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There may be evidence of a western ice shed at some stage. While Scandinavian ice, or ice from an eastern ice shed, may have moved sandstone erratics across the high ground early in the last glaciation, or during a previous one, the issue of their preservation during deglaciation, or subsequent glaciation, arises. If the deglaciation of the peninsula was highly erosive it is unlikely that previously deposited or entrained sandstone erratics could have survived tliis later stage on the narrow western coastal strip. There is no doubt that the most recent glacial phase on the eastern coast was highly erosive. What drift there is usually takes the form of a barely assimilated glacitectonite (figure 5.4). That this erosive situation reflects the most recent, local phase of glaciation, is shown by the involvement of metamorphic erratics from the high ground to the west (plate: figure 5.11). If this stripping also occurred on the west coast, it is difficult to explain the preservation of erratics in areas such as St Ninian's Isle (figure 5.2). One possible explanation is given in figure 5.5. If the central ice shed was displaced to the west of the high ground itself, and retreated towards it, then there may have been a zone of inactivity beneath its centre where previously deposited or entrained erratics would be preserved. This model would not account for erratics emplaced during a previous glaciation, since they would have suffered during englaciation of the most recent, but if, during the last glaciation, erratics were

transported in basal ice to the west coast of the area, from whatever source, and then deglaciation progressively occurred in such a way as to limit erosion here, then their

preservation is less remarkable. This discussion is inevitably speculative, but a western ice shed may be borne out by evidence from other areas (see chapter 6), also from the southern tip of the south Mainland peninsula (see below), and possibly even in consideration of the Bum of Mail moraine (see below).

On the southern tip of the peninsula, there are source areas for sandstone, schistose, and granitic erratics, and here again it is suggested that ice moved first from east to west across the area, and was subsequently shed from Fitful Head in the west33 (e.g. figure 3.9). The present writer's suivey suggests that the situation here is more complex than this. Although unable to confirm the sense of direction of the striae to the east of Ward Hill, the observations would appear to be consistent with an ice stream moving eastwards across the narrow strip of land here, and a clast

32Peach and Home, 1879

fabric of lodgement till at Quendale suggests an even more northerly azimuth (figure 5.2). The till at Quendale includes sandstones, granite, and metamorphics, consistent with ice moving broadly west to east, although there are possible source areas for all these types offshore to the

southwest (figure 3.2). The striae on the east coast around Voe may reflect east to west or west to east ice movement. Those on the west coast appear to show divergence around Fitful Head. The writer is not able to confirm the sense of direction of these, however the fabric at Noss Hill also suggests a broadly northwest-southeast azimuth of ice movement, consistent with the striae at the north end of Fitful Head. The clastic content of the till here is important, containing subsidiary quantities of both sandstone and metamorphics in the largely granitic composition. All along the coast between Spiggie Bay and Fitful Head there are erratics of all three rock types but in this particular location, if the fabric analysis is accepted, or assuming that ice from Fitful Head would have flowed under topographic constraints, then it is likely that the ice shed lay to the northwest, offshore, and crossed metamorphic outcrops to the northwest of Noss Hill. The sandstone clasts were either entrained earlier from the east, or from the West Fair Isle basin in the west. The implied western ice shed configuration reflects that suggested above (figure 5.5). Furthermore, to the northwest of Noss Hill, there is a dendritic pattern of now dry channels, running under the influence of topography, and converging on one channel containing a misfit stream running around the north side of the hill (figure 5.2; plate: figure 5.12). The heads of this system run close to the coastline and have no catchments commensurate with their size. It is difficult to explain these phenomena except as a proglacial meltwater system emanating from ice to the northwest of the coast. Although the metamorphic erratics at Noss Hill might have been transported by a piedmont-style glacier from Ward of Scousborough there is no other evidence of such a system and it does not explain the fabric analysis or the meltwater channel system. However, not only are there clear movements of particular sandstone erratics from east to west in this southern area34, but the movement of sandstone and local syenite up Fitful Head is recorded and this must be due to an ice shed to the east, since basal ice from the west would be unlikely to ascend the sheer cliffs facing it.

There is, therefore, some evidence for ice crossing this southern area from both eastern and western or northwestern ice sheds. Clearly these movements did not occur at the same time but there is no evidence that they reflect discrete glaciations. At Quendale, for example, both metamorphic and granitic erratics are found in sediments associated with the most recent ice movement (plates: figures 5.13 and 5.14).

