuded by a series of eastward-flowing streams, 

 which produced the pre-Glacial valleys of which the Tyne, Wear, 

 ' Wash/ and i Sleekburn ' are the principal remnants. The course 

 and connections of these have been discussed ; and although the 

 present slope of the rock-surface may not be always such, as on 

 this hypothesis it should be, the whole structure of the valleys, as 

 well as all the field-evidence, is in support of this view of their 

 origin. It is also what we should expect from the hypothesis 

 regarding the development of the rivers of the East of England, put 

 forward by Prof. W. M. Davis (9) ; and it is in accordance with 

 the ideas of Mr. Jukes- Browne, regarding the evolution of the 

 British Isles in Tertiary times (7). Eurther, considerable support 

 is given to it by the work embodied in the ' Geological History of 

 the Rivers of East Yorkshire,' by Mr. Cowper Reed (13), whose 

 main conclusions are in agreement with those of the Yorkshire 

 geologists (14). 



If the courses of the pre-Glacial valleys are compared with those 

 along which the present rivers flow, it will be seen that changes of 

 considerable magnitude have taken place in the number, direction, 

 and connections of the rivers that flow over the area under 

 discussion. 



The valleys of pre-Glacial development are broad, and their sides 

 slope gently upward from the thalweg, thus bearing testimony 

 to the long period during which the forces of subaerial denudation 

 must have acted, while they were being produced. The valleys, 

 however, which have been developed since the Glacial Period are 

 deep and narrow, except where they correspond with those that 

 were already in existence, and have been denuded comparatively 

 quickly by stream-action, chiefly during and after the uplift that 

 produced the raised beaches. Subaerial denudation has had but 

 little effect in altering their contours. The tests for a valley 

 developed through the rock after the Glacial Period, are that it is 

 narrow, straight, and (in the district under consideration) deeply cut, 

 and that it has no Boulder-Clay at its base, although the clay may 

 occur on the top of the rock through which the valley has been 



