The Pamphlet Collection of Sir Robert Stout: Volume 22
The Boulder-clay and morainic deposits confirm in a remarkable manner the conclusions already established regarding the double system of glaciation in Shetland. The rich variety of rocks, not only in the Mainland, but also in Unst and Fetlar, enables us to test the truth of these conclusions by noting carefully the distribution of the included stones and the sources whence they were derived. If it be true, as has just been stated, that the ice moved from the North Sea to the Atlantic during the primary glaciation, it naturally follows that the Boulder-clay or moraine profonde occurring to the west of the serpentine areas in Unst and Fetlar should contain a certain percentage of stones derived from those areas. The very same reasoning is also applicable to the Mainland; and in order to show how completely the dispersal of the stones in the Boulder-clay substantiates this conclusion, we shall briefly describe a series of traverses we made in Unst, Fetlar, and the Mainland, where the rocks vary in lithological character, indicating the variations in the Boulder-clay and the distribution of the included stones.
Round Balta Sound, in Unst, this deposit is sparingly distributed, only occasional sections being visible on the north and south sides of the bay, the included blocks being almost wholly derived from the page 795 gabbro and serpentine areas. Duo west of the Sound, in the hollow along which flows the Baliasta burn, there is a considerable covering of Boulder-clay, the included stones being mainly composed of serpentine and dark graphitic schist, though the underlying rock consists of green chloritic schist and gneiss. Ascending the Vallafield ridge, the slope is found to be covered with heather and peat, and well-nigh destitute of drift. Where this covering has been removed, numerous bleached fragments of serpentine are to be found; while near the top of the ridge, where the slope is more gentle, occasional patches of Boulder-clay are met with in which well-striated fragments of serpentine, gabbro, and black schist occur. No Boulder-clay is to be seen on the watershed, which reaches a height of over 600 feet at this point; still, where the peat is worn away, a few bleached fragments of serpentine are observable.
Along the western coast, from Woodwick to Wick Bay, a narrow ledge or terrace intervenes between the rock-slope and the coastline, which is covered with Boulder-clay more or less continuously. Excellent sections of it are exposed at the heads of the numerous geos. At Collaster it consists of a tough fawn-coloured clay, full of striated stones of all sizes up to blocks 2 to 3 feet long. The following percentages wero taken from the banks of the voe at this locality:—
|North side of bay||South side of bay.|
|Gneiss and schist (underlying rock)||53 per cent.||60 per cent.|
|Serpentine (from east side of watershed)||32 per cent.||22 per cent.|
|Gabbro (from east side of watershed)||11 per cent.||9 per cent.|
|Black schist (from east side of watershed)||1 per cent.||3 per cent.|
|Vein-quartz||2 per cent.||3 per cent.|
|Granite (from Lambaness)||1 per cent.||2 per cent.|
|Ycin-granite||1 per cent.|
|North side of Collaster Voe||32 per cent.||11 per cent.|
|Houlon Ness||21 per cent.||22 per cent.|
|Wick Bay||22 per cent.||26 per cent.|
This relative distribution of the stones is not a mere accident; for a glance at the map will show that to the E. and E.N.E. of Collaster the serpentine occupies a much greater breadth of ground than the gabbro, while to the east of Wick Bay the conditions are reversed. Such a direct relationship is inexplicable on the hypo- page 796 thesis that the primary glaciation of the island was due to floating ice.
If we traverse the southern shore from Muness Castle to Belmont, similar evidence is obtained from the Boulder-clay regarding the ice-carry. Again, in the north part of the island, in the lee of Saxavord hill, this deposit occurs on the east bank of Burra fiord, about 300 feet above the sea-level, where it reaches 50 feet in depth. The material is mainly derived from the talc-schist and quartzose bands which constitute the hill; but a considerable proportion of the stones likewise consist of the peculiar granite of Lambaness. Now it must be borne in mind that, ore these granite-fragments could have reached this position along the path-line indicated by the striæ, they must have been transported in the moraine profonde across the shoulder of Saxavord hill, where it attains a height of 800 feet; whereas none of the Lambaness granite occurs in situ at a greater height than 150 feet.
On the west coast of Fetlar, blocks of gabbro and serpentine, derived from the centre of the island, occur in the Boulder-clay north and south of Burgh Hall; while striated fragments of the same rocks, from Unst, are found in this deposit on the north-east coast of Yell. We likewise observed smoothed fragments of gabbro from Fetlar in this deposit on the east coast, between Mid Yell and Basta Voe.
