Genetic influences on the nature and properties of basal melt out tills

• Main Author: McGown, A.
• Published: University of Strathclyde 1975
• Subjects: 624.15
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.464143

The glacier regime is described and the classifications of glacier types are outlined. The modes of acquisition and transportation of the debris in glaciers is considered and the deposition process briefly considered. The various systems of classification of unstratified and stratified drift are given and the several morphological expressions of the unstratified drift are described. The crushing mechanism producing basal tills, of which the materials studied are examples, is reviewed in terms of their transportation environment in the glacier. The mechanical crushing of rocks is also considered and the particle size distribution laws developed to describe the products of this industrial process considered for application to glacial comminution products. The nature of glacial comminution is then further reassessed in terms of the shape of particles developed by the process. A full description of the locations of study sites and samples included in the investigations is then given including a detailed description of the local geological and glaciological environments where these are known. Previous investigations of the textural variations in tills are reviewed and the sampling and testing techniques for particle size distribution are outlined. The textural variations measured, or previously reported for Scottish, Norwegian, American and Canadian Pleistocene tills are analysed and compared to the measured variations in tills taken from three active modern glaciers in Norway, Iceland and Antarctica. These variations are then interpreted in terms of the mode of formation of the tills. Close correlations between these glacial products and industrial crushing products are thus shown to exist. Based on this, it is suggested that till variability can be much more easily understood if the material is treated as two size fractions mixed in variable proportions. The size fractions in any one till are then found to be fairly consistent with the mix proportions varying often very locally. Lithological variations are, however, found to have much less effect than might be expected. The organisation of the particulate matter in the tills is then considered in terms of the mode of deposition of the basal melt out tills. The orientation of clasts in the tills were measured in the field. This study was then extended in the laboratory using the contact goniometer to include the orientation of particles down to the sand size grade. Further qualitative studies using the Scanning Electron Miorosoope then extended the appreciation of the particle organisation down to the clay size particles. This attempt at determining the nature of particle organisation at all size levels was then proven to be most useful in determining the actual mechanisms operating during deposition. Essentially it was shown that the clasts react to the englacial stress field whereas the fines are organised during the melt out process and record in a more sensitive manner than the clasts, the stress history of the till during and after deposition. Some of the microfabrics in the tills were found to be very open and field data on in-situ density confirmed both the variability and the very low densities that can be found in the tills. Investigations of the factors influencing achieved density when recompacting tills showed that the variations in grading in these soils could be as important as differences in compactive effort. It is suggested that for engineering situations where quality control of compaction is required it is necessary to include gradational variations in the assessment procedures otherwise variations in compactive effort will be masked. A review of the factors affecting permeability and shear strength of soils was also undertaken with particular reference to the tills. Detailed investigation of some tills were undertaken and comparisons made to recorded data for tills used in Norwegian and Canadian dams. The interrelated influences of grading, density and particle arrangement were shown to critically affect permeability and shear strength and a system of classification based on percentage fines and the identity of the fines was suggested which attempted to include many of the variables. Limited success was met with respect to permeability but none with shear strength, the many other variables influencing the tills strength properties overriding the basic parameters used in the system. The use of percentage fines and the identification of the coarse and fine fractions in the tills was still however shown to be the best means of understanding the variations within anyone till. The inherent variations in the basal melt out tills were therefore shown to be attributable to their modes of formation and deposition and by recognising the fundamental nature of the till composition and particle organisation it is considered that a better understanding of the variations has been achieved.