Northwest Britain Research Articles

Glacially- and tidally-influenced shallow marine sedimentation of the late Precambrian Port Askaig Formation, Scotland

The late Precambrian Port Askaig Formation forms part of the thick succession of metasediments and volcanics of the Dalradian Supergroup of northwest Britain. The glacianegnic Port Askaig Formation is well exposed on the Garvellach Islands in western Scotland and consists of a series of thick and regionally extensive diamictites interbedded with sandstones, siltstones, dolomites and minor conglomerates. Diamictite facies are similar to glaciomarine diamicts described from the Late Cenozoic Yakataga Formation in the Gulf of Alaska, formed by settling of fine-grained suspended sediment and ice-rafting in relatively shallow water (< 250 m) . Two particularly distinctive diamictite units, the Great Breccia and the Disrupted Beds are interpreted as the product of downslope resedimentation events; it is significant that the Port Askaig Formation was deposited during a period of increasing tectonic activity in the Dalradian Basin. Thick and extensive sand sequences interbedded with the diamictite facies are interpreted as tidally influenced shallow marine shelf deposits; sands may have been derived from deltaic sources on the basin margin and transported into the basin during storms. The entire Port Askaig Formation appears to represent an overall shallowing upwards sequence punctuated by smaller scale transgressive (diamictite) and regressive (sandstone) events. Interpretation of the Port Askaig Formation as a succession of glacially and tidally-influenced shelf sediments has significant implications for the reconstruction of palaeoclimatic histories and regional palaeogeographies during late Precambrian time.

Loran-C Chains: Determination of Extent of Cover

A mainly theoretical study has been undertaken to demonstrate how the extent of cover from a hyperbolic navigation system chain can be evaluated. The impetus for the study was the need to assess how Loran-C could be extended over Western Europe, particularly in the South western Approaches, North Sea, English Channel and Bay of Biscay sea areas.The technique described in this article leads to an accurate determination of the electric field strength at a distance from each transmitting site and takes into account the complexities of the ground-wave propagation path. This field-strength contour is combined with the geometric effects of station siting (expansion factors) to yield a constant S/N contour (–10 dB) which defines the ¼n.m. error and hence the limit of cover for the hyperbolic chain under study.In order to exercise the analytical methods a hypothetical Loran-C chain was studied comprising a master station at Lessay (France), with secondary stations at Soustons (also in France), at Sylt (dual rated; off the North Sea coast of Germany, near the Danish border) and at a fourth station located in north-west Britain on the Hebridean island of Barra. The study indicated that such a hypothetical chain would significantly improve Loran-C cover over much of western Europe.

The status and conservation of the butterfly Plebejus argus L. (Lepidoptera: Lycaenidae) in North West Britain

A survey was made of the declining butterfly, Plebejus argus L., in North West Britain in 1983. Twenty-six colonies were found on limestone grassland, many of them large. Twenty colonies were found on heathland, but most were relatively small. One large colony that inhabits mossland was rediscovered. P. argus occupies early succesional habitat at low altitude, predominantly on sheltered southerly facing slopes. Through biotope loss and fragmentation and declining traditional management, suitable habitat is created only sporadically in modern Britain. Some of these habitats (especially on heathland) are short lived. Thus, many colonies are transient. As P. argus forms closed colonies and rarely flies far, the decline appears to be a compound effect of reduced amounts of suitable habitat and inability to colonise them: the distances are often too great. Active conservation is required to check the decline of P. argus, especially habitat management and also introduction to suitable habitat now beyond the dispersal range of P. argus. Recommendations are made.

