152 Professor J. W. Gregory — 



and they were enlarged by denudation until their floors were but 

 little above sea-level. During these movements the climate was 

 becoming colder, as is shown by the disappearance from the British 

 seas of those Lower Pliocene moUusca that are characteristic of 

 warmer conditions. Frost action must have attacked the higher 

 peaks and especially the slopes facing north and east ; and the 

 continual freezing and thawing of water in the joints of the granite 

 led to the formation of corries along the margins of the snow-fields. 

 Dr. Mort's memoir on Arran gives the first adequate description of 

 its thousand-foot platform, which is also well developed in Ayrshire. 

 Dr. Mort has advanced good reasons for the opinion that it was 

 probably due in Arran to marine denudation.' If so, this platform 

 ■ is evidence of an uplift of at least 1,000 feet ; and that it was several 

 hundred feet greater than that amount is shown, as Dr. Mort remarks, 

 by the buried valleys in South-Western Scotland. If this platform 

 be a plain of marine denudation the uplift in Arran was not uniform. 

 The platform varies in height from 800 to 1,200 feet around the Goat- 

 fell Mountains, and to the north-west of Brodick. It is from 

 700 to 1,000 feet beside the western coast from lower Glen lorsa 

 northwards to near Pirnmill ; from 750 to 1,000 feet north of North 

 Glen Sannox ; from 500 to 750 feet in the south-western part of the 

 island, between Blackwaterfoot and Glen Scorrodale ; and between 

 500 or 600 feet and 900 feet in the south-eastern jDart of the island. 

 That the uplift was differential agrees with the aspect of the land 

 as seen from the steamer when approaching Brodick, as the platform 

 slopes gently downward from the Goatfell Mountains both to 

 south and north. That the greatest uplift occurred north of Brodick 

 was recognized by Sir Andrew Ramsay (1841, section i, opposite 

 p. 8), who represented the island as part of a great anticline with 

 its axis ending on the eastern coast at North Sannox. 



The uplift of the thousand-foot platform was unquestionably 

 pre-Glacial, for deep valleys with glaciated floors, on which are well- 

 preserved moraines, had been cut through it to the level of less than 

 100 feet above the sea. The effects of post-Glacial denudation on 

 the island have been comparatively trivial ; gorges, sometimes 30 feet 

 deep, have been cut by streams through easily denuded rock. The 

 post-Glacial period has been very short compared to that occupied 

 by the formation of the major valleys. 



In Glacial times Arran was a local glacial centre. Ice formed 

 over the mountains of North Arran and flowed radially to the sea. 

 Dr. Mort quotes with approval Gunn's remark that Arran had a 

 " local icecap which shed material all round " (Gunn, 1903, 



^ Professor Davis (1909, p. 289) gives as a criterion for the distinction of 

 marine from subaerial plains that in sea-cut plains " the border of the un- 

 reduced masses should have been a sea-cliff " ; while in subaerial plains "there 

 would presumably be a gradual transition from unreduced masses to the 

 reduced plain ". As Dr. Mort points out, the abrupt ending of the platform 

 against the mountains favours the marine origin of this plain. 



