Glen Tilt Research Articles

The Glen Tilt diorite: crystallization, petrogenesis and relation to granitic rocks

Synopsis The Glen Tilt diorite forms a significant component of a late Caledonian, calc-alkaline, predominantly granitic intrusive complex on the north-west side of the Loch Tay Fault. It intrudes Grampian Group and lowermost Appin Group Dalradian metasedimentary rocks. The diorite is massive and contains common to abundant hornblende, variable amounts of biotite and clinopyroxene and late-stage intergranular quartz and K-feldspar. Opaque oxide and titanite ar e minor components. Associated granitic rocks comprise biotite granodiorite/granite and muscovite-bearing leucogranite. Dominantly north-easterly trending porphyritic microdiorite dykes are common in the area and cut elements of the granitic plutons. Petrographical, mineralogical and chemical variation in the diorite is the result of crystal fractionation and accumulation processes. Comparison with experimental studies together with thermobarometric estimates suggests the body crystallized at 2–4 kbar and under hydrous conditions; early clinopyroxene exhibits a reaction relationship with later crystallizing hornblende. Pressure estimates are consistent with phase assemblages developed in contact metamorphosed pelitic rocks. Bulk rock and mineralogical chemical comparison suggest the porphyritic microdiorite minor intrusions are co-magmatic with the main diorite. Field relations and petrographical observations indicate that biotite granodiorite/granite is emplaced early in the intrusive sequence and that localized melt remobilization and back-veining accompanied diorite emplacement. Leucogranite probably occurs later in the intrusive sequence although the complex as a whole was probably emplaced in a short time span. The chemical compositions of the main plutonic elements suggest they are not directly related by differentiation of a common magma but more likely represent contribution s from differing mantle and crustal sources. The complex as a whole probably represented the site of a magma chamber that fed coeval Siluro–Devonian volcanism, the ascent of magma probably being focused along the Loch Tay Fault and an intersecting NW trending structure.

John MacCulloch's ‘Millstone Survey’ and its consequences

Summary During the Napoleonic Wars there was a shortage of suitable millstones for the British Ordnance gunpowder mills. John MacCulloch spent five summers searching Western Britain for a source of non-siliceous limestones to be used as gunpowder millstones. His search was authorized by the Board of Ordnance, which also paid all his expenses. By 1812 MacCulloch had found suitable limestones in Sutherland, Skye, and at Glen Tilt, but the Board of Ordnance were unable to exploit any of the sources until 1815 when unsuccessful quarries were opened on Skye. After the defeat of Napoleon at Waterloo the British millstone project was abandoned. The military objective of the millstone survey was not achieved, but MacCulloch learned much about the geology of the older rocks of Britain and he built a reputation as an expert geologist through the papers he submitted to the Geological Society of London.

Thermal evolution of metamorphic rocks in the Central Highlands of Scotland

Summary Conditions of the metamorphic maxima in lower Dalradian kyanite zone pelites from Kinloch Rannoch, Schichallion, Glen Tilt, Glen Ey and Glen Avon are calculated from analyses of coexisting kyanite, garnet, biotite, plagioclase and quartz (staurolite or chlorite also coexist in some samples). Temperatures of 550–620°C were reached at pressures of 9–12 kb corresponding to depths of burial of 35–40 km. Compositional gradients in zoned garnets indicate that pressures were falling at the time maximum temperatures were reached. The high values of pressure are confirmed by observations of the sub-assemblages kyanite-zoisite (not anorthite-margarite) and kyanite-garnet-chlorite-quartz (not staurolite or chloritoid). The pressure of intrusion of a Newer Igneous complex in upper Glen Tilt is 5–6 kb. The present results are contrasted with conditions at a similar structural/stratigraphical level 50 km WNW where comparable temperatures were reached at depths around 20 km. It is concluded that either erosion rates early during metamorphism were lower or the tectonic cover was greater in the Central Highlands than to the west.

