John R Tudor.

The Orkneys and Shetland; their past and present state online

. (page 34 of 59)
Online LibraryJohn R TudorThe Orkneys and Shetland; their past and present state → online text (page 34 of 59)
Font size
QR-code for this ebook

the western shore south of Dales Voe.

In the north and western portions of the INIainland there is a
splendid development of highly siliceous intrusive rocks, which
occupy the most elevated ground in the island. They extend
from a point on the north end of the Mainland, opposite the island
of Uya, southwards to Rooeness Voe, culminating in the dome-
shaped mass of Rooeness Hill. Thence they cross the peninsular
tract to the Heads of Grocken, west of Hillswick, where they
are brought into conjunction with the schists by a fault. The
Drongs and the western part of Muckle Rooe are formed of
this material, and likewise the north-eastern headlands of
Vcmentry, while the small area of quartz-porphyry at Melby
must also be included in the same great intrusive series.
A second extensive sheet occurs in Sandsting, between Grut-
ing Voe and Selie Voe, while still a third is met with in
Papa Stour. The Northmaven mass consists mainly of a
binary granite composed of quartz and pink orthoclase fel-
spar, shading occasionally into pink and salmon-coloured
quartz-felsite. As a rule, the rocks are coarsely crystalline,
and there can be no doubt that they must have consolidated
underneath the surface. Further, the marked columnar
structure which meets the eye along the banks of Rooeness
Voe and from Sand Wick to Brei Wick, suggests the idea of
a great intrusive sheet. Indeed, as the result of detailed
examination of the Rooeness Hill plateau, we have come to
the conclusion that it is an intrusive sheet which forced its
way upwards and laterally between the metamorphic strata on
the one hand and the members of the Old Red Sandstone on
the other, at the time when the Mainland lay buried under the
sedimentary deposits which accumulated during that period.
That such is the explanation of this mass seems all the more
probable from the evidence supplied by the pink felsite of
Papa Stour. This sheet covers the greater part of the island.
The same columnar structure is everywhere apparent in the
sea-clift's, and at various points on the shore it may be seen


cutting across the sandstones and lavas from a lower to a
higher horizon. At the north-west corner of the island, at the
Horn of Papa, a portion of the once superincumbent strata is
still to be seen. They consist of red sandstones, which show
signs of alteration where they rest on the pink porphyry.
Originally they must have covered the whole of the sheet, and
must have been continuous with the strata on the Mainland.
But only a fragment now remains. Those who wish to study
the structure of these sheets would do well to sail down
Rooeness Voe, or along the western shore of Papa Stour.
Along the cliffs the observer is confronted by symmetrical
columns rising from the sea-level, which are traversed by a
series of vertical joints. Hence it follows that the vertical face
of the clitf is preserved, though constantly assailed by the sea
and subjected to continual recession by the removal of huge
slices of rock. Frequently the columns are isolated, and they
are left to battle with the denuding agencies as best they may.
The columns of the Drongs are beautiful relics of the Rooeness
Hill sheet, which have hitherto been able to resist complete

The relations of the granite mass of Sandsting to the altered
Old Red strata, in which it occurs, are well seen on the
shores of Gruting Voe, at the foot of Culswick Hill. Here the
junction-line between the two has nearly the same inclination
as that of the quartzites, which dip to the north at an angle of
about 20°. The two rocks, however, are not perfectly con-
formable, as the granite here and there cuts across the bedding,
indicating the intrusive nature of the rock. Between Reawick
Ness and Selie Voe this granite mass is brought into conjunction
with the metamorphic rocks by the fault which bounds the
altered Old Red area on the east side.

Close by the entrance to the Noss Sound, on the Bressay
shore, a series of " necks " occurs arranged in a linear manner,
which seem to have come to the surface along a line of fissures.
Similar "necks " are met with on Noss, on the opposite side
of the Sound. It is highly probable that these volcanic


orifices served merely as vents for the discharge of steam,
witli occasional showers of triturated materials derived mainly
from the sides of the vents. The adjacent bed of tuff, asso-
ciated with the grey flags, to which reference has already been
made, as well as the nature of the agglomerate which fills these
" necks," supports this view.

