NORWEGIAN BOULDERS IN HOLDERNESS.
ALFRED HARKER, M.A., F.G.S.
(First published in The Naturalist in 1893)
(First published in The Naturalist in 1893)
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.
Hull Geological Society 2020
(republished from The Naturalist with permission of the Yorkshire Naturalists Union.)