Maw — On Sibbaerial aud Marine Denudation. 



445 



portant a part in their re-excavation. K a progressive series of land 

 contours superimposed on new surfaces of deposit are enabled to 

 faintly transmit, in 'part, their structure upwards (see Figs. 1 and 2) 

 to succeeding surfaces, each surface differing in its details from that 

 which precedes it, a complex structure of surface would be gradually 

 accumulated, the individual changes of surface being lost in the 

 denudation of surface through the superimposed strata, and the accu- 

 mulated result impressed on the single fresh denuded surface (ff), 

 of the older formation. 



"Y" -shaped Gullies and Vertical Gorges. — The sudden transition 

 that frequently takes place between the form of the immediate water 

 channel and that of the main valley containing it, is I think capable 

 of a satisfactory explanation compatible with a common cause. In 



Fig. 6. 



nearly all the mountainous valleys of Wales the immediate water- 

 channels are bounded by either vertical cliff-sides (as in the Aber 

 valley, and by the way, here you have a fine example of cliffs pro- 

 duced by river action) or by steep sides having a regular and uniform 

 slope, and in both cases joining into the general contour of the 

 valley by a hard and well-defined outline. It must be borne in mind 

 that on the theory of subaerial denudation as illustrated in Fig. 6, 

 not only the amount, but the rate of excavation is greater towards 

 the bottom, than the confines of a valley towards the watershed ; near 

 the watershed you have nothing but the rain that falls on the spot, 

 but as you proceed downwards a progressive concentration of water 

 and consequent power of excavation takes place, and in the actual 

 water-channel at its base there is a kind of force in its erosive inten- 

 sity which is unlike in its effect that of water distributed over the 

 surface. The progressive modification of the contour will be rej^re- 

 sented by the succession of dotted lines in Fig. 6, The waterflow 

 depression being lowered at a greater rate than the watershed, the 

 tendency towards the vertical will fixst commence at its very bottom. 



