Vol. 58. | THE RIVER-SYSTEM OF SOUTH WALES. QD 1 
Oligocene, inasmuch as Oligocene strata have been uplifted by 
them in the Isle of Wight; they were pre-Pliocene, inasmuch 
as no Pliocene strata have been tolded into either syncline.’ The 
uplift of the Chalk, therefore, along the Caledonian axis, the position 
of which has been inferred by reference to the Chalk-escarpment, 
took place between the deposition of those formations. 
But in view of the close analogy which holds between the Cale- 
donian disturbances in South Wales and this Caledonian uphtt of 
the Chalk, not only in direction, but in their effect upon the river- 
drainage, these also may be inferred to have been contemporaneous, 
and the initiation of the whole of the river-systems 
referred to in this paper would have taken place be- 
tween the Oligocene and Pliocene Periods. 
Passing in review the principal movements which have affected 
the regions referred to in this paper since the Carboniferous Period, 
we may construct a chronological table as follows :— 
. | 
| Date. Character. | Direction. 
| 
| . | 
| Post-Carboniferous | Elevatory movement. Armotican. | 
Piero Eriassic: ........: | Impulse from the south. 
1 
MPs OG a2 Sona | Subsiding. Charnian. 
| 
re Eee | Hlevatory. Armorican. 
“| Impulse from the south. 
2, 
| Post-Cretaceous and 
| perhaps Eocene... } SS : Charnian. 
| Post-Oligocene...... | Elevatory. Caledonian. 
Pre-Pocene . 2.3.5... Impulse from the north. 
bs: {| Elevatory. Armorican. | 
| Do. do. ......... |. Impulse trom the south. | 
| 
Of all these, the post-Oligocene movements alone have 
had any direct influence in determining the existing 
river-systems. There may be reluctance in accepting this con- 
clusion, on the ground that it assigns a late date to great movements 
and to denudation on a sweeping scale. Not only, however, have 
movements as great or greater taken place at a still later date in 
other parts of the world, but we have in the Isles of Wight and Purbeck 
direct evidence of the date of the disturbances and of the great erosion 
consequent upon them. It should be remembered also that, of the 
vast mass of Paleozoic strata which has been removed from South 
Wales, by far the greater part was denuded in pre-Triassic times. 
Post-Oligocene denudation need be called upon to account for little 
more in that region than the removal of the Upper Cretaceous rocks, 
and the excavation of the valleys occupied by the existing rivers. 
1 C. Reid, Mem. Geol. Surv. ‘ Pliocene Deposits of Britain’ 1890, p. 69. 
Prestwich’s view, that the Chalk- and Oolite-escarpments were of Pleistocene 
age, has not met with acceptance. It was ably combated by Prof. Gregory in his 
paper on the Evolution of the Thames, ‘ Natural Science’ vol. v (1894) p. 97. 
