The nature and derivation of glacial till in part of the Tweed basin

• Main Author: Kerr, Robert J.
• Published: University of Edinburgh 1978
• Subjects: 551.4
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.653328

Initial studies were associated with a gas pipeline trench (average depth c. 2.3m) cut obliquely across the Tweed drumlin field for 26 km west-northwest from near Coldstream (NT 858413). These studies include stratigraphy, atone-counts (50 Bitesin the "basal series" and 28 from intermediate levels), macro-fabric orientation analyses (29 sites), particle size and heavy mineral analyses (48 and 15 sites respectively). Surface stone-counts were made in association with pipeline sampling, and counts of a finer size-fraction (100-160 mm) made on most basal samples. Surface stone-counts were also carried out away from the pipeline within the middle Tweed and Teviot basins, regional implications being interpreted in the light of relationships and patterns observed in pipeline-associated studies. Around Black Hill near Earleton, detailed erratics studies recorded changing concentrations of an indicator stone in surface tills, from which local variations in basal ice flow were inferred. Deposition processes are considered and melt-out tills, often with associated sub-till fluvioglacial sequences are identified locally. Such tills, part of a suggested pattern of ice sheet stagnation, are not recognisable by the normal characteristics of ablation tills. Variations in till character and composition are noted vertically within sections, and regionally in relation to changing geology. Increasing dominance of exotic Silurian erratics towards the surface of all sections and at increasing distances down-glacier of source, is interpreted in the light of ice stagnation. A threefold zonation is suggested within any drift sequence, this zonation having been controlled by position in the glacier bed or basal ice immediately prior to stagnation. The development of individual zones varied locally with changing conditions at the glacier sole. Geological, orientation and erratics evidence is also related to drumlin form and occurrence. Theories are offered on drumlin formation as part of the depositional sequence within the Tweed basin.