Crustal structure of the continental shelf off northwest Britain from two‐ship seismic experiments

Two‐ship multichannel seismic profiles, using expanding spread and constant offset source‐receiver configurations, were shot with explosive charges and a 16.4 ℓ air gun to investigate the structure of the continental shelf off northwest Britain. A long‐range (69 km) expanding spread profile reveals that the crystalline basement off northern Scotland is covered by sedimentary sections up to 2.5 km thick, and is divisible into two seismic units with velocities of 6.1 km/s and 6.6 km/s. Prominent supercritical Moho reflections indicate a crustal thickness of 26.7 km. Strong sedimentary and basement refractions, together with oblique reflections from the vicinity of the Moho, have been profiled at a constant air gun‐receiver offset of 10 km across a sedimentary basin west of the Orkney Islands. On the Outer Hebridean shelf to the west of mainland Scotland, the metamorphic basement lies within 250 m of the sea floor. P- to S-wave conversion occurs at the basement surface; [Formula: see text] gives a Poisson’s ratio of 0.27–0.31 at depths of 300–1 000 m. Marked changes in mode conversion efficiency are observed on constant offset profiles and are attributed to variations in the velocity structure of an uppermost low‐velocity (4.9 km/s) layer of weathered basement. The deeper crustal velocity appears uniform with depth, although there is some evidence of significant lateral velocity changes (6.28–6.61 km/s). In contrast to the shelf north of Scotland, reflections from within the basement and from near the base of the crust are recorded only sporadically on constant‐offset profiles. A strong event at 10.4 s two‐way reflection time appears to have arisen from a seismic discontinuity within the upper mantle. The differences in seismic character of the basement on the two‐ship profiles suggest significant variations in crustal structure within the Caledonian foreland of northern Britain.

Crustal Structure of the Continental Shelf off Northwest Britain from Two-Ship Seismic Experiments

Two-ship multichannel seismic profiles, using expanding spread and constant offset source–receiver configurations, were shot with explosive charges and a 16.4 I air gun to investigate the structure of the continental shelf off northwest Britain. A long-range (69 km) expanding spread profile reveals that the crystalline basement off northern Scotland is covered by sedimentary sections up to 2.5 km thick, and is divisible into two seismic units with velocities of 6.1 km/s and 6.6 km/s. Prominent supercritical Moho reflections indicate a crustal thickness of 26.7 km. Strong sedimentary and basement refractions, together with oblique reflections from the vicinity of the Moho, have been profiled at a constant air gun–receiver offset of 10 km across a sedimentary basin west of the Orkney Islands. On the Outer Hebridean shelf to the west of mainland Scotland, the metamorphic basement lies within 250 m of the sea floor, P- to S-wave conversion occurs at the basement surface; Vp/Vs gives a Poisson’s ratio of 0.27–0.31 at depths of 300–1000 m. Marked changes in mode conversion efficiency are observed on constant offset profiles and are attributed to variations in the velocity structure of an uppermost low-velocity (4.9 km/s) layer of weathered basement. The deeper crustal velocity appears uniform with depth, although there is some evidence of significant lateral velocity changes (6.28–6.61 km/s). In contrast to the shelf north of Scotland, reflections from within the basement and from near the base of the crust are recorded only sporadically on constant offset profiles. A strong event at 10.4 s two-way reflection time appears to have arisen from a seismic discontinuity within the upper mantle. The difference in seismic character of the basement on the two-ship profiles suggests significant variations in crustal structure within the Caledonian foreland of northern Britain.

Alkyl esters, mid chain ketones and fatty acids in late glacial and postglacial lacustrine sediments

C 28−C 52 straight chain alkyll esters, maximizing in abundance near C 32 and at C 44, were isolated from contemporary sediments and deposits up to 14,000 years old from Coniston Water (North-west Britain). The molecular composition was determined by gas chromatography-mass spectrometry. Branched chain esters, also present in the profile, differed in molecular composition from those of a previously studied site. This feature was attributed to differences in microbial populations of the two environments. Mid chain ketones, not previously reported in lacustrine sediments, were attributed to higher plant waxes, based on molecular composition. Variations in the distribution of free monocarboxylic acids were related to input sources and extent of diagenesis.

Seismic velocities of Lewisian metamorphic rocks at pressures to 8 kbar: relationship to crustal layering in North Britain

Summary P- and S-wave velocities on a set of rocks from the Archaean Lewisian metamorphic complex, north-west Britain, have been measured in the laboratory at confining pressures to 8 kbar. From estimates of velocities at crustal pressures and temperatures, it is concluded that the composition of the three crustal layers below the Scottish Caledonides are most likely to be, in order of increasing depth: (1) mixed metamorphic rocks in amphibolite or lower facies, plus granites, (2) mixed pyroxene-granulite facies rocks, of overall intermediate composition and (3) mafic garnet granulites. Poisson's ratios of isotropic aggregates of mafic amphibolites and serpentinized peridotites in the Lewisian are high (0.28-0.31) because of the presence of hydrous minerals. Only a highly preferred orientation of minerals in such rocks in the lower crust would lead to observed values of Poisson's ratio of about 0.25.

