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The Pamphlet Collection of Sir Robert Stout: Volume 22

V. Mobainic Deposits belonging to the Later Glaciation

V. Mobainic Deposits belonging to the Later Glaciation.

On referring to the striæ-map, it will be seen that numerous instances occur along the east coast of the Mainland, from Lerwick to Dunrossness, running in a S.S.E., S.E., and E.S.E. direction, the trend being affected by certain local influences. Now, along this tract there is an irregular covering of a loose morainic deposit, passing into an ordinary Boulder-clay, resting on the areas of the Old Red Sandstone, and containing fragments of the schists from the Cliff Hills, along with the stones derived from the underlying formation. These sections are exposed on the shore between Lerwick and Rovey Head, in the Clickamin bay, Wick Sound, Gulberwick Bay, and in the Old-Red-Sandstone areas from Sandlodge to Boddom. After a carcful examination of these sections, we felt convinced that the small fragments of blue and grey schists occurring in the Boulder-clay and morainic matter have been derived from the hills which extend from the Wart of Skewsburgh northwards by Scalloway to Dales Voe. It has been already shown that these schists and clay-slates form the highest ground between Dunrossness and Dales Voe north of Lerwick; and the occurrence of fragments of these rocks in the drift along the eastern seaboard points merely to a local radiation of the ice, after the great mer de glace that overflowed Shetland had melted back and was no longer confluent with the small glaciers that lingered on during the later glaciation.

That this is the explanation of the foregoing phenomena seems still more likely when we examine the eastern sea-board of North- mavine between Colifirth Voe and Fethaland Point. At certain localities, in the narrow strip occupied by the metamorphic rocks, we found similar deposits mainly made up of the debris of the underlying rocks, but likewise containing stones derived from the Roeness plateau. These sections occur on the north bank of Colifirth Voe in the North-Rooe bay; and again round the Fethaland fishing-station and in the Sand Voe. Now from Colifirth Ness northwards to Calsta extends a well-marked ridge parallel with the Biurg range, against which the local glaciers abutted, which were shed from the Roencss plateau. A similar ridge runs from North-Rooe Bay to Fethaland, culminating in the Lanchestock hill (416 feet). These ridges deflected the later glaciers, and caused them to move southwards into Colifirth Voe, and northwards into North-Rooe Bay and Sand Voe. Hero and there, however, where the ice must have been heaped up, it streamed across the lower portions of the ridge flanking the sea. From the configuration of the ground it is apparent that the Roeness plateau must have shed its largest glacier in the direction of North Rooe. Hence it follows that only at certain localities reached by the local glaciers are fragments of the quartz-porphyry to the west associated with the schists in the page 802 drifts. The same remarks are applicable to the erratics of pink granite and quartz-felsite scattered over the surface between Fethaland and Colifirth Voe.

These accumulations were in all probability extruded at the snouts of the local glaciers when the great mer de glace had melted away from the immediate vicinity of Shetland. This is rendered all the more likely from the number of striated stones in the deposit, and its tolerably coherent nature, differing somewhat from the loose debris of the ordinary surface-moraines. But, in addition to these later deposits, there is abundant evidence to show that when the hilltops had emerged from the icy covering which so long held sway during the primary glaciation, the severe frosts which prevailed caused an accumulation of blocks and rubbish on the surface of the attenuated glaciers. In course of time, as the glaciers melted back, loose heaps of rubbish were laid down, sometimes as isolated mounds, but frequently in concentric lines indicating pauses in the retreat.

As might be expected from the size of the valleys and the limited elevation of the hills, the moraines are not largo; but they are nevertheless very abundant; indeed there are few of the important valleys draining a mass of high ground which do not contain well-marked groups. They consist of loose debris with angular and subangular stones; and in some cases the deposit is merely an assemblage of small stones without any matrix. Numerous examples occur, however, where the heaps show distinct traces of stratification and the stones are somewhat waterworn.

In a small valley about two miles in length, draining the cast side of the Bonxie hills and flowing into the bay below Connings-burgh, a fine series of moraine heaps occurs, displaying the usual concentric arrangement. They vary in length from 5 to 10 feet; and numerous blocs perches of the Bonxie-hill schists rest on the mounds.

In the district of Delting, moraines are to be found in the main valleys and round the heads of the larger sea-lochs, as for instance the Dales, Colifirth and Swining Voes on the cast coast, and near Voe, North Brae, and Voxter on the west coast. And so also on the banks of Vidlon and Dourye Voes in Lunnasting, similar deposits are seen resting on the Boulder-clay. We frequently observed that the moraines became more numerous where two or more valleys converge at the head of a sea-loch, which is just what might be expected when the tributary glaciers must have coalesced at this point. At the head of Swining Voe two streams join the sea; and at the point where the valleys converge a well-marked series of concentric mounds is met with. The dividing ridge is strewn all over with innumerable moraines, doubtless the lateral moraines of both glaciers; and in the various burn-sections the morainic drift is found to rest on the Boulder-clay. The evidence derived from the disposition of these mounds shows clearly that at the time they wore deposited the glaciers did not fill the valleys to any great extent, neither did the trunk glacier extend very far down the voe.

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The scarcity of strife produced by the later glaciers in the district of Delting, when compared with their great abundance on the eastern seaboard, near Lerwick, is deserving of note; but their absence is doubtless due to the fact that the glaciers slipped forward over a terrace of Boulder-clay which effectually buried the underlying rocks.

Abundant moraine heaps with enclosed tarns and innumerable blocs perchés were also observed between Colifirth and Sand Voes, and in the valleys draining into Rooness Voe, and likewise in the districts of Weesdale and Sandness.

The islands of Unst, Yell, Whalsey, and Bressay nourished a similar series of local glaciers, as is evident from the moraines now strewn on their slopes. In the morainic deposit found on the northwestern shore of Bressay, near Heogan, while the groat majority of the stones consist of grits and liver-coloured quartz rocks belonging to the Old Red Sandstone, there is also a considerable number composed of grey schists, which we identified as belonging to the hills round the head of Dales Voe in the Mainland. This deposit resembles in every respect the sections occurring in the opposite side of the Sound near Grimmester, and again in the bays between Lerwick and Brenista. It is clear therefore that this deposit points to the advance of the local glaciers of the Mainland; they must have been powerful enough to cross the northern portion of the Sound, and to override the north-western part of Bressay. We searched carefully for traces of this more recent deposit in other parts of Bressay, hut failed to discover them. The evidence in proof of the existence of local glaciers in the neighbourhood of the Wart at the south end of the island seems to indicate that their further advance would be chocked on this account. We may well believe, therefore, that as the local glaciers of the Mainland streamed into the Sound, they wore met by the small sheets of ice shed from the Wart, and thence moved southwards along the path of least resistance.