A traverse across the district of Northmavine, in the Mainland, from Ollaberry on the east coast, by Hillswick, Braewick, Tanwick, to the Grind of the Navir, furnishes admirable opportunities for examining the distribution of the stones in the Boulder-clay. A glance at the map will show the variety of rock-formations which occur along this lino; and the marked lithological characters of the rocks fortunately prevent any possibility of mistaking them. It is particularly observable that the till partakes of the physical character of the rock-formation on which it rests, though there is also a percentage of foreign stones derived from localities which lay in the path of the ice-sheet. The distribution of the stones in the Boulder-clay along this line of section places beyond all doubt that the ice-sheet, as it impinged on the Mainland, moved in a W.S.W. direction, and as it left the Mainland it veered round towards the N.W. and N.N.W.
|Diorite (in situ to the east of section)||71||per cent.|
|Felsito and hornblendic porphyry||17||per cent.|
|Vein granite||6||per cent.|
|Vein orthoclase felspar||4||per cent.|
It may seem strange that none of the underlying schists are represented in the above percentage; but it so happened that the stones we selected high up in the section averaged about 4 inches across. In another percentage of stones measuring about 2 inches across, the underlying schists number about 15 per cent. The prominent ingredient in this section is the diorite, which occurs to the east of Hillswick; but it ought to be remembered that not a single fragment of the lavas and ashes to the west are to be found in this deposit.
About two miles to the west of the foregoing locality, in the north-east corner of Braewick Bay, a section of Boulder-clay, about 12 feet high, is exposed resting on the intrusive quartz-felsite, containing diorite, schist, and felsite stones; while still further west, within the limits of the contemporaneous volcanic rocks, sections of Boulder-clay occur in the bays of Tanwick and Stennis, the included stones being dull purplish porphyrite, blocks of tuff, quartz-felsite, schist, and diorite. Further, along the storm-swept cliffs of the Grind of the Navir, a thin deposit is traceable containing the same ingredients as at the localities last mentioned. The diorite stones, however, are comparatively rare at the Grind of the Navir: in fact they gradually diminish in number in proportion to the distance from their parent source; and the very same remark applies to the other ingredients.
We traversed the south bank of Roeness Voe from the head of the sea-loch to Ockren Head, where similar phenomena were observed, viz. the invasion of the quartz-felsite area by the diorite stones, and the invasion of the area occupied by the porphyrites by the diorito and quartz-felsite stones. Indeed the evidence obtained along these lines of section completely refutes the theory that these northwesterly striæ could have been produced by ice coming from the North Atlantic.
Another traverse, from Vidlon Voe westwards by Swining Voe page 798 and across the high grounds to North Brae, indicates in an unmistakable manner the direction of the ice-movement during the primary glaciation. In passing out of the Vidlon valley, across the watershed into Swining Voe, the eye readily fixes on a rocky ridge or, rather, a scries of semi-detached roches moutonnées, which present their bare slopes to Vidlon Voe, in the lee of which lie well-marked "drums" of Boulder-clay, whose long axes coincide in direction with the trend of the striæ. This deposit covers the whole of the gentle peat-covered slope which forms the eastern boundary of Swining Voe; and it contains numerous fragments of a band of nodular gneiss, which crosses Lunnasting in a north-and-south direction about midway between Lunna and Lunna Ness.
But, further, the Boulder-clay in both the valleys draining into Swining Voe consists of a tough tenacious clay, full of striated stones, derived mainly from the underlying schists, quartzites, and dark hornblendic rocks; and associated with these are fragments of the coarse gneiss of the promontory of Lunna and the nodular band already referred to.
Now it is interesting to note that both in the Vidlon and Swining Voes, which lay across the path of the ice-sheet, the Boulder-clay is found to have the greatest development on the eastern shores; while the western slopes, which were exposed to the full sweep of the abrading agent, axe finely moutonnées and striated, and well-nigh destitute of drift. But if we take the adjoining Colafirth and Dales Voes, which coincide very nearly with the direction of the ice-markings of the primary glaciation, we find well-marked Boulder-clay slopes on both sides of the sea-loch, indicating that the deposit was distributed more or less equally along the bottom and sides of the valley.
These features remind one very much of the familiar terraces of Boulder-clay in the high-lying valleys in the south of Scotland; while the deposit itself is in all respects identical with the ordinary Scotch till. Indeed, whether we consider the resemblance in the mode of occurrence, or the character of the deposits in Scotland and the Shetland Isles, we cannot resist the conclusion that both have a similar origin.
But even in the Dales and Colafirth Voes it would seem that the deposit steals further up the slopes, and attains a greater thickness on the north than on the south banks—a phenomenon which may be accounted for by the supposition that the ice, as it moved up the sea-lochs, had a greater erosive effect on the one seabank than the other. This supposition is confirmed by a glance at the stri;e-map, which shows that the markings are not quite coincident with the banks of the voes, but cross the southern shores at a gentle angle.
After crossing the Leas of Deal and descending the valley between the Duddon and Gallows hills towards Busta Voe, the boundary-line of the diorite is again crossed, when fragments of this rock are found abundantly both in the moraines and the underlying Boulder-clay. Not a single block of this rock, however, is to be met with on the surface or in the drifts to the east of the boundary-line.page 799
In the district which stretches from Weesdale westwards to Walls, and thence to Melby, the Boulder-clay sections furnish corroborative evidence of the north-westerly movement of the ice in that region. In the vales of Tingwall and Weesdale there is no trace of the altered Old-Bed-Sandstone rocks which occupy the peninsular tract of country to the west. But as soon as the line of the great fault is crossed, which bounds these strata between Aith Ness and Selie Voe, abundant fragments of the gneissose rocks of Weesdale and adjoining tracts, as well as blocks of the porphyritic granite, are found in the Boulder-clay resting on the altered Old-Bed rocks.