A Late-Glacial Ice Cap in the Central Grampians, Scotland

Using evidence provided by hummocky moraines, end moraines, till sheets, drift limits, erratics, meltwater channels, outwash deposits, ice-contact fluvioglacial forms and major periglacial forms, an ice cap (with associated outlet glaciers) that developed in the central Grampians in Late-glacial times is reconstructed using a three-dimensional approach. The former ice surface is contoured at 50 m intervals. The ice mass had an area of nearly 3oo km2, a volume of 32 km3 and an average thickness of about 10 m. Former firn lines are determined and inferences made about contemporary temperatures and the distribution of precipitation. THE area investigated lies in the centre of the Grampian Highlands astride the major east-west watershed (for location see inset, Fig. 2). It is bounded in the west by the Pass of Drumochter glacial breach (D. L. Linton, I951) and in the east by the straight, fault-guided Glen Tilt trench. Part of the area is known as Gaick Forest, but for brevity the whole will be referred to as the Gaick area. The main relief feature is a plateau at 700-930 m that is limited almost everywhere by a sharp 200-450 m descent to lower ground (Fig. I). The plateau is especially well developed in the western half of the area, although it is here cut clean across by a major glacial breach 400 m deep (Loch an Duin breach). The bounding slopes of the plateau are broken by numerous valleys that trend perpendicular to its edge and thus collectively form a radial system. Unrelated to this radial pattern, on the northern edge of the area, is the Glen Feshie glacial breach (Linton, 1949), immediately beyond which are the Cairngorms. Apart from Linton's contributions on glacial breaching and a brief account by J. K. Charlesworth (I955), the only published work on the glaciation of the Gaick area is by G. Barrow et al. (1913). These authors concluded that glacier ice that accumulated in the Gaick area was subsequently largely surrounded and partly overwhelmed by an ice sheet centred in the western Grampians. Local ice later became important again in parts of the area as the exotic ice became less powerful. The limits of local ice were not specified and a readvance was not proposed. BASIC FIELD EVIDENCE

The Diorites and Associated Rocks of the Glen Tilt Complex

AbstractThe diorites and Sron a' Chro' granite of the Glen Tilt complex enclose hornblende-schist xenoliths derived from the Dalradian epidiorites of the country rock. Small hornblendite xeoliths are found in the complex but are restricted to the diorite outcrop. Porphyroblastic hornblende is developed within the schist xenoliths and reaction zones rich in hornblende are formed around the xenoliths. The mineralogical and textural characteristics of these reaction products suggest that the typical two-hornblende generation appinites of the complex represent modified hornblende schist. A similar origin for the quartz-orthoclase-appinite of the complex is rejected and their development from hornblendite is indicated.

The Diorites and Associated Rocks of the Glen Tilt Complex, Perthshire. II. Diorites and Appinites

AbstractThe south-eastern margin of the Glen Tilt mass consists of a series of diorites showing a considerable variation in mineralogical and chemical composition. Associated with, and enclosed within, the diorites are a number of small masses of coarser hornblende-felspar rocks which are described as appinites. There is an absence of sharp contacts between the various rock types of the complex and a hybrid origin is suggested to account for the mineralogical assemblage of the appinites.

The composition and paragenesis of the hornblendes of the Glen Tilt complex, Perthshire

The south-eastern margin of the Glen Tilt complex consists of a long strip of diorites with a number of small associated patches of rocks of appinitic type. Xenoliths, both of hornblendite and hornblendeschist, the latter belonging to the Perthshire series of the Dalradian, are found enclosed within the dioritic rocks, which range petrographically from diorite to quartz-mica-diorite. In all these rocks hornblende is the most important fcrromagnesian constituent always predominating over pyroxene in the basic members and frequently persisting to the exclusion of biotite in the intermediate rocks of the intrusion. The hornblendes were examined chemically because of the difficulty of estimating their composition even from a thorough optical investigation. As they play such an important role in the complex it is considered that they may give some indication of the history and mode of formation of the rocks in which they occur. Such a chemical investigation of a series of related hornblendes will also indicate the possible range of composition of common amphiboles within the diorites and related rocks of a single complex.

The Diorites and Associated Rocks of the Glen Tilt Complex, Perthshire

The Glen Tilt Complex, one of the larger masses of the Newer Granites of the Central Highlands, is included in sheet 64 of the Geological Survey of Scotland. The greater part of this complex is a granite which is bounded on the south-west and southeast by an earlier series of intermediate and basic rocks. The granites described in this contribution are restricted to a small area at the south-eastern margin of the large granitic intrusion generally known as the Beinn Dearc granite. The smaller and independent intrusion of the Sron a ‘Chro’ granite and a number of smaller masses of granite associated with the marginal strip of diorites on the north-western side of Glen Tilt have also been examined. These small isolated areas appear to be contemporaneous with the intrusion of the main Beinn Dearc mass and have been intruded between the earlier diorites and the margin of the intrusion, a feature not uncommon in many of the other Scottish Newer Granites. A small independent mass of muscovite-biotite-granite intruded into quartz-mica-diorite occurs on Conlach Mhor. Although these rocks are completely isolated from both the biotite- and muscovite-biotite-granites of the main Beinn Dearc intrusion their essential similarity with the latter leaves no reasonable doubt of their common origin.

The first geological map of Scotland

The earliest geological map of Scotland1 was published in Paris in the year 1820. It formed one of the illustrations to Dr Ami Boué’s ‶Essai Géologique sur L’Écosse,″2 and measured about 10 inches by 8 inches, being on the scale of about 40 miles to an inch. The map was roughly hand-coloured and in consequence all the existing copies do not agree, but it is of considerable importance to us as the first attempt to show the whole of Scotland on one sheet, and it gives us a very clear idea of the state of geological knowledge of our country 115 years ago. Before that time much work had been done on Scottish geology and many scientific papers had appeared on that subject. Probably the earliest writings which are still of use were Sinclar’s (1672, p. 262) ‶Account of the Midlothian Coalfield,″ which is quite modern in its point of view. There is also an excellent account of the Clackmannan coalfield by Robert Bald (1811). We must not forget, of course, the work done by Hutton and Playfair in Arran, Glen Tilt, and in Berwickshire. These memoirs are still of enduring interest. Much, however, that had been published was of a different order. For many years it had been known that the west side of the island of Staffa possessed a marvellous range of hexagonal columns formed of a rock which was recognised as basalt. The first authority who discerned the importance of this occurrence was apparently Sir Joseph Banks

VI. Notes on a Series of Specimens illustrating the Effects of Differential Weathering on some Scottish Metamorphic Rocks

List of Specimens exhibited. 1. Loch Tay Limestone, 1 mile west of Crianlarich. 2. Limestone with Actinolite, Glen Elg. 3. Limestone, Glen Tilt. 4. Junction of Limestone and Gneiss, Gilbert's Bridge, Glen Tilt. 5. Quartz veins, in folded Gneiss, Gaick Forest, Inverness. 6. Compact Limestone, showing irregular weathering. 7. Cordierite-Actinolite Schist, Dubh Visge, Garabal Hill. Many of the specimens have a banded appearance, with sharp, salient, more or less parallel ribs on the weathered surface, due to the differential weathering of alternations of harder and softer layers. In 1, flakes of mica are distributed through the more resistant layers. On weathering, these stand out as sharp, parallel ribs, an appearance characteristic of the Loch Tay and other Scottish metamorphic limestones. In 3, the alternations are layers varying slightly in degree of purity. In 2, 4, and 5, bands of actinolite, gneiss, and quartz constitute the harder layers. No. 7 is a specimen of a peculiar schist found as a block in the bed of the Dubh Visge, a burn near the Garabal Hill igneous complex. Its weathered surface shows large nodular lumps, often an inch across, separated by deep indented depressions. A thin slice shows that the rock consists mainly of biotite and cordierite, the latter being quite fresh, with the characteristic trillings and yellow pleochroic halos. A few crystals of actinolite break across the foliation of the rock. Sillimanite, apatite, and iron ores occur as inclusions in the cordierite. A section across one of the nodules, however, shows an almost total This 250-word extract was created in the absence of an abstract

Observations on the Tiree Marble, with Notes on others from Iona

I. Introduction. Although so well known, and represented in many mineralogical collections, the Tiree Marble has never been described in detail. Macculloch says that the Tiree flesh-coloured marble ‘resembles the greater number of the primary limestones found in gneiss and mica-slate, and may be considered as a large nodule ... The nodule of limestone ... appears to be an irregular mass of about 100 feet in diameter, and is surrounded on all sides by gneiss.’ [The marble] ‘occasionally contains imbedded lumps of granite or gneiss, similar to those which occur in the limestone of Glen Tilt. These are always visible at the surface, from their superior power of resisting the action of the atmosphere.’ He also describes the exposures of limestone in a field south-east of Balephetrish Hill. He says that ‘it is possible that the masses of limestone thus found in gneiss have once been stratified, and that they have suffered some posterior changes by which the appearances of this disposition have been obliterated.’ Prof. Bonney has given a valuable description of the dynamic phenomena so well illustrated in thin sections of the Tiree Marble. Prof. Cole & Prof. Sollas have suggested that the Tiree Marble may originally have been a wind-blown coral-sand rock, with abundant rounded crystals of detrital augite, etc., such as was found on the east side of Mer, one of the Murray Islands. This theory appears untenable; for it is impossible to imagine a volcanic origin for the majority of the accessory minerals, which, with

Theory of the Earth with Proofs and Illustrations

AS we learn from Sir Archibald Geikie in his interesting preface, the history of the later portion of James Hutton's great work on the “Theory of the Earth” is a perplexing question. In 1795 the well-known two volumes appeared, containing the first and second parts, but the title-page bears the words “in four parts.” Of those two the first is little more than a reprint of the essay on the same subject read to the Royal Society of Edinburgh in 1785. The second part, dealing with the operation of natural causes on the surface of the globe—or dynamical geology, as it is now sometimes called—was new matter. These volumes are without preface or preliminary sketch, so that no clue is given to the plan of the remainder of the work, while the fact that Hutton ends his second volume with an elaborate summary suggests that he contemplated a pause of some duration before issuing the remainder. At his death, in 1797, the third volume, according to Playfair, was practically complete; and we do not know why his friends did not publish it. Perhaps, as Sir A. Geikie suggests, they waited for certain illustrations, which Mr. John Clerk, Hutton's great friend, had promised to furnish. Gaps are left for these in the text; but, at any rate, Playfair and Lord Webb Seymour quote from the manuscript in a paper on Glen Tilt, read to the Royal Society of Edinburgh in 1814. It was then lost sight of—the earlier portion, including three chapters, has vanished; the other was a parting gift from Lord Webb Seymour to Leonard Horner, and was presented by him to the Geological Society of London in 1856. In its charge it has remained, forgotten by most of the Fellows, till Sir Archibald Geikie urged its publication on the Council, promising to take upon himself the laborious task of editing. Needless to say, this has been admirably done. The manuscript is printed as though it had followed on the preceding volumes. A few small lacunæ or matters needing explanation are dealt with in explanatory notes, which are models of terseness and a great help to the reader, who, in addition, has to thank the editor for an index, not only to this volume, but also to the two others.

On the Plutonic Rocks of Garabal Hill and Meall Breac

The schists of the Southern Highlands of Scotland have been traversed in many places by igneous rocks. Sills and dykes are widely distributed throughout the district, and here and there, as for example in Glen Tilt, Glen Lednock, and near the head of Loch Lomond, extensive areas are composed of plutonic rocks. Our object in this communication is to describe some of the phenomena which may be observed in the last-mentioned locality. The plutonic rocks in question form the belt of high ground stretching in a south-west direction from Inverarnan. They vary considerably in chemical and mineralogical composition. Different parts of the mass can be proved by the phenomena of veins and inclusions to belong to slightly different periods, but regarded as a whole it must evidently be referred to one geological epoch. Whenever two portions in juxtaposition are seen to differ in age, the more acid, so far as our observations go, is invariably the younger. Thus the veins are more acid and the inclusions more basic than the material surrounding them. Rather more than half a mile above Inverarnan tonalite (quartz-diorite) appears in the bed of the Arnan and on each side of the stream. From this point it extends in a south-westerly direction for about miles, as shown on the accompanying map, and possesses a fairly uniform character. Garabal Hill, south-west of Inverarnan and immediately south of the place where tonalite appears in the Arnan valley, is composed of diorite. In some places, as, for instance

