290 PROF. W. J10RRIS DAVIS ON [Aug. I9O9, 



VIII. Date and Altitude oe Ramsay's Plain. 



As to the date of origin of the ancient plain, Ramsay implies 

 that it was ' older than the New Red Sandstone ' (1866, p. 236). I 

 venture to suggest a much more recent date, such as the beginning 

 or middle of the Tertiary Era. Geologists half a century ago seem 

 not to have been impressed with the great duration of the later 

 divisions of geological time : they assumed tbat several geological 

 periods would be needed for the erosion of large valleys ; but on 

 this matter present opinion appears to have been reversed. For 

 example, there is good reason to believe that since Eocene time, 

 thousands of feet of strong and weak strata have been removed by 

 subaerial erosion from broad areas in Arizona, and tbat in late 

 Tertiary time, during and after a strong uplift of the lowland thus 

 produced, the Colorado cafion was eroded through it. Again, 

 there is much reason for thinking that since Cretaceous time, more 

 than a thousand feet of Mesozoic strata bave been removed by 

 subaerial erosion from a belt of country stretching across England 

 from north-east to south-west, and that during and after a moderate 

 uplift of the lowland thus produced, many open valleys and broad 

 lowlands of a second generation have been excavated in the Mid- 

 lands. It is true that the Mesozoic strata of the Midlands are 

 much weaker than the Palaeozoic formations of North Wales ; but 

 if examples of the mature dissection of resistant structure in modem 

 geological times are wanted, we need go no farther than Cornwall, 

 where many of the valleys eroded since the planation and uplift 

 of that district are thoroughly mature, although the date of plana- 

 tion is given as late Tertiary. Messrs. J. B. Hill & D. A. MacAlister 

 have recently stated that the upland about Ealmouth and Truro, 



' devoid of craggy features, is suggestive of a plain of marine denudation that 

 was upheaved in comparatively late geological times . . . The scenery of the 

 area, therefore, has been mainly sculptured since the early part of the Pliocene 

 period ' (1906, p. 3) ; 



and Mr. C. Reid & Dr. J. S. Flett bave assigned a Pliocene date to 



' a strongly marked shelf, or plane of marine denudation .... bearing no fixed 

 relation to the limits of the granite' (1907, p. 70) 



in the Land's End district. Mr. H. B. Woodward (1906) and 

 Mr. A. J. Jukes-Browne (1907) give an earlier (Tertiary ?) date for 

 the uplands of Eastern Devon. Smooth uplands of even greater 

 altitude and of presumably earlier origin have been observed in 

 Cornwall by Mr. G. Barrow (1908, p. 384) ; but they do not require 

 consideration in the present connexion. A mid-Tertiary date for the 

 peneplanation of the uplands of Wales is, therefore, not inconsistent 

 with the erosion of open valleys in the uplifted peneplain after its 

 elevation. 



As to the altitude at which the uplifted plain of Wales now 

 stands, it is difficult to secure information for the Snowdon district ; 

 but the uplands to the eastward of Bettws-y-Coed appear to have 

 an altitude of 1000 or 1200 feet ; farther south, around Builth, 