The intrusive rocks of this period also occur in the form of
dykes. From the great slieets of granite and quartz-felsite,
numerous veins of granite, felsite, and rhyolite penetrate the
surrounding strata, which are of the same age as the large
intrusive masses. But there is also a later series of dykes
which intersect the granites and quartz-felsites. They consist
of dark green diabase-porphyrite which are easily distinguished
from the bright salmon-coloured acidic series. Hibbert noted
the occurrence of these dykes in Rooeness Hill, and during
our traverses in the districts of Northmaven, Delting, and
Muckle Rooe we came across many similar intrusions, varying
in breadth from two feet to several yards. Along the cliffs of
Rooeness Voe and in Muckle Rooe these dykes are strikingly
exhibited, forming great wall-like masses, running generally in
a north and south direction. Sometimes they project above
the acidic rocks ; while, again, they weather more rapidly, form-
ing great clefts in the face of the cliff. From the fact that
they traverse the lavas and tuffs, as well as the sheets of granite
and felsite, there can be little doubt that they form the last
indications of volcanic activity during the Old Red Sandstone
period in Shetland.

Throughout the islands there are abundant traces of glacial
action. The sea-worn islets along the shore, the striated sur-
faces on the low grounds, and the abraded appearance of the
highest hills alike point to the action of a thick mass of
ice which must have enveloped the islands. The striated
surfaces are very plentiful in some districts ; indeed, we recorded
upwards of three hundred examples during our repeated visits
to Shetland. The geological map of Shetland (Plate 6)
clearly indicates the trend of the ice during the primary and


later glaciation. It will be sufficient for our present purpose if
we summarise the evidence and show how it clearly indicates
two distinct periods of glaciation. In Unst, Fetlar, Whalsay,
the Out Skerries of Whalsay, Bressay, and along the eastern
seaboard of the Mainland and Yell, there is one uniform
system of ice markings trending west, west-south-west, south-
west, and in some cases south-south-west ; while in the western
districts of the two latter islands, as well as in Muckle Rooe,
Papa Stour, and Foula, the strise veer round to the north-west
and north-north-west. The various examples belonging to this
system were produced by ice which crossed the islands from
the North Sea to the Atlantic. They belong to the primary
glaciation. But in addition to these instances we find a series
of ice markings indicating a local radiation of the ice, when,
in fact, Shetland nourished a series of independent glaciers.
These are splendidly developed near Lerwick, where the
average trend is south-east, which is very nearly at right
angles to the direction of the earlier ice movement in that
neighbourhood. At certain localities the ice-markings of the
primary glaciation were completely effaced by the later move-
ment. Several interesting examples occur, however, where the
stris belonging to the two periods can be seen on the same

The evidence derived from the boulder clay and the mo-
rainic deposits confirms in a remarkable manner the double
system of glaciation in Shetland. If it be true that the ice
crossed the islands from the North Sea to the Atlantic during
the primary glaciation, then it naturally follows that the dis-
persal of the stones in the boulder clay should be in complete
harmony with this movement. An examination of the various
sections throughout the islands places this conclusion beyond
doubt. On the western seaboard of Unst the boulder clay
contains fragments of serpentine, gabbro, and graphitic schists?
all of which occur /« sihc on the east side of the Vallafield
range. Moreover the relative distribution of the serpentine
and gabbro stones in this deposit on the western shore is in


direct proportion to the relative areas occupied by these rocks
to the east of the watershed. It follows, therefore, that the
agent which glaciated Unst must have crossed the watershed,
carrying the bottom moraine up the slope and depositing it
under the lee of the range. In Fetlar, blocks of gabbro and
serpentine are likewise found in the boulder clay on the west
coast ; while along the east coast of Yell blocks of gabbro
and diorite occur in this deposit which have been brought
from Unst and Fetlar, testifying alike to the same westerly

