The location of the Monadhliath Mountains in the middle of the Grampian Highlands
places them in a central zone with respect to ice flow pathways during the maximum
extent of the last British and Irish Ice Sheet (BIIS) in the Late Devensian. At the ice
sheet maximum the Scottish mainland was probably entirely submerged beneath ice,
which flowed north-westwards out to the continental shelf break, merging with
Scandinavian ice occupying the North Sea basin (Bradwell et al., 2008). This period
roughly equates with the global Last Glacial Maximum (LGM), 28,000-22,000 years
ago (Mix et al. 2001). The most recent model of the BIIS (Clark et al., 2012) places
the Monadhliath Mountains immediately to the east of the main north-south ice divide
of the Scottish ice sheet, and north of a subsidiary west-east divide, centred over the
East Grampian and Cairngorm mountains. Geomorphological evidence for ice
streaming in the Great Glen and Spey Valley to the northeast and southwest of the
Monadhliath massif indicates a general ice flow direction towards the northeast across
the region, supporting this ice-divide positioning.
Synopsis High strain zones, characterized by shear fabrics and mylonites, have been identified within lower amphibolite-grade, coarse granular metasandstones of the Glen Lethnot Grit Formation of the Southern Highland Group (Dalradian) along the Highland Border, south-west of Stonehaven. The rocks comprise part of a polydeformed, regionally metamorphosed, Neoproterozoic sequence, bounded to the north by the post-orogenic Mount Battock Granite (Late Caledonian) and to the south-east by the Highland Boundary Fault Zone. Early regional D1 and D2 structures have been rotated by a major monoform, the Highland Border Downbend (D4), to produce a steeply dipping, downward facing succession. The mylonitic metasand-stones contain a variety of kinematic indicators, including asymmetrical pressure shadows, S-C fabrics, shear bands and an extensional crenulation cleavage, which yield a sense of shear of top-towards-the SE (present structural position). Regional syn- to post-D2 metamorphism resulted in the development of prograde pelitic mineral assemblages which overgrow earlier Dl structures. Mylonitic fabrics within the high strain zones, developed during D2, both deform and are also overgrown by syn- to post-kinematic biotite porphyroblasts. The high strain zones may represent part of a major ductile structure active during the Dalradian D2–D3 tectonothermal event, which contributed to the development of strong metamorphic gradients and narrowing of mineral assemblage zones within the Highland Border.
Groundwater in upland floodplains has an important function in regulating river flows and controlling the coupling of hillslope runoff with rivers, with complex interaction between surface waters and groundwaters throughout floodplain width and depth. Heterogeneity is a key feature of upland floodplain hydrogeology and influences catchment water flows, but it is difficult to characterise and therefore is often simplified or overlooked. An upland floodplain and adjacent hillslope in the Eddleston catchment, southern Scotland (UK), has been studied through detailed three-dimensional geological characterisation, the monitoring of ten carefully sited piezometers, and analysis of locally collected rainfall and river data. Lateral aquifer heterogeneity produces different patterns of groundwater level fluctuation across the floodplain. Much of the aquifer is strongly hydraulically connected to the river, with rapid groundwater level rise and recession over hours. Near the floodplain edge, however, the aquifer is more strongly coupled with subsurface hillslope inflows, facilitated by highly permeable solifluction deposits in the hillslope–floodplain transition zone. Here, groundwater level rise is slower but high heads can be maintained for weeks, sometimes with artesian conditions, with important implications for drainage and infrastructure development. Vertical heterogeneity in floodplain aquifer properties, to depths of at least 12 m, can create local aquifer compartmentalisation with upward hydraulic gradients, influencing groundwater mixing and hydrogeochemical evolution. Understanding the geological processes controlling aquifer heterogeneity, which are common to formerly glaciated valleys across northern latitudes, provides key insights into the hydrogeology and wider hydrological behaviour of upland floodplains.