In the north of the area evidence of changes in ice movement direction is significant. When the evidence is considered as it is above, the second hypothesis of the glaciation of Bressay, that Mainland ice crossed northeast over it, is the more probable. This requires a relatively late ice

shed to the south-west of Lerwick. Topographically it might lie as in figure 5.3 (ice shed 2), leaving ice to travel across non-inetamorpliic rocks, although a more significant ice cap over or to the west of the main high ground might result in a zone of inactivity over potential western erratic sources. Either way, such an ice-shed is incompatible with the pattern around

Gulberwick, where the northwest-southeast dominance is approximately perpendicular to submarine contours and also shows some correspondence with local topography (figure 5.2). The writer has observed the pattern into Brei Wick and, again on the morphochronological grounds of dominant pattern and the preservation of older, laterally more extensive patterns through deglaciation in an erosive context, these striae probably post-date those observed on the east shore of the bay. It can be reasonably argued therefore that the northeast Bressay pattern pre-dates that into Gulberwick and may reflect containment to a deeper bathynunetric contour than the Gulberwick pattern. The northern Bressay pattern may reflect regional northeasterly movement if a more considerable ice sheet's zone of erosion was restricted to source areas within the sandstone around Lerwick (as suggested above), or any further-travelled metamorphic erratics were swept off Bressay subsequently. Figure 5.3 attempts to combine the ice movements for which there is evidence, although the relative timing of ice shed 1 cannot be ascertained. It may be earlier because if there are striae corresponding to the Kebister Ness set on Bressay, they are not dominant and therefore probably older. If the two sets are unrelated and ice crossed Kebister Ness and did not reach Bressay, then ice shed 1 would be the most recent, a more reasonable explanation if the ice cap as a whole is retreating to the high ground of central Mainland.

Most of the rest of the evidence north of Channerwick is strongly suggestive of local ice moving under the influence of topographic and bathynunetric contours (figure 5.2). Striae are notably not perpendicular to topographic contours in the northwest of the area and as far south as West Burra. Tliis can be explained by ice moving perpendicular to bathynunetric contours around the -100m level, i.e. into the West Fair Isle basin. There is no apparent shift to the -50m level, suggesting that by the time the ice front reached this location, little erosion was occurring. This is easily understood given the narrow spinal source area and sleep slopes and may again suggest a late ice shed slightly west of the spine (see above and figure 5.5 - northeast). Around

Gulberwick, it is suggested above that ice movement may reflect the influence of shallower bathynunetric contours. Continued supply of ice from the northwest to a later stage in deglaciation in this more northerly location (represented by a greater degree of topographic containment) is readily envisaged, with the ice cap retreating to the wider and (on average) higher altitude areas of central Mainland. The implication is that the southern spine of south Mainland may have been deglaciated earlier than central Shetland. To some extent tliis may be reflected in exposed possible solifluction deposits (figure 5.4 and plate: figure 5.15) on the hills above Bum of Mail (although it is not clear how widespread these are beneath peat) and rare cryoturbation structures near St Ninian's Isle (plate: figure 5.16). The age of these features is,

however, not known, and their distribution may be limited. This latter point perhaps indicates that they are not of Lateglacial age since, there being no evidence of widespread glacial activity during this time, more widespread periglacial deposits would be anticipated if they were.

In the northwest of the area there is some evidence of changes in ice flow through the last glaciation (figure 5.2). Striae show that ice, at least at high level, crossed Whiteness and Stromness voes. However the distribution of drift suggests that ice has moved down Weisdale Voe (figure 5.4), in particular where a protrusion sticks out into the voe there is lodgement on the up Voe side and thin drift or glacitectonite on the down glacier side. This evidence of changing ice flow directions is more readily observed in the adjoining central Mainland area (figure 7.2).

In Dales Voe the golf course is built on a series of sloping, terrace-like, drift slopes. Sections are very limited but the upper surfaces appear to be underlain by diamict, whilst the lowest consists of fine gravels. A kilometre up valley front the coastline, a birdsfoot fan extends from a deep channel that drains a shallow enclosed basin above the valley to the west (figure 5.2 and plate: figure 5.17). The fan is clearly inactive and the minor stream in the voe flows around it. It is likely that it, and the lowest terrace of the valley, represents drainage from melting ice in the basin after the voe was deglacialed and the cutting passes through several metres of valleyside diamict. This assemblage of features suggests retreat of ice across Dales Voe to the west, possibly offering further evidence of a more western, late, ice shed.