70 Mackintosh— The Sea v. Rain and Frost. 



hard materials. A hollow excavated by rain-torrents would assume 

 more or less of the V form, and would necessarily shallow out towards 

 the summit of the escarpment. But in many of these combes the 

 innermost part of the surrounding cliif is the steepest, showing that 

 they must have been formed by an inwardly-directed cause. They 

 frequently display a geometrical regularitj'^, combined with other 

 unmistakeable indications, that they were literally scooped out by a 

 sweeping, undermining, and gyratory action. These semi-cup- 

 shaped hollows are not confined to the escarpment. They exist, 

 though in smaller numbers, on the south side of the Downs. About 

 two miles, on the left-hand side of the road leading from Brighton 

 to Lewes, a very deep and striking combe gradually opens to the 

 view of the traveller, which is as much a sea-cove as if it were still 

 washed by the spray. There is another called Kingiey Bottom, in 

 the neighbourhood of Chichester. In forming an opinion relative 

 to the origin of combes, it is advisable to retire to some distance, 

 so that their entire outline and connection with neighbouring phe- 

 nomena may be embraced at one view.^ 



In a future article I intend to consider the indications presented 

 by river-channels and valleys, of the limited extent oi fluviatile denu- 

 dation in England and Wales. From what has been already said I 

 think it must follow that rain and frost can only be justly regarded 

 as supplementing the denudation effected by the sea ; that their 

 capacity to lower the earth's surface is comparatively small, unless 

 immediately assisted by streams of sufficient transporting jDower ; 

 that the sea, by its laterally-excavating agency, and by uniting in 

 itself, at the same time and on the same spot, a power of detaching 

 and removing, can alone j)rove equivalent to the production of such 

 a series of escarpments, cliffs, mural precipices, rocky pillars, ter- 

 races, headlands, and combes, as those composing the more abrupt 

 inequalities of the surface of the earth. 



' I think it is difficult to conceive of an explanation more perfect, simple, har- 

 monious, and convincing than Sir Charles Lyell's marine theory of the denudation of 

 the Weald. I'hat the same cannot be said of the atmospheric theory -will, I think, 

 appear from the following extracts : — " The rate at which the escarpment will be 



worn back will depend on the rate at which the river deepens its valley The 



excavating power of the stream in the longitudinal valley will depend on that of the 

 transverse valley, and if the sea-level remains constant, the transverse stream will go 

 on deepening its bed and lessening its excavating power, until at last it ceases to 

 have any at all. A slight elevation of the land would once more give the tj-ansverse 

 stream an excavating power, which in time would be communicated to the longitudinal 



streams It must not be inferred, however, that we consider escarpments to be 



river-cliffs. The longitudinal streams, though running parallel to these escarpments, 

 do not run immediately below them, but often, as with the Medway itself, at a con- 

 siderable distance. No rnw -gravel in this area is ever found on the face of the 

 escarpment, nor can we discover thereon any traces whatever of river-action" (Foster 

 and Topley, Quart. Journ. Geol. Soc, vol. xxi.. No. 84, 1865, pp. 471, 47^?;). I do 

 not think that Dr. Foster and Mr. Topley have succeeded in showing that the gravel- 

 terraces in the basin of the Medway (containing Tertiary pebbles and Greyweather 

 fragments, and dipping lower than the inclination of the river) are of fluviatile 

 origin, or that a vertical extent of 300 feet has been scooped out by rain nnd rivers ; 

 but admitting a considerable amount of denudation along the courses of rivers, the 

 bays and combes of the great Chalk escarpments would still form a distinct class of 

 phenomena, and require a separate explanation. 



