Vol. 70.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxXXYU 



gradient, traversing the declivity in a steep-sided valley which has 

 perhaps an approximate average depth of 100 feet and a width of a 

 quarter of a mile. Here, therefore, appears to be a post-Glacial 

 excavation, of which the capacity may he measurable. 



The river, however, on reaching the Boulder-Cky plain was greatly 

 weakened in transporting power, and was unable to roll its burden 

 of gravel farther. The contents of the gorge were consequently 

 shot on to the low ground and there spread abroad. The fan thus 

 formed extends outwards from the foot of the declivity for about 

 2 miles, and occupies an area of between 3 and 4 square miles. It 

 will be seen that the cubic contents of the delta, plus the amount 

 of fine sediment which the river could cany off in suspension, 

 should equal the cubic capacity of the gorge. 



I have mentioned this case, not because I have founded any 

 calculation upon it myself, but because it forms an example where 

 two estimates could be made, and checked one against the other. 

 It w T ill be remembered that a calculation of the amount of material 

 removed by the Dee under similar circumstances, in the process of 

 excavating its post-Glacial valley below Cefn Viaduct, has been 

 made by Mr. Wills in his paper on ' Late Glacial & Post-Glacial 

 Changes in the Lower Dee Valley.' l If the Alyn gorge were to be 

 used for the purpose of estimating post-Glacial time, the rapidity 

 with which the river is now working, and has worked in the past, 

 would have to be determined and estimated. It is still adding to 

 the delta, but its operations are increasingly retarded by the fact 

 that it has cut down to the rock in several places, and that its 

 gradient becomes less steep in proportion as excavation proceeds. 



There are, moreover, many deltas throughout the country which 

 mierht furnish data for estimating the rate at which denudation is 

 proceeding. Many of them are being formed contemporaneously 

 with alluvial deposits and inosculate with them in such a way as to 

 be partly inaccessible for measurement, while others are being 

 pushed forward under water. Deltas of the former kind abound, 

 but become specially noticeable where a tributary enters a steep- 

 sided flat-bottomed valley. In the Vale of Neath, a deep trench 

 which traverses the South Wales coal-field from north to south, 

 every tributary stream has shot its delta on to the alluvial meadows. 

 For the most part these gravel-fans present a symmetry and an 

 appearance of finish, as though they had ceased to grow. They are 

 clothed, too, with vegetation, and even partly built over, but it is only 



1 Q. J. G. S. vol. lxviii (1912) p. 188. 