The Speyside area of the southern Monadhliath Mountains displays an exceptionally welldeveloped
suite of landforms and deposits that formed during the latter stages of Late
Devensian glaciation in this part of Scotland (Young, 1978; Hinxman and Anderson, 1915;
Merritt et al., this guide). This section of the field guide describes the glacial landforms and
sediments present in the valley of Raitts Burn (NH 760 047 – 796 036). The burn flows
south-eastwards across the north-western flank of Strathspey to join the valley of the River
Spey c. 3 km downstream of Kingussie (Fig. 1). Exposed in the cliff sections cut by the burn
is a sequence of rhythmically laminated clays, silts and sands, interbedded with ice-proximal
diamictons and matrix-rich gravels (Auton 1998; Phillips and Auton, 2000). Detailed
examination of the complex suite of sedimentary and glacitectonic structures developed
within these sediments has revealed a complex sequence of depositional and deformational
events recording the formation and overriding of an ice-dammed lake (Phillips and Auton,
2000). The glacial landform record in the area clearly indicates that this complex pattern of
meltwater ponding and ice advance into the lake occurred in response to active ice retreat
during the deglaciation of the Raitts Burn basin.
Controversy has surrounded the origin of the sediments that underlie the generally
flat–topped asymmetric ridge, up to 60 m in height, which extends from the western
side of Gleann Ballach into the valley floor at (NH 6513 0106) (Fig. 60A; Boston
and Trelea-Newton, this guide: 17). The most recent description of the deposits by
Gheorghiu et al. (2012) is based principally on the sequence exposed in a river cliff
of the Allt Ballach at the eastern end of the ridge at c. 490 m OD. The 6 to 7 m-high
exposure is dominated by well-sorted, fine- to medium–grained sands with planar
lamination and ripple cross bedding, with lenses of till, laminated clay, and sparse
dropstone cobbles; all indicating deposition in a glaciolacustrine setting. Gheorghiu
et al. (2012, p. 137) supplemented the information from the exposed sequence
by augering to the west of the exposure, which they suggest confirms that similar
lacustrine sediments underlie the main part of the ridge. This reaches an elevation
of c. 560 m OD and coincides in altitude with a ‘horizontal notch cut into bedrock’
that they interpret as an ‘ice-marginal meltwater route’. Significantly, cosmogenic
ages on boulders ‘on top of the lake deposit at 550 m OD and 528 m OD’ yielded
exposure ages of 11.6 ± 1.1 ka and 10.9 ± 1.0 ka, with a mean age of 11.2 ± 1.1 ka.
These ages have now been revised to 13.7 ± 0.8 ka and 12.8 ± 0.8 ka respectively,
with a mean age of 13.2 ± 0.8 ka (Gheorghui and Fabel, this guide: 18).
SUMMARY A lithostratigraphical framework has been erected for Quaternary deposits of the Sellafield district in west Cumbria. It has enabled a sequence of glacial and deglacial events to be established that is one of the most detailed for the last glacial-inter-glacial cycle onshore in Britain and provides new constraints on models of the evolution of the Irish Sea basin during the last 120 000 years. The products of at least one major pre-Devensian (Oxygen Isotope Stage 6 or older) glaciation have been recorded in cored boreholes around Drigg and Nether Wasdale. Ice flowing from Wasdale terminated in proglacial lakes on two occasions during OIS 3 and 4 and an intervening cold-water marine incursion occurred that reached a height of at least 20 m below present sea level, probably during OIS 3. Glaciers emerged again from the Lake District during the build-up of the main Late Devensian ice sheet, which appears to have reached its maximum extent early in OIS 2, when the whole district was glaciated. Scottish ice flowing into the northern Irish Sea basin eventually deflected local ice southwards. The Irish Sea ice stream became dominant again during several readvances that followed a significant deglacial event. The first readvance (Gosforth Oscillation) over-rode most of the coastal plain. The last major one (Scottish Readvance) proglacially tectonized the glacigenic sequence along the coast, forming a push moraine at St Bees. Thick sequences of glaciolacustrine sediment accumulated within the major valleys of the district, both before and after the Gosforth Oscillation, when ice at the coast caused considerable ponding of meltwater inland.
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