Mackintosh— On the Cliffs and Valleys of Wales. 397 



feet, and the height of the escarpment below from 40 to 50 feet ; 

 and tliis may be regarded as about the average size of the main 

 terraces. The platform C, where I examined it, was very nearly 

 level, both longitudinally and transversely. It was covered to a 

 considerable depth with clay and loam, mixed with well-rounded, 

 semi-rounded, and angular small stones. The platform B is covered 

 with similar silt, and is here and there more or less swampy. These 

 terraces, which are probably the most elevated of any yet discovered 

 in Great Britain, furnish a proof of numerous and long-continued 

 pauses, and a consequent presumption in favour of considerable de- 

 nudation, during the intermittent rise of the land above the glacial 

 sea. But the advocate of marine denudation is not necessarily limited 

 to the glacial period of submergence. The general form communi- 

 cated by the sea to the rocky surface of any particular area, may be 

 capable of resisting atmospheric denudation "for an indefinite period.' 

 The style of architecture may remain after the details of the structure 

 have been laid in ruins ; but during a secondary submergence of the 

 land, the sea may be able to repair the havoc committed by the 

 powers of the air, and re-impress its seal on the blanched and shat- 

 tered monuments of its primeval sway. 



Note. — Since the above was vrritten, Mr. Maw's able article on 

 Watersheds has appeared in this Magazine (August, 1866). I have 

 only space to express my opinion that his observations on the gra- 

 duated series of levels extending from watersheds to the sea are 

 applicable only to certain areas where the sea may first have formed 

 an inclined plane of denudation, and afterwards hollowed it out into 

 valleys. I do not believe in the existence of systems of valleys as a 

 general rule. In most districts large valleys run approximately 

 parallel to the lines of watershed, and are only transversely con- 

 nected at long and irregular intei-vals by narrow and apparently 

 accidental gorges. It is through these gorges that rivers, after much 

 wandering about and change of course, find their way to a lower 

 level ; and they often «eem (the Severn for example) to have the 

 greatest difiiculty in reaching the sea at all. With regard to marine 

 denudati-on, it ought to be remembered, that on many coasts the sea 

 is not only running up previously-excavated valleys, but forming 

 fresh inlets, frequently long and winding ; and that its excavating 

 power in these inlets is often increased rather than diminished. 

 It is not true that the sea acts on the principle of making straight 

 lines of coast, for we everywhere find it takuig advantage of the 

 slightest crevice or inequality in the composition of rocks as a com- 

 mencement of indentations which are small in protected situations, 

 but extensive beyond any assignable limit on coasts that are fully 

 exposed. I believe that coast indentations increased by current 

 action during gradual submergence, and current indentations in- 



1 Excepting at intermediate levels where streams hare had space to acquire con- 

 siderahle volmne, -nithoiit descending so far as to lose sufficient inclination of channel. 

 This may be called the zone of maximum atmospheric denudation — the higher and 

 lower lands the minimum zones. 



