GLACIITION OP THE OEKNET ISIANDS. 651 



and at the base of the Old Man of Hoy. The whole series rests 

 itnconformably on the flagstones ; and in the south-west portion the 

 upper division is brought into conjunction with the lower by a 

 fault which extends from Melsetter to the coast-line opposite Risa 

 island *, 



III. Glaciatiok. 



The glacial phenomena of Orkney completely establish the double 

 system of glaciation which we found to obtain in Shetland. There 

 is satisfactory evidence for maintaining that during the primary 

 glaciation the Orkney Islands must have been overridden by a mass 

 of ice which moved from the North Sea to the Atlantic ; but 

 towards the close of the Glacial period, when the great mer de glace 

 had retreated from the Orcadian coast-line, local glaciers must have 

 lingered for a time in the valleys of Hoy and in some of the more 

 elevated parts of the Mainland. 



Though these islands do not comprise any districts that might be 

 compared with North Mavine or the promontories of Lunnasting in 

 Shetland, which are dotted all over with finely preserved roches 

 moutonnees and rock-basins, nevertheless a careful search along the 

 cliff-tops reveals numerous instances of glaciated surfaces and ice- 

 markings. The latter, however, are not so abundant as we found 

 to be the case in Shetland, which may be satisfactorily explained by 

 the rapid disintegration of the flagstones when long exposed to 

 atmospheric waste. 



In the island of Westra the average direction of the striae in the 

 eastern part of the island is W. 20°-30° N. Close by Noltland 

 Castle, at the roadside, the trend is W. 20° N., on the north-west 

 face of Cleat hiU N.W., and immediately to the east of the same 

 hill W. 30° N. At Eackwik, on the eastern shore, the ice-markings 

 vary from W. to W. 20° N., while in Tuquoy Bay they point 

 W. 10° S. 



A careful examination of the striated surfaces on the hills west of 

 Pierowall proves that the ice must have been slightly deflected as 

 it impinged on the eastern slopes, the lower portion moving in the 

 direction of the northern coast-line, while the higher strata streamed 

 westwards over the hill-tops towards Nonp Head and Eussitaing. 

 On the north-eastern face of the hill south of Ourness several 

 examples were noted pointing N. 30°-35° W., but in the gap be- 

 tween the hills the direction is W. 5° S. 



Perhaps one of the most interesting features connected with 

 the glaciation of Westra is the freshness of the ice-markings on 

 Nonp Head (240 feet) and along the cliff-tops to the south. A few 

 yards to the north and south of the highest point of this bold head- 

 land, finely preserved strise were observed on grey flags, where the 

 thin Boulder-clay had been recently removed by the action of the 

 sea, trending W. to W. 3° N. Above Eamna Gio the direction 



* The geological structure of Hoy was solved by Professor Greikie and 

 Mr. B. N. Peach in 1874. See " The Old Eed Sandstones of Western Europe," 

 Trans. Eoy. Soc. Edinb. vol. xxviii. p. 411 ; also '' The Old Man of Hoy," Geol. 

 Mag. decade ii. vol. v. p. 49. 