The evidence derived from the boulder-clay sections on the
Mainland is equally conclusive ; for it matters not whether we
cross the northern, central, or southern portions of the island,
we are compelled to admit that the ice-flow during the primary
glaciation must have been towards the Atlantic. A traverse from
Ollaberry on the east coast by Hills Wick, Brei Wick, Tang Wick,
to the Grind of the Navir furnishes excellent opportunities for
examining the distribution of the stones in the boulder clay.
In the neighbourhood of Ollaberry and along the road to
Fund's Water, the stones in this deposit are composed of the
underlying gneissose and schistose rocks. None of the frag-
ments of the diorite, nor any of the lavas and ashes along the
western shores, occur in it. But when the diorite area is
reached near Ura Firth the schists and gneiss to the east are
represented in the boulder-clay patches. West of the diorite
section again, in the lee of the ridge of the metamorphic rocks
of Hills Wick, one of the finest boulder-clay sections on the
Mainland occurs. It is upwards of 100 feet in depth, and
contains smoothed and striated stones of diorite, felsite, schist,
granite, &c., but not a single fragment of the lavas and ashes
between Stenness and Ockren Head is to be found in this
section. When we move westwards to the bays of Tang Wick
and Stenness within the area occupied by the volcanic rocks,
the included stones consist of porphyrite, tuff, felsite, schist,
and diorite. The very same phenomena are observable in
the boulder-clay sections on the south bank of Rooeness Voe,



viz. the invasion of the felsite area by the diorite stones, and
the invasion of the area occupied by the porphyrite by the
diorite, granite, and quartz-felsite stones. In short, the evi-
dence obtained along these lines of section completely refutes
the theory of an ice movement from the North Atlantic.

In the centre of the Mainland, blocks of the gneissose series
in the Weisdale district have been carried westwards to the area
occupied by the altered Old Red strata, while striated frag-
ments of the latter rocks occur in the boulder-clay sections on
Papa Stour. Again, on the west side of the watershed, north
of West Quarfif, there is a deposit of boulder clay in the Sandy-
banks Burn containing striated grits, red flags, and shales derived
from the Old Red Sandstone area on the east side. But further,
where the Sandybanks Burn enters the sea, large blocks of the
Lerwick sandstones and well-rounded conglomerates were
found both on the surface and in the boulder clay. A hundred
yards to the south of this locality fragments of the Brenista
flags appear, and close to West Quarff blocks of the basement
breccia are met with, associated with pieces of the Brenista
flags and Rovey Head conglomerates in the thin coating of
boulder clay on the slope and on the shore. If we cross from
Channer Wick to the west coast and traverse the shore section
from May Wick to Loch Spiggie, numerous blocks of Old Red
Sandstone occur which have been carried from the areas along
the east coast. 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.
And so also 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 observe smoothed stones of syenite and
coarse grits in situ to the east, while on the hilLtop (928 feet)
blocks of syenite were noted, which must have been carried
up the slope.

All the facts now adduced unquestionably point to the
westerly flow of the ice. They prove in fact, that the glaciating
agent must have been powerful enough to override the water-


shed of the Mainland. It is rather remarkable that, so far as
our observations went, no trace of marine shells was to be
found in the boulder- clay sections,- nor any fragment of the
secondary rocks from Scotland, which are so conspicuous in
the Orcadian deposit.

But there are certain deposits still to be discussed which
belong to a later glaciation. Along the east coast of the Main-
land, between Lerwick and Dunrossness, there is an irregular
covering of a loose morainic deposit shading into an ordinary
boulder clay resting on the Old Red Sandstone areas. These
deposits contain striated fragments of the clayslates and
schists, derived from the hills extending from the Ward of
Skewsburgh northwards to Dales Voe. Similar deposits occur
on the eastern seaboard of Northmaven between Colifirth
Voe and Fethaland Point, containing granite stones derived
from the Rooeness plateau. These accumulations were, in all
probability, extruded from the mouths of the local glaciers
which radiated from the Mainland. 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. In addition to
these later deposits, however, there is abundant evidence to
show that when the hill-tops had emerged from the icy
covering which so long held sway during the primary glaci-
ation, the severe frosts which prevailed caused an accumu-
lation of blocks and rubbish on the surface of the attenuated
glaciers. In course of time as the glaciers melted back, loose
heaps of rul)bish 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 large, but they are nevertheless very abundant. 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. In the district of Dclting mo-
raines are to be found in the main valleys and round the heads

D D 2


of the larger sea lochs, as, for instance, the Dales, Colifirth,
and Swining Voes, on the east coast, and Near Voe, North
]^>rae, and Voxter, on the west coast. Similar moraine heaps
occur on the banks of Vidlon and Dourye Voes, in Lunnasting,
and in the valleys draining the Rooeness plateau. The islands
of Unst, Yell, Whalsay, and Bressay nourished a similar series
of local glaciers, as is evident from the moraines now strewn on
their slopes.