Again, in the sections round the coast-line in the neighbourhood of Melby, the fragments in the subglacial deposit entirely consist of the underlying sandstones and the red quartzites and shales of Sandness Hill, along with some pink quartz-felsitcs; but none of the purplish porphyrites which occur in Papa Stour are represented in these sections. Had the movement been from the north-west, then assuredly some fragments of the porphyrites would have been met with round Melby. Instead of this being the case, however, the Boulder-clay of Papa Stour contains numerous fragments of the altered Old-Red-Sandstone rocks from the Mainland.
Another traverse across the island, from Gulberwick to West Quarff, reveals phenomena no less remarkable. On the slopes of the hills above Gulberwick, fragments of the red flags of Brenista and grits are met with; and they also occur in some patches of Boulder-clay near the head of the burn draining into the bay at East Quarff. On the west side of the watershed the Sandybanks bum is reached, which flows into Cliff Sound about a mile and a half to the north of West Quarff. In this hollow there is a deep covering of Boulder-clay, attaining a thickness near the farmhouse of 20 feet. Following this burn to its source, the deposit is found to consist of tough tenacious clay, with well-scratched stones, many of which consist of grit, red sandy flags, and shales of Old-Red-Sandstone age, associated with grey schists derived from the underlying rock. But further along the western seaboard, between the mouths of Sandybanks burn and West Quarff, similar phenomena are observable. Where this stream enters the sea, large blocks of the Lerwick sandstones and well-rounded conglomerates, measuring 2 feet across, were met with both in the Boulder-clay and on the surface. A hundred yards to the south of this locality fragments of the Brenista flags appeared, and became more numerous as we followed the coast-lino southwards. Not far from West Quarff blocks of the basement breccia were met with, associated with fragments of the Brenista Flags and Rovey-Head conglomerates, in the thin coating of Boulder-clay on the slope and on the shore.
We have already indicated the relative areas occupied by these subdivisions of the Old-Red-Sandstone rocks between Rovey Head and East Quarff, for the special purpose of showing the analogous distribution of the stones in the Boulder-clay on the western seaboard.
On referring to the map it will be seen that the members of the page 800 Old-Red-Sandstone occupy the strip of low ground from Levenwick southwards by Loch Spiggie to Quendale bay. Now from Channer-wick southwards along the hill-tops to the "Wart of Skewsburgh (854 feet), smoothed blocks of the red flags, varying from 2 inches to a foot across, are to be found in those places where the peat has been worn away. These blocks are readily detected on the top of Skewsburgh hill, in spite of their being bleached by the peat.
Further, if we cross from Channerwick to the west coast, and traverse the coast-lino from Maywick to Loch Spiggie, numerous blocks derived from these areas are likewise met with. In the hollow which runs south from Maywick to Bigton, striated blocks from the red flags are strewn on the eastern slope overlooking the valley, the largest of which have been used as building-material by the villagers of Maywick. Again, on the hill-slope about a mile cast from Bigton, blocks of flaggy sandstone are very numerous; and they likewise occur very abundantly in the Boulder-clay on the top of this hill. On both sides of Bigton Bay, the sections of Boulder-clay contain numerous fragments of red flags, though the majority of the stones are made up of the underlying schists. Close to the point where the sand-bar joins the island of St. Ninians to the Mainland, a similar admixture of stones, derived from the red flags on the cast side of the island, is to be seen in the Boulder-clay underneath the blown sand. And so, too, southwards towards Loch Spiggie, wherever patches of Boulder-clay have escaped denudation, the same phenomena are observable.
Again, on the slope of Fitful Head, at a height of 800 feet by aneroid measurement, there are small patches of this deposit, in which we observed smoothed stones of syenite and coarse grits in situ to the east; while on the hill-top (929 feet) blocks of syenite were noted, which must have been carried up the slope. These facts unquestionably point to the same westerly flow of the ice; but at the base of the slope, along the margin of the syenite area, there is an excellent section of morainic stony clay, in which blocks of schist, syenite, and Old-Red grits are commingled. This deposit is evidently the product of a later glaciation, when the Fitful Head shed its own glacier, and when the detritus which had accumulated on the slope during the primary glaciation was rolled downwards to the low ground at the foot of the hill.
From the evidence now adduced it cannot be doubted that, during the primary glaciation, the great mer de glace crossed the Mainland from the North Sea to the Atlantic. We might have multiplied the evidence considerably by referring to the Boulder-clay distributed over the other islands; but we have confined our observations to those lines of section where there is the greatest variety of rock-formations, in order to show both the strength and harmony of the evidence. Ere leaving this division of the subject, we ought to state that, though we carefully searched the numerous sections of Boulder-clay in the different islands, we found no traces of shells in the deposit.
There are certain phonomena still to be discussed, which indicate page 801 the gradual retreat of the great ice-sheet when this northern archipelago was no longer influenced by the ice-sheets of adjoining countries, but nourished a series of local glaciers which deposited their moraines as they shrank back into the hills.