102 



W. B. Wright — Preglacial Shoreline in the 



narrow straits between (see Pig. 4). The platform of erosion among 

 these islands is in a few places still well preserved, as on the plateau 

 known as L6n M6r to the south-west of Loch Staosnaig. A distinct 

 shelf can also be traced round Beinn Eibhinn, and it is again 

 particularly well marked on the south side of Beinn Oronsay. 



Some outlying hills in the north of Colonsay were also isolated, 

 while the big inland cliffs south of Balnahard marked, at this stage, 

 the coast of the main island. 



On the east side of the islands the effect of full exposure to the 

 impact of the ice is noticeable in the planing down of the platform 

 and the rounding off of the old cliff feature. This is, of course, 

 especially marked in the softer, more pelitic beds, but where the rock- 

 shelf is cut in the harder grits, as in the case of some of the headlands 

 north of Scalasaig, both cliff and platform are sometimes well 

 preserved (see Pig. 3). 



Fig. 3. The shores of Colonsay looking north from Port Olmsa, showing the 

 preglacial cliff and rock-platform and the ice-planed slopes of the east coast. 1 



In addition to this well-marked high-level platform there exists 

 another, first pointed out by Mr. Bailey, which lies a little above 

 sea-level, and may perhaps also be taken as indicating a preglacial 

 plain of marine denudation. This lower platform includes a con- 

 siderable portion of the low-lying ground in Oronsay, and also the 

 ice-worn skerries, many of them covered at high tide, which render 

 the western coast so dangerous to navigation. There is also a suggestion 

 that the fine sea-cliffs along the coast from Kilchattan north are not 

 entirely postglacial. The evidence, however, for this lower preglacial 

 plain is very poor, and little reliance can be placed oh it. It rarely 

 or never has a well-defined inner margin or a cliff that can be traced 

 any distance. One point, however, which lends it some support should 

 be mentioned here. A plain at a similar level has been observed in 

 the Torridonian rocks on the east coast of Ion a along the shores of the 

 sound separating Iona from the mainland of Mull. This shelf is 

 overlain by the gravels of the 25-foot beach, but it is clear that it does 

 not belong to this beach, since there is no cliff at the back of it at 

 all equivalent in magnitude to the extent of the plain, but instead 

 a rounded ice-worn slope. 



1 See note 2, p. 101. 