We must now consider how the glacial phenomena of Shet-
land and the Orkneys may be accounted for. Doubtless there
are some who would not hesitate to ascribe them to the
action of icebergs. We have elsewhere stated our reasons
for believing that the phenomena connected with the boulder
clay can be accounted for satisfactorily only by the action of
land ice. The land ice which glaciated Shetland could only
have come from Scandinavia, as the striated surfaces clearly
point in that direction. If we take the estimate given by our
friend, Mr. Amund Helland of Christiania, for the minimum
thickness of the ice in Sogne Fjord during the period of
extreme cold, it follows that instead of the ice breaking up in
the form of bergs, it must have invaded the North Sea and
moved in a westerly direction towards the Shetland Islands.
He gives 6,000 feet as the estimate at this point, and when we
remember that the average depth of the German Ocean is about
240 feet, we can readily understand how such a mass could
never have floated between Norway and Shetland.

When this mer de glace impinged on the Shetland frontier it
would necessarily be deflected to some extent by the opposing
mass of high ground. Hence, as we move southwards from
Unst, where the average trend is 10° W. 20° S. towards Bressay
and Lerwick, the deflection increases to S.W., and in some
cases to S.S.W. But as soon as the ice reached the crest of the
Mainland it would naturally veer round to the W. and N.W.
This north-westerly movement on the western seaboard of
Shetland, however, was no doubt largely due to the influence
of the Scotch ice-sheet. Recent investigations seem to show


that the sheet which radiated from the Highlands of Scotland
was about 3,000 feet thick. The latter must have coalesced
with the Scandinavian mer de glace on the floor of the North
Sea, and the combined ice-field would naturally take the path
of least resistance. In other words, one portion would flow
north-westwards towards the Atlantic by the Orkneys, while
another part would flow southwards towards the English
coast. We can quite well understand, therefore, how the
Scotch ice-sheet, as it crept outwards along the bed of the
Moray Firth towards the North Sea must have pushed along
the marine shells and silt which it encountered on the sea-
floor. These would be commingled with the boulder clay
which had gathered underneath the ice-sheet, and the shells
would ultimately be smoothed and striated precisely like the
stones in the bottom moraine. Hence the occurrence of
Scotch rocks, together with the shell fragments, is what we
would naturally expect, as the Orcadian group would be over-
ridden by the Scotch portion of the ice-field, and Shetland by
the Scandinavian portion.^ The absence of marine shells in
the Shetland deposit probably indicates a greater extension of
land in that neighbourhood in pre-glacial times. As the great
mer de glace retreated from the coast-line of Shetland ami
Orkney, local glaciers lingered for a time, but eventually, as
the climatic conditions ameliorated, they shrank back into the
hills, sprinkling the slopes with debris and moraine heaps.

Ere closing this chapter we must refer to the interesting
question of the origin of the freshwater lochs and voes. The
freshwater lochs abound chiefly m the Mainland ; and in
certain districts they occur in great numbers. They are due
either to the irregular deposition of the boulder clay or moraine
matter, to hollows in the peat, or to rock basins which have
been eroded by the ice. Indeed, they are so abundant in some of

1 Our friend and former colleague, Dr. Croll, first suggested the proba-
bility of the North Sea being filled with ice, enveloping alike the Orkneys
and Shetland. A full exposition of his views is given in his remarkable
work on Climate and Time in their Geological Relations, chap, xxvii.


the rocky districts, as to recall portions of the north-west of
Sutherlandshire. At present we are only concerned with those
which occupy rockbound hollows and which are the result of
glacial erosion. These occur most abundantly in the diorite
area of Northmaven, on the rocky plateau of Rooeness, on the
headlands north and south of Vidlon Voe and in the district of
Walls. At each of these localities the sheets of water, with cer-
tain exceptions, fill eroded hollows in the rocks ; and from the
manner in which their rocky margins are grooved and polished,
from the freshness of the 7vches moutonn'es which encircle
them, there can be little doubt that they have been eroded by
the ice during the general glaciation. From one of the hills
north of Magnusetter Voe, in Northmaven, we counted about
twenty small lochs in the heart of the diorite area. On the
promontory of Lunnasting they likewise occur in great
numbers, varying in size from basin-shaped hollows to lochs
more than a mile in length.

