Western Isles of Scotland. 103 



III. ISLAY. 



Along the north-west coast of Islay between Loch Gruinart and the 

 lludh' a' Mhail the preglacial platform of marine erosion has a very 

 magnificent development. The relations here, however, are obscured, 

 and have escaped the notice of former geologists owing to the fact 

 that the rock-platform of the preglacial shore is everywhere overlain 

 by the copious gravels of the postglacial 100-foot beach. The platform 

 came thus to be regarded as the work of the 100-foot postglacial sea. 

 Apart, however, from the fact that nowhere else has this sea been 

 able to erode anything beyond the most trifling notch in rocks of the 

 hardness of the Islay quartzite, there is ample evidence that the rock- 

 platform is of much earlier date. The cliff at the back of the 100-foot 

 beach is in many places composed of boulder-clay, and the rock- 

 platform on which the gravels rest passes beneath this boulder-clay; 

 while from behind the boulder-clay there rises again a steep hill face 

 of rock, evidently the old cliff corresponding to the rock-platform. 

 The boulder-clay thus lies packed into the angle between the cliff and 

 the platform. Moreover, if we follow the present sea-cliffs, which 

 show in section the outer portion of the platform capped by the 100-foot 

 gravels, we find here and there between the gravel and the platform 

 lenticles of boulder-clay which lie on slightly lower portions of the 

 platform, and which have been planed off above by the action of the 

 100-foot sea. 



Both these phenomena are well seen in the neighbourhood of the 

 Mala Bholsa. Immediately north of the small stream which enters 

 the sea south of this hill the gravel of the 100-foot beach rests on 

 boulder-clay, which lies in a slight hollow of the preglacial platform. 

 The matrix of the boulder-clay is here a quartzite meal of the nature 

 of pasty sand, but the structure of the deposit leaves no doubt that it 

 is boulder-clay. Also about 200 yards north-east of this the cliffs of 

 the Aonan na Mala immediately below the Mala Bholsa show the 

 following section : — 



Angular gravel . . . 4 feet \ inn foot be-ich denosits 

 Stratified sand ... 4 ^ j -WU loot beacli deposits. 



Boulder-clay . . . . 7 ,, 

 Rock-platform. 



A great mass of drift is banked up on the west side of the Mala 

 Bholsa, and in the cliff of the Aonan na Mala the preglacial platform 

 is seen passing horizontally beneath it. The great geo or sea-cut 

 gully, which a little to the north runs up towards the Mala Bholsa, 

 shows in section this remarkably even platform beneath the drift to 

 within 30 yards of its junction with the cliff-slopes of the hill. 

 Measurement with an Abney level from high-water mark of spring- 

 tides gave 105 feet as the level of the platform at the innermost 

 visible point, i.e. about 30 yards from the rock-cliff. A hollow in it 

 contains a small patch of quartzite rubble beneath the boulder-clay. 



From the Mala Bholsa magnificent views can be obtained both to 

 the south-west and east of this striking plain of marine erosion with 

 its overlying but utterly unconnected sheet of postglacial beach-gravel. 



On the west coast of the Oa of Islay some remnants of the preglacial 

 platform are also preserved, although along the greater part of the 



