Hummocky moraine has been mapped in various locations and it has been argued that it represents debris associated with static collections of snow and ice during the Loch Lomond stadial47. The areas are shown in outline in figure 7.4. There is no evidence of the age of these features, although for them to be associated with the Loch Lomond stadial then the regional equilibrium line altitude would need to be less than 200 m above modern sea level. They do not appear to be associated with static accumulations of snow and ice however. Their size and morphology is highly variable and some are certainly significant constructional features clearly associated with active ice margins. Tliis is best illustrated with the two most well developed areas which are on the west side of the Scallafield Ridge, and west of Souther Hili. In the former case there are ridges 8-10 m high, and up to 100 m long, parallel to the Scallafield ridge and with some cross sectional asymmetry (plate: figure 7.17) suggesting glaciation from that high ground and the formation of a series of terminal moraines. In the only section the author has been able to find, a coarse, angular, diamict was observed, again inconsistent with the washed sediments anticipated by formation associated with inactive ice. The hummocks in tliis case occur in topographic lows between north-south trending ridges. This is to be expected of the interaction between glaciers and cross-flow topography and the location of the moraines is not necessarily of climatic significance. Similarly to the west of Souther Hill there are examples of hummocks with clear cross-sectional asymmetry, the shallow side upslope and upglacier (plate: figure 7.18). These hummocks reach 5 m in height and 10 m long. Elsewhere the size of hummocks is much less significant and it is often difficult to tell what contribution peat, bedrock, hummock and post-depositional drainage channels make to the visible morphology. In places such as Dales Voe and Colla Firth, the broad disposition, as shown in figure 7.4 may again suggest debris accumulation from ice entering the valley from the side rather than at its head, consistent with glaciation from the highest ground. It seems likely tliat these features indicate progressive retreat to the highest ground in the area during a deglaciation but without dating control it is not possible to rule out a Loch Lomond stadial age. The chronostratigraphic issue as a whole is discussed in chapter 10 but the evidence given there cannot be related to the morphology discussed here.
7.4. Conclusions
There is widespread evidence of erosional activity in central Mainland and all observations of glacial action probably relate to the degiacial period. If Scandinavian ice crossed Shetland in the early part of the glaciation it is therefore not surprising that no evidence of its passage remains, although there is evidence of an early, relatively eastern, ice shed. Evidence of striae on the
Scallafield ridge and other ice direction indicators further north and the pattern of ice movement into Yell Sound points to an ice shed to the east of the main high ground in Mainland (figure 7.5). If this is not Scandinavian glaciation then it probably reflects relatively early deglaciation from the west. Whilst a local ice cap would build up on the high ground and the ice shed would ultimately migrate west if precipitation was from the west, early western deglaciation would account for an eastern migration.
It is not possible to consistently account for all ice direction indicators on eastern peripheral areas in a single, atemporal, reconstruction. On the grounds that laterally and vertically peripheral locations preserve older patterns, a tentative reconstruction is given in figures 7.5 to 7.8. On the eastern coast early deglaciation was probably from the north and northeast (figures 7.6 and 7.7) and later from the south east (figure 7.7). The pattern suggests early deglaciation at the northern and northeastern margins of the ice sheet followed by increased topographic containment. The glacimarine sedimentation of the East Shetland Platform does not appear to be reflected in inshore areas. Tliis again suggests glacimarine deglaciation to some limit following which deglaciation was terrestrial, with sea level rise to modern levels after the disappearance of ice at least to the high terrestrial ground. The step change in the nature of deglaciation may coincide therefore with bathymmetry and the British Geological Survey's work on sea bed morphology (above) also shows some correspondence with water depth. There are difficulties with this reconstruction. Firstly, it is unlikely that deglaciation occurred in the discrete steps shown - more likely there was some continuum of progressive deglaciation and topographic containment, although this does not preclude step changes. Given this, it is difficult correlate local patterns from different areas. This is particularly tme of incorporating the west coast reconstruction into the time frame of the east. It has been suggested that deglaciation started here early but its progress at each point in the east coast story is difficult to assess.
There is evidence of morainic features around the high areas of central Mainland. Their location appears to reflect topographic control and they remain undated.