Late Quaternary solifluction sheets in the British uplands

Solifluction sheets are large-scale and extensive valley-floor and valley-side landforms developed widely in the British Isles from mass-wasting of glacial and periglacial sediments during late-glacial times. We describe their geographical distribution and review the processes that have led to their development. We use data from the Cheviot Hills, the one site in the British Isles where sedimentology and optically stimulated luminescence dating have been combined, to assess their age and nature of deposition. We also present data from central Wales where a new mapping and resistivity survey has reconstructed the nature of valley-side solifluction sheets. We explore the relative lack of recent research on these landforms and argue that solifluction sheets represent a clear example of how upland geomorphological systems have responded to late-glacial climate change. We end by identifying a number of areas where research on these enigmatic features could be focussed, including better understanding of their distribution, sedimentology and age.     

The Perseid meteor shower

The Perseids usually appear, with no remarkable variation in numbers, every August, maximising about the 12th. They have a parent, Comet Swift-Tuttle (1862 III), and, with the exception of planet Earth, the causative meteoroids keep away from perturbing planets. Of all the known streams the origin and evolution of the Perseids should be the easiest to understand. The meteoroids are fast and are prone to fragmentation.     

An observational program for a unified optical-radio reference frame

An observational pilot program is described which provides a first step towards the construction of an inertial extragalactic reference frame from optical and radio measurements. Candidate objects will be selected from extragalactic compact radio sources which display optical counterparts. Galactic radio stars will be secondary reference objects. The reference frame will be defined by the radio positions of the extragalactic objects. Based on a coordinated effort in both hemispheres, a global homogeneous net of about 400 quasars and 100 radio stars is expected to result from a five year campaign.     

Riparian management and stream bank erosion in New Zealand

Improvements in riparian management, such as shrub/tree planting and livestock exclusion, are often assumed to result in reduced stream bank erosion and associated catchment sediment yield. Studies that quantify the effectiveness of riparian interventions aimed at reducing bank erosion and river sediment yields are, however, rare. This paper discusses how bank erosion processes can vary throughout catchments (with particular reference to their scale dependence) and hence how the effectiveness of different riparian interventions can be variable. The findings of known published accounts of the effectiveness of riparian management interventions for reducing stream bank erosion in New Zealand are also summarised. Only nine relevant studies were identified and most used qualitative or semi-quantitative analysis methods. Most studies compared stream banks in pasture catchments (with unlimited livestock access) with stream banks where livestock were excluded and riparian shrubs/trees were present. Many studies reported that managed stream banks were in better condition than unmanaged banks. The exclusion of livestock from riparian areas was generally reported as the principal factor in the measured improvements or differences. Only two studies specifically attributed reduced stream bank erosion to the presence of riparian vegetation. The dearth of research identified here highlights the need for further quantitative studies to determine the effectiveness of riparian management measures for reducing bank erosion.     

The effect of the atmosphere and ionosphere on long period magnetospheric micropulsations

A study is made of the screening effect of the atmosphere and ionosphere which lets only part of a magnetospheric micropulsation signal reach the ground. We obtain analytical expressions relating the magnetic field in the magnetosphere to that on the ground, which are confirmed by computing a full solution. Only the part of the signal with no vertical current will be seen on the ground; this agrees with previous work. Using this result, we find that the reflection condition of a transverse hydromagnetic wave incident on the top of the ionosphere indicates a ‘fixed-end’ boundary condition for the field line. To a first approximation, micropulsation polarisations observed on the ground must be rotated through a right angle to obtain those in the magnetosphere. The strength of the electric field predicted in the ionosphere agrees with radio aurorae observations.     

Anatomy of a granophyre intrusion

Various structural and textural features of a drusy granophyric ring-dyke intrusion can be explained in terms of the behaviour of magmatic water. Initial crystallization of water-rich magma close to ternary eutectic composition resulted in textures grading from granophyric to microgranitic. Eventual entrapment of water-rich pockets resulted in crystallization from a silicate liquid and a hydrothermal liquid simultaneously, probably at magmatic overpressure, to form isolated bodies of pegmatite containing giant euhedral quartz crystals. Later aplite, rhyolite breccia, and tuffisite veins reflect the escape of magma and magmatic gas through the congealed parts of the intrusion, at first permissively, but later with increasingly explosive effects.     

