Late- and Post-Glacial Shorelines, and Ice Limits in Argyll and North-East Ulster

Abstract

EVER SINCE the raised beaches in the northern part of the British Isles were first described there has been considerable speculation as to their age and origin. Originally regarded as indicating a fall of sea-level, they are now generally accepted as proving the isostatic uplift of those areas most deeply buried by the ice-cap of the last glaciation. They are most conspicuous as great terraces of sand and gravels commonly in close association with terminal moraine debris in the different highland glens. At an early stage these 'beaches' were shown to be dissimilar from the modern ones in that they are composed of much more poorly sorted material, consistent with the view that they accumulated as deltas and fans of outwash debris from glaciers occupying the upper reaches of the highland glens. In Scotland, the highest deltas of this type were found in the Oban area at some I30 feet above sea level. Similar features to the north and south of Oban could be found only at considerably lower levels (60 feet at Ullapool, in Ross-shire), beyond which points they disappeared with remarkable suddenness. At the more exposed localities, particularly in western Mull, western Jura and northern Islay, massive storm beaches were found with crests rising as high as I 5 feet O.D. There has been only very limited consideration of these Late-Pleistocene beaches since the pioneer work of W. B. Wright (1911, 1928, 1937), and the partial mapping carried out by the Geological Survey of Scotland. Originally the view was held that the different Late-glacial 'beaches' could be correlated by means of altitude alone. Thus two main levels were postulated to be at Ioo feet and 50 feet. A similar method of correlation was also applied to the Post-glacial beaches, the highest one being regarded as a synchronous feature at 25 feet above sea-level. Although Wright succeeded in modifying this argument somewhat, the concept of horizontal synchronous strandlines persisted. The 25 foot 'beach' was regarded as being horizontal throughout much of the glaciated area, but it plunged beneath present sea-level towards the periphery of glaciation; for no raised beach was recorded in such marginal areas as that south of Dublin in the Irish Sea basin, in the Orkneys, and in the Outer Hebrides. However, the concept of horizontal synchronous beaches would appear to be difficult to accept in view of Wright's isokinetic theory (Wright, 1937, pp. 404-37). Using archaeological methods Wright found that the highest Post-glacial beach became progressively younger when traced outward from the area most deeply buried beneath the ice-cap. Even if his archaeological dating is considered questionable, this hypothesis accords with the findings of the Scandinavian workers. The original Scandinavian ice-cap was larger, and consequently the isostatic rebound was also