lO THE NEW RAILWAY BETWEEN UPMINSTER AND ROIMFORD. 



spot becoming merged in a vague slope in the course of a few yards. 

 But, on the other hand, the position of the highest gravels towards 

 the northern edge of the broad plain, the very gradual descent thence 

 towards the present channel of the river, and the whole aspect of the 

 valley, is simply that of a district in which the Thames has been 

 slowly cutting its way laterally from north to south, and vertically 

 from a higher level to a lower one. The absence of well-defined 

 terraces is entirely caused by the softness of the strata in which 

 they, have been cut, and occurs, as I can personally testify, wherever 

 a river has cut its way through soft and incoherent beds such as 

 London Clay. But where the same stream has been eating its way 

 through harder rocks, distinct terraces appear. Thus, the Geologists' 

 Association, in July, 1891, visited Henley-on-Thames. There, on 

 an eminence known as No Man's Hill, the members stood on a 

 gravel terrace overlying Chalk, 315 feet above Ordnance Datum, and 

 210 feet above the Thames at Henley, and saw, most clearly and 

 sharply cut, in the distance, at Remenham, another gravel terrace cut 

 out of the Chalk, a little more than 100 feet lower than that on which 

 they were standing. 



Similarly in the low, drift-covered districts of Cumberland, near 

 the Solway, I found it impossible to map the terraces on the left 

 bank of the Eden below Carlisle, because they were but vaguely 

 indicated h6re and there, precisely like those of southern Essex. 

 But a few miles away, on the Esk at Longtown and Netherby, 

 two or three clearly marked terraces could be traced throughout 

 their course. The explanation of this difference was to be found 

 in the facts that, on the Esk, a soft sandstone underlying the Glacial 

 Drift existed to some height above the surface of the stream, and the 

 terraces were cut in it ; while on the Eden, at the spot mentioned, the 

 Glacial Drift came down nearly to the water's edge. Consequently 

 the terraces cut in it had become as vague and doubtful, and as 

 impossible to trace for any distance, as those in the London Clay 

 of the Lower Thames. But the mapping of alluvial flats and river 

 terraces is part of the duty of a worker on the Geological Survey, 

 while it is a matter unlikely to attract the attention of most other 

 geologists. 



The second point on which I wish to touch is this. Previous 

 to the discovery -of Boulder Clay beneath Thames Valley Gravel at 

 Hornchurch, the most southerly exposures of Chalky Boulder Clay 

 known were those north of Rojnford and at Finchley. In each 



