OR/14/052 Discussion

Boon, D, Kirkham, M, and Scheib, A. 2014. Physical properties of till deposits from Anglesey, north west Wales. British Geological Survey Internal Report, OR/14/052.

The results of the Particle Size Analysis tests on red and grey tills from Cemlyn Bay and Hen Borth suggest these tills are texturally similar. The matrix of the Cemlyn Bay Red till has a slightly higher fines (clay & silt) content than the lower Grey till. The red till from Beaumaris is slightly more gravelly than the underlying basal grey till.

Direct shear box tests on sand from the drumlin at Cemlyn Bay provide an effective cohesion value, c’, of 13 kPa. This cohesion is likely provided by calcium carbonate cement, possibly derived from the carboniferous limestone content within the till. Similar cements have been described in sand and gravel soils in drumlins in the USA and attributed to precipitation due to reduction of hydrostatic pressure and CO₂ outgassing of subglacial meltwater (Menzies & Brand, 2006). This cement and the strength it provides may indeed become reduced over time, as acidic meteoric rainwater and soil water percolates through the slope, and this process may be a controlling factor in the development and style of slope failures in natural and cut soil slopes.

The silty fine sand grain-size of the hydrofracture sand material makes it potentially susceptible to liquefaction under cyclic loading conditions (e.g. under an ice sheet or during earthquake shaking), which may inform conceptual models for its mode of emplacement, although the clay content decreases the liquefaction potential slightly.

The XRFS analysis shows the lower grey tills from Cemlyn Bay contain very high levels of Ca and Sr. By comparison the brown till above contains low levels of Ca with around half the Sr. This is explained by incorporation of Carboniferous limestone into the Grey till. Alternatively this could also result from calcium carbonate leaching from the red till as a result of the near surface weathering processes.

The geotechnical properties of the bedrock material vary markedly across the island; with weak fissured mudstone, strong limestone and extremely strong metamorphic and igneous rocks (Greenley, 1919) present. It has been postulated that the shear strength and permeability of the bedrock may control sub-glacial landform distribution and processes (Phillips et al, 2010). The influence of bedrock physical properties on processes at the deforming bed is relevant for modern day ice sheet evolution. The