Callum R. Firth

Ice-proximal glaciomarine sedimentation and sea-level change in the inverness area, Scotland: A review of the deglaciation of a major ice stream of the British Late Devensian ice sheet

Abstract Evidence of both rising and falling relative sea levels and glacitectonic movements is preserved in two formations of raised glaciomarine deposits that were laid down in front of an oscillating ‘grounded’ tidewater glacier in the Inverness Firth. These changes occurred during the latter stages of the disintegration of the Moray Firth ice-stream, one of the major ice streams that drained the British main Late Devensian ice sheet. Most of the glaciomarine deposits antedate a sequence of glacio-isostatically tilted Late Devensian marine shorelines and associated littoral and estuarine deposits. The shorelines began forming at about 13,000 BP and record a progressive fall in relative sea level. A new model for the deglaciation of the Moray Firth region is proposed after a critical appraisal of published accounts of both onshore and offshore Quaternary sequences. The disintegration of the Moray Firth ice stream involved several rapid phases of retreat to pinning points, caused by iceberg calving and triggered by rising global sea level. Each retreat was followed by minor readvances or stillstands, possibly caused by short-lived accelerated periods of glacio-isostatic rebound and concomitant temporary falls in relative sea level. Two such events occurred in the Inverness Firth: the Ardersier Oscillation and the Alturlie Stillstand. Substantial differences (lower relative sea levels, later deglaciation) are apparent between the pattern of ice-retreat in the Moray Firth region and published accounts of the deglaciation of the Irish Sea basin. These differences require a reassessment of some current hypotheses concerning the disintegration of major ice streams associated with high relative sea levels. Furthermore, geological and geomorphological evidence suggesting both rising and falling sea levels in the Inverness area, prior to ca. 13,500 BP, is not fully compatible with recently published computer simulations of the dissolution of the British main Late Devensian ice sheet.

The altitude and age of the main postglacial shoreline in Eastern Scotland

Abstract This paper discusses the patterns of altitude and age variation of the Main Postglacial Shoreline in eastern Scotland from the Firth of Forth to the Dornoch Firth. For reasons of comparability, attention is confined to the carseland areas. The mean altitude of the shoreline declines eastwards from 14.8 m to 6.1 m OD in the Forth area, from 11.2 m to 8.1 m in the Earn-Tay area, and northwards up the coast from the Tay area to ca. 6.4 m in the Montrose Basin area, ca. 3.9 m around the Ythan estuary and ca. 2.0 m near Fraserburgh. In the Beauly Firth/inner Moray Firth area it declines northeastwards from ca. 9 m to 7.1–7.9 m, and declines to 5.7 m in the Dornoch Firth. The isobase pattern for eastern Scotland, as represented by a quadratic trend surface, conceals a significant regional variation in the Tay area. The age of the shoreline has been investigated at 14 sites grouped in 8 locations at varying distances from the centre of uplift. The ages range from ca. 6800 14C years BP in the upper Forth carselands to 6095 ± 75–5700 ± 90 BP near Fraserburgh. With the exception of one site, the data are consistent with the hypothesis that the shoreline is measurably diachronous, having been abandoned progressively later with increasing distance from the centre of uplift. The implications of these findings for understanding isostatic-eustatic relationships are discussed.