NORWEGIAN BOULDERS IN HOLDERNESS.
ALFRED HARKER, M.A., F.G.S.
The geologist whose lot is cast in East Yorkshire
has no opportunity of examining igneous and crystalline rocks in place
without making somewhat extended excursions. As a compensation for this,
however, he has close to hand, in the boulders of the Holderness
Boulder-Clays, specimens of many rocks of unique interest easily collected
and studied. These are the spoils of the great Norwegian ice-sheet which,
during the maximum glaciation of the region, crossed the North Sea, grounded
in face of the chalk cliffs of Speeton, invaded the then bay of Holderness,
and mingled its burden of foreign material with that brought by native ice
from the English uplands. The boulders from Norway may be collected from the
clays, especially the Basement-Clay of Dimlington and Bridlington, or from
the beach south of Flamborough Head. They have been recorded by the officers
of the Geological Survey, by Mr. Lamplugh and the present writer, and by
various local workers. Many of them are of such characters as to be
distinguished at a glance from all rocks of local origin, or from the
igneous rocks transported from Teesdale, the Cheviots, or the Lake District.
Two or three leading types are worthy of brief remark.
(I.) Angite-Syenites. — After the close of the
Silurian period there were intruded among the Silurian strata of southern
Norway a group of crystalline igneous rocks of peculiar types. Chemically
they are remarkable for their richness in soda, and the mineralogical and
structural characters of some of them distinguish them, in the eyes of the
petrologist, from the rocks of all other districts. One marked type, largely
developed in the coast-stretch between Christiania and Langesundstjord, is a
variety of augite-syenite which Brogger has named laurvikite, from the town
Laurvig. Boulders of this rock are not difficult to find on the Holderness
coast. It is rather coarsely crystalline, and presents a handsome appearance
on a broken surface from the broad cleavage-faces of the felspars, the
lustrous black augite, and the occasional flakes of dark or golden-brown
mica. The felspars are often grey or dark in colour, and some- times show a
beautiful iridescence on their planes. This, with the rather coarse texture
of the rock and the tendency of the augite to assume a diallagic appearance,
points to deep-seated consolidation of the rock-mass from which our boulders
are derived. A microscopic examination of thin slices of the rock reveals
some interesting features, especially the curious intergrowths of different
kinds of felspar to form composite crystals.
(II.) Rhomb-porphyries. — Another type of rock
associated in its home with the preceding, and also recognisable as boulders
in Holderness and Norfolk, is that which Norwegian geologists have long
known under the name Rhombenporphyr, Its distinguishing feature is the
occurrence of abundant porphyritic crystals of a felspar crystallised with
an unusual habit, such that its outlines, as seen, for instance, upon the
smooth surface of a boulder, have often the form of a rhomb. The crystals
may be sharp-angled or rather rounded at the edges, and they frequently show
irregular patches in their interior of different material, as if affected by
corrosive action. They usually have a dark-grey colour. The fine-grained
ground-mass in which these crystals are embedded is of a paler grey colour
with a violet tone, but, when more weathered, it often assumes a reddish
tint. It is chiefly felspathic, but a microscopic examination discovers
other minerals, such as augite, apatite, and little flakes of dark mica.
(III.) Saussurite-Gabbros. — These rocks belong to
another set of post-Silurian intrusions, found in western Norway and
especially in the district around Bergen. They present considerable
variations in appearance, but, speaking generally, they are evidently
crystalline rocks of moderately fine to rather coarse texture, and in hand-
specimens show little more than dark green hornblende set in a dull white
felspathic-looking substance. This latter is partly decomposing felspar,
never showing the bright cleavage-planes of the minerals in the
augite-syenites, partly the minutely granular material rather vaguely
denominated 'saussurite’ by Reusch and other workers in Norway the rocks are
conveniently termed saussurite- gabbros : the great alterations they have
certainly undergone render their original character a matter of some doubt.