New X.R.F. data retrieval techniques and their application to U.S.G.S. standard rocks

Accurate and consistent X-ray fluorescence analysis of major and trace elements using a single pressed pellet for each sample has been achieved with a wide variety of rock compositions. Counting techniques for use with data reduction and correction programs in Standard Fortran IV are described; the latter depend throughout on data developed from a wide range of standard rock compositions. A comparison of ten major and fifteen trace element analyses for U.S.G.S. standard rocks with averages taken from the literature testifies to the precision of these techniques.     

Geomorphology,acoustic backscatter,and processes in Santa Monica Bay from multibeam mapping

Santa Monica Bay was mapped in 1996 using a high-resolution multibeam system, providing the first substantial update of the submarine geomorphology since the initial compilation by Shepard and Emery [(1941) Geol. Soc. Amer. Spec. Paper 31]. The multibeam mapping generated not only high-resolution bathymetry, but also coregistered, calibrated acoustic backscatter at 95 kHz. The geomorphology has been subdivided into six provinces; shelf, marginal plateau, submarine canyon, basin slope, apron, and basin. The dimensions, gradients, and backscatter characteristics of each province is described and related to a combination of tectonics, climate, sea level, and sediment supply. Fluctuations of eustatic sea level have had a profound effect on the area; by periodically eroding the surface of Santa Monica plateau, extending the mouth of the Los Angeles River to various locations along the shelf break, and by connecting submarine canyons to rivers. A wetter glacial climate undoubtedly generated more sediment to the rivers that then transported the increased sediment load to the low-stand coastline and canyon heads. The trends of Santa Monica Canyon and several bathymetric highs suggest a complex tectonic stress field that has controlled the various segments. There is no geomorphic evidence to suggest Redondo Canyon is fault controlled. The San Pedro fault can be extended more than 30 km to the northwest by the alignment of a series of bathymetric highs and abrupt changes in direction of channel thalwegs.     

The dust distribution within the inner coma of comet P/Halley 1982i: encounter by Giotto's impact detectors

Analysis of the data from Giotto's Dust Impact Detection System experiment (DIDSY) is presented. These data represent measurement of the size of dust grains incident on the Giotto dust shield along its trajectory through the coma of comet P/Halley on 1986 March 13/14. First detection occurred at some 287000 km distance from the nucleus on the inbound leg; the majority of the DIDSY subsystems remained operational after closest approach (604 km) yielding the last detection at about 202000 km from the nucleus. In order to improve the data coverage (and especially for the smallest grains, to approximately 10(-19) kg particle mass), data from the PIA instrument has been combined with DIDSY data. Flux profiles are presented for the various mass channels showing, to a first approximation, a 1/R2 flux dependence, where R is the distance of the detection point from the cometary nucleus, although significant differences are noted. Deviations from this dependence are observed, particularly close to the nucleus. From the flux profiles, mass and geometrical area distributions for the dust grains are derived for the trajectory through the coma. Groundbased CCD imaging of the dust continuum in the inner coma at the time of encounter is also used to derive the area of grains intercepted by Giotto. The results are consistent with the area functions derived by Giotto data and the low albedo of the grains deduced from infrared emission. For the close encounter period (-5 min to +5 min), the cumulative mass distribution function has been investigated, initially in 20 second periods; there is strong evidence from the data for a steepening of the index of the mass distribution for masses greater than 10(-13) kg during passage through dust jets which is not within the error limits of statistical uncertainty. The fluences for dust grains along the entire trajectory is calculated; it is found that extrapolation of the spectrum determined at intermediate masses (cumulative mass index alpha = 0.85) is not able to account for the spacecraft deceleration as observed by the Giotto Radio Science Experiment and by ESOC tracking operations. Data at large masses (>10(-8) kg) recently analysed from the DIDSY data set show clear evidence of a decrease in the mass distribution index at these masses within the coma, and it is shown that such a value of the mass index can provide sufficient mass for consistency with the observed deceleration. The total particulate mass output from the nucleus of comet P/Halley at the time of encounter would be dependent on the maximum mass emitted if this change in slope observed in the coma were also applicable to the emission from the nucleus; this matter is discussed in the text. The flux time profiles have been converted through a simple approach to modeling of the particle trajectories to yield an indication of nucleus surface activity. There is indication of an enhancement in flux at t approximately -29 s corresponding to crossing of the dawn terminator, but the flux detected prior to crossing of the dawn terminator is shown to be higher than predicted by simple modelling. Further enhancements corresponding to jet activity are detected around +190 s and +270 s.