100 W. B. Wright — Preglacial Shoreline in the 



shoreline is slightly lower in Yorkshire and the Isle of Man than 

 further south. In Yorkshire the inner angle of the preglacial plat- 

 form lies a few feet above high-water mark, in the Isle of Man 

 approximately at high-water mark. The former of these localities 

 lies a short distance outside the area of distribution of the so-called 

 '25-foot' postglacial raised beach, the latter fairly well within it. 

 This may possibly be an indication that towards and within the area 

 of the postglacial raised beaches the preglacial raised beach has suffered 

 a depression from which it has not yet recovered. 1 Its apparent 

 absence over the greater part of Scotland might therefore be due to 

 its being entirely below sea-level and thus unobservable. 



Recently, however, some discoveries in the Western Isles of Scotland 

 have shown that the problem is a much more complex one than had 

 formerly been supposed. Throughout an area embracing Islay, 

 Colonsay, Mull, and the Treshnish Islands there is a pronounced and 

 locally well-preserved shoreline of preglacial age at a height varying 

 from 90 to 135 feet above sea-level. This rather startling disclosure 

 has of course been the basis of many speculations, but it is doubtful 

 if these have as yet any permanent value. The existence of the shore- 

 line is, however, beyond question, as is also its preglacial age. We 

 shall devote the succeeding paragraphs to an account of its develop- 

 ment in the different islands, and conclude with some general remarks. 



II. Colonsay and Okonsay. 



The old plain of marine erosion can be traced throughout the 

 greater part of Colonsay and Oronsay, and is one of the most 

 remarkable physiographical features of the islands. It has suffered 

 somewhat from glacial and perhaps also subaerial denudation, and 

 large areas of it have been removed at various stages by the sea. 

 Nevertheless here and there wide stretches are preserved, and it is 

 possible in many places to trace its inner margin, which is frequently 

 marked by a line of lofty cliffs. 



A pronounced feature of the platform is the manner in which it is 

 locally dissected by slacks and hollows. It is to be noted, however, 

 that many of these may have pre-existed the cutting of the platform, 

 and therefore do not necessarily represent the extent of subaerial and 

 glacial erosion since its formation. If there has been any marked 

 subaerial modification of the original form of the plain, which may 

 well be doubted, it has clearly been effected in preglacial times, for 

 the postglacial denudation has been very slight. One finds few or no 

 hollows of obviously postglacial date, and the amount of atmospheric 

 weathering since the lee Age has also been quite trifling. Everywhere 

 one finds the surface of the plain ice moulded, and in many places it 

 is striated and covered by boulder-clay ; the old line of cliff also 

 frequently shows marks of the passage of the ice, and one cannot but 

 be struck by the slight modification which these much exposed glaciated 

 surfaces have undergone. 



1 As opposed to this, however, it should be noted that the scanty remnants of 

 the preglacial platform found at Bray Head in Wicklow, which lies just within 

 the area of distribution of the 25-foot beach, do not show any marked depression. 