The so-called saussurite is an aggregate of albite, epidote, zoisite,
actinolite, etc., often requiring very thin slices and high magnifying
powers to resolve it; for the most part it must be formed by the destruction
of a lime-soda-felspar. The patches of hornblende are no doubt in great
measure secondary too, and this is indicated in hand- specimens by their
green colour and frequent fibrous structure. Several boulders of
saussurite-gabbros have been collected from the beach between Bridlington
and Flamborough.
(IV.) Granites. — Among the granites we find less
strongly distinctive features to warrant identification of the specimens
with particular masses in situ ; still there can be no doubt that the great
majority of the granite boulders in the Holderness clays must be referred to
Norwegian sources. The only British granites that we should naturally expect
in this connection are those of Shap Fell and of the Cheviots; the former
and apparently one variety at least of the latter can be recognised, but the
greater number of the specimens belong to types widely different. Many are
grey granites with brown mica (biotite) as a characteristic mineral; others,
often of greenish grey colour, show both brown and silvery white micas.
These rocks are of medium to fine texture. In thin slices they show bending
of the micas and felspars and disturbance of the optical properties of the
quartz, causing it to give only incomplete extinctions when rotated between
crossed Nicol's prisms. These are well-known effects of the stresses which
accompany great crust- movements, and the granites in question doubtless
formed part of the gneissic areas of southern and western Norway. We also
find among the boulders a red granite of coarser grain than the preceding,
consisting essentially of flesh-coloured felspar with a smaller proportion
of grey quartz and a little brown mica. The microscope shows that the
felspars are mostly microcline and microperthite ; the evidences of violent
mechanical disturbance seen in the other granites are here wanting. This
type of rock may be referred with considerable probability to the district
west of Christiania, where similar granites occur in intrusive masses of
rather later date than the augite-syenites and rhomb-porphyries.
(V.) Gneisses and crystalline schists. — Rocks
belonging to these divisions are found among our boulders in great variety,
and there can be no doubt as to their Scandinavian origin, but we have not
enough information respecting the great development of crystalline rocks in
Norway to enable us to refer individual specimens to precise localities. One
type well represented is a banded horn- blende-gneiss showing lenticular
white and dark streaks about half an inch wide, rich in felspar and in
hornblende respectively. The quartz is partly interstitial, partly in
rounded grains enclosed by the hornblende. Another type has the dark streaks
composed largely of deep brown mica, with some silvery white mica in
addition, the flakes set parallel to the general direction of banding.
Another rock is richer in felspar and of finer texture, showing a compact
white mass enclosing grey quartz-grains and black crystals, about one-fifth
of an inch long, of hornblende with parallel arrangement. Still another type
is fine-grained and dark, the most conspicuous element being dark brown mica
in little glistening parallel flakes, while a lens shows black hornblende in
addition. Hornblende-schists and mica-schists are found also of various
types, one not uncommon among the boulders being a dark mica-schist
enclosing dark red garnets round which the streaks of filmy brown and pale
micas bend like the grain of wood around knots. A finer-grained type has
smaller garnets, and the white mica more prominent relatively to the dark.
Other types met with among the Holderness boulders might be referred to, but with less certainty as to the precise locality of their home; such, for instance, as certain quartzites possibly from the quartzite-conglomerates of the Bergen district; and no doubt many of the remarkable metamorphic rocks there studied by Reusch may have furnished specimens to the ice-sheet which reached our shores. Further examination would be certain also to detect more types from the Christiania district ; probably the red quartz-syenite which Brogger has named 'nordmarkite’ which covers a considerable area of ground, and his 'laurdalite' ,a rock allied to the augite-syenites noticed above, but containing, in addition, the minerals elseolite and sodalite. Indeed an English mineralogist might, perhaps, profitably search among our boulders for examples of the 'syenite-pegmatite veins ' in which the geologist just named has found so long a list of rare and remarkable minerals. I have said enough, however, to show that these strangers among our local boulders, although they form but a small percentage of all those embedded in the clays and washed out on to the modern beach, may usefully occupy a collector in the district and afford material for interesting petrological studies, while illustrating one of those links between Yorkshire and Scandinavia of which another writer has spoken in a recent volume of 'The Naturalist’.
Hull Geological Society 2020
(republished from The Naturalist with permission of the Yorkshire Naturalists Union.)