STRANDFLAT DEVELOPMENT AND QUATERNARY SHORELINES ON TIREE AND COLL, SCOTTISH HEBRIDES

The islands of Tiree and Coll in the Scottish Inner Hebrides are domin ated by staircases of glaciated rock-platform surfaces in Lewisian gne iss that are interpreted here as areas of strandflat. The rock platfor m surfaces exhibit differential glacio-isostatic uplift from east to w est and are considered to indicate at least four separate periods of s trandflat formation during prolonged intervals of Quaternary cold clim ate. It is suggested that many of the well-known areas of high rock pl atform located farther east in the Scottish Inner Hebrides represent e roded remnants of the strandflat surfaces described here. The existenc e of the Tiree and Coil strandflat surfaces in conjunction with their formation during periods of Quaternary glaciation implies that they we re produced during periods when mainland Scotland was ice-covered yet when the western margin of the ice sheet was normally located east of these islands. A prominent Late-glacial raised shoreline also occurs o n both islands and declines in altitude to the west, from 22 m in nort heastern Coil to 9 m in western Tiree. It is believed that this shorel ine, whose regional gradient is 0.39 m km(-1), may represent part of t he Main Wester Ross Shoreline that was formed in the Northwest Highlan ds of Scotland during a period of glacial advance that interrupted the decay of the last (Late Devensian) ice sheet. The general similarity in altitude between the Late-glacial shoreline features and rock platf orm surfaces implies that during each period of Quaternary strandflat formation, relative sea-level returned to the same approximate positio n. This would appear to indicate that the glacio-isostatic and glacio- eustatic components affecting the positions of relative sea level in S cotland have remained in approximate equilibrium throughout the durati on of the Quaternary.