The voes, or sea-lochs, are among the most interesting
features of the Shetland Islands ; and the question of their
origin is not free from difficulty. Flowing, as they do for
miles, into the heart of the country, it sometimes happens that
only a narrow isthmus is left to prevent the waters of opposite
shores from uniting. Yell is nearly bisected by the Whale-
firth and Reafirth Voes ; and a submergence of only a few feet
would separate Northmaven from the southern portions of the
Mainland and allow the waters of Sullam Voe to flow westwards
into St. Magnus Bay. Sometimes the voes are flanked by
gentle slopes of boulder clay ; at other times they are bounded
by steep walls of rock, as in the well known Rooeness Voe.
Many of the most characteristic sea-lochs run along the line of
strike of the metamorphic rocks, of which the Weisdale,
Stromness, Whiteness, Dales, and Laxfirlh Voes may be cited
as the best examples ; but there are others which have no con-
nection with the lines of stratification. As a rule they are found
to merge into narrow valleys draining the high grounds, the
width of the voes being in direct proportion to the size of the


valleys. This relationship would seem to indicate that these
narrow fjords are submerged land valleys which existed long
before glacial times. Their origin therefore would be analogous
to that of the fjord valleys on the western seaboard of Norway
and Scotland. In that case, the voes must have been carved
out by the ordinary agents of denudation, when the floor of
the sea which now surrounds Shetland formed dry land. Both
in Scotland and along the east coast of England, the evidence
derived from the buried river channels would lead us to believe
that these countries stood at a higher level in preglacial times
than they do now, and we may well believe that Shetland
shared in the same Continental conditions. The absence of
shells in the boulder clay seems to strengthen this conclusion.
At any rate, the agents of denudation would be guided in their
operation in a large measure by the strike of the metamorphic
rocks, and if there was a wide area of land round what now
constitutes the Shetland archipelago, they would accomplish
greater results, as the size of the rivers would be in proportion
to the area of drainage. We have seen also that some of the
voes and inland valleys coincide with the outcrops of bands of
limestone, the erosion of which would be aided by chemical

There can be no doubt, however, that the sea lochs in
Shetland were deepened, during the primary glaciation, by the
great mer de glace which crossed the islands. Of this, we
may adduce two striking examples. The soundings given in
the Admiralty chart show that SuUam Voe, which is one of the
largest of the sea-lochs in the Mainland, measuring upwards of
seven miles in length, varies from ten to fifteen fathoms in
depth between Foula Ness and the mouth of Voxter Voe.
Beyond the latter point, to the head of the voe, the depth
suddenly increases to twenty-one and twenty-five fathoms.
This increase of sixty feet in depth is doubtless owing to the
intense abrasion caused by the ice as it impinged on the rocky
isthmus of Mavis Grind at the head of the voe. The eastern
face of Mavis Grind still retains the finely-polished surface


along with the ice markings. Another instance occurs in Rooe-
ness Voe, for at the bend north of Ura Firth the depth varies
from T02 to 138 feet, while about two miles further down the
loch shallows to 42 feet.

The widespread covering of peat throughout many of the
islands is rather remarkable in a region which is now destitute
of trees. When the Shetland peat is viewed in connection
with similar deposits in the Orkneys, Scotland, and Scandinavia,
we are led to the conclusion that during the period of its
vigorous growth the land stood at a higher level, and probably
enjoyed more genial conditions than at present. Since this
post-glacial elevation the land has been submerged. Nowhere

Online LibraryJohn R TudorThe Orkneys and Shetland; their past and present state → online text (page 34 of 59)