XVII.—Chapters on the Mineralogy of Scotland. Chapter Fourth.—Augite, Hornblende, and Serpentinous Change

Malacolite was first observed in Scotland by Dr Macculloch, who included it with the second sub-species, under the name of Sahlite.He, in his different works, noted its occurrence—“near the church of Rodel in Harris,”- “in Tiree,”-“Glen Elg,”-“Glen Tilt,”-and “in Rannoch.” With his usual power he describes the occurrence, varieties, and even the crystalline form of the mineral,—the clearness and precision of his descriptions being only equalled by the utter want of precision and frequent mystifications as to their localities.The above sentence conveys all the direct information which Dr Macculloch vouchsafes as to the five localities indicated by him, though in a vague manner he may occasionally lead one to conjecture that certain particular spots are meant.By the time that the mineralogist, led by Dr Macculloch to Glen Elg, Tiree, or Rannoch, has walked some twelve or twenty miles in an unsuccessful search, he begins to think that if he be successful, he has some claim to be himself regarded as an independent discoverer.

V.—Additional Remarks on Glen Tilt.

The observations which were made on the structure of this valley in 1814, appeared in the third volume of the Transactions of the Geological Society. Since that period, I have had (in 1817) an opportunity of traversing the same ground, and in somewhat greater detail. In so doing I have found no occasion to change the opinions then formed respecting its structure. On the contrary, a greater intimacy with its parts and with their disposition, and an increased practice in observation, have tended to confirm the remarks originally made. A slight correction only is required, resulting from a much wider investigation than the former; it respects gneiss, and will be introduced in its proper place. Several additional circumstances have however occurred, which are not only interesting in themselves, but will probably be acceptable to the Society, as tending to render the account of this spot more complete. I need make no apology for the detached form in which they are recorded, as they do not admit of any useful arrangement. Granite Veins Since these veins have here been traced to the great mass of granite, it would be interesting to know how far they extend from it, as there are so few opportunities of pursuing the courses of granite veins to a central mass. Those formerly described are immediately lost in the hills that form the south-eastern boundary of the valley; although it is not possible to discover whether they actually terminate near the junction, or are prolonged far into

VIII. A Geological Description of Glen Tilt.

It is well known that the appearance of the granite in various parts of the valley of the Tilt afforded to Dr. Hutton one of the original arguments on which his theory was founded. Hence this spot has acquired among geologists a sort of classical celebrity, sufficient to render it an object of interest, independently of that which is excited by the intricate and remarkable disposition of its rocks. Observations, now more numerous than in his day, have brought to light so many facts similar to those which appeared to him so important, that there is no difficulty in pointing out abundant instances of an occurrence once supposed rare. But having had occasion while examining these rocks to observe other interesting appearances, which have been either misapprehended or over-looked, I consider that a notice of them will not be unacceptable to the Society. The notice is accompanied by such specimens and drawings as are requisite for the illustration of the subject. In describing Glen Tilt I have been under the necessity of differing on certain points from those who have preceded me. An examination of many of these points of difference would lead to discussions as endless as they would be fruitless, and give this paper the aspect of a controversial essay, rather than of that to which it alone pretends, a descriptive one; for which reason I have chosen to describe it precisely as it appeared to me. Where my description differs from that of others, the differences will

XI. An Account of Observations, made by Lord Webb Seymour and Professor Playfair, upon some Geological Appearances in Glen Tilt, and the adjacent Country

1. The river Tilt is a principal branch of the Tay, which rises on the borders of Aberdeenshire, and runs towards the south-west, through the north-eastern part of the county of Perth. A portion of the valley along its course, for about ten miles above Blair of Atholl, is called Glen Tilt.The adjacent country presents the common character of the Highlands. It is mountainous and rugged, and the surface, except in the lower parts of the valleys, is chiefly covered with heath. Peat-moss frequently occurs.