Seaweeds and rocky shores of the Outer Hebrides

SynopsisThe Outer Hebrides are oceanic in character, bathed by comparatively warm waters derived from the North Atlantic Drift. The spring tide range is 3·5 m (west coast) and 4 m (east coast).The southern and western coasts include some of the most exposed shores in Britain and the steep rocky shores exhibit species and zonation patterns typical of north-west Britain. The sublittoral zone is mainly rocky and is dominated by beds of Laminaria hyperborea, which are very extensive west of the Uists.The eastern sides of the larger islands are much more sheltered and have numerous sea lochs many of which have great lengths of very sheltered rocky shore, dominated by extremely vigorous growths of fucoid algae, particularly Ascophyllum nodosum. Some of the lochs have extensive ramifications, with localized tidal tidal rapids supporting a rich and luxuriant flora and fauna.The paper reviews the literature on the ecology of the rocky shores, on the seaweeds in particular, and includes a full list of all seaweeds recorded in the islands with their distribution by island. (Two hundred and sixty-four species are recorded and this is about 38% of the British total.) Thus the seaweed flora is rich and diverse; the presence of selected species is discussed.The large brown seaweeds grow very luxuriantly in the Outer Hebrides and have been used by man there for many centuries. The history of this utilization is briefly reviewed. The fucoids and Laminaria spp. were formerly used extensively as a source of alkalis (sodium and potassium carbonates) and iodine. Nowadays very large tonnages of Ascophyllum are regularly harvested for the production of alginates; quantities of cast up Laminaria hyperborea are also used for this purpose.

Seismic velocities in the Lewisian metamorphic complex, northwest Britain—‘in situ’ measurements

Synopsis Sixteen reversed seismic refraction profiles, each 1 km long, in Sutherland between Kylesku and Durness have demonstrated a difference in P-wave velocity between the northern and central belts of the mainland outcrop of the early Precambrian Lewisian rocks. The variably-retrogressed pyroxene granulites of the central belt (of andesitic-dioritic composition) yielded a velocity of 6.34 ± 0.17 km/s (6 observations); the quartzo-feldspathic gneisses of the northern belt yielded a velocity of 5.64 ± 0.18 km/s (6 observations); the rocks of the boundary region, near the Ben Stack line, yielded a velocity of 5.28 ± 0.23 km/s (4 observations). Correlations are made with the velocity layering of crustal models derived for the area, confirming the wide occurrence in the upper crust of granulite-facies rocks. Such high values of velocity in surface rocks place strict limits on velocity gradients in such rock types in the deeper continental crust.

Evidence for two discrete centres in Skye

ONE of the best-developed igneous areas in North-West Britain is on the Isle of Skye: in addition to extensive flood basalt lavas and their feeding dykes there are three adjacent intrusive centres across the south-east of the island1. The Eastern and Western Red Hills, usually identified as two almost contemporaneous centres, comprise surface granite outcrops in a mass 10 km across, intruded against the older basic-to-ultrabasic Cuillin centre which is of similar size and lies to the west (Fig. 1a). Using aeromagnetic, gravity and isotopic evidence we will argue that there are two structurally discrete centres at depth, which probably were intruded several million years apart.

Irish colonists in south‐west Britain

Abstract By assessing the results of excavations, inscriptions, personal and place names, settlement and land use patterns, two phases of Irish colonization into south‐west Britain can be recognized. The first affected north‐east Cornwall and west Devon from the late fifth century A.D. and the second, west Cornwall a century later. Though less visible than the Irish colonization of northwest Britain, these immigrants had a lasting effect on subsequent cultural patterns in the south‐west.

Cultural Connections in North-West Britain and Ireland.

Cultural Connections in North-West Britain and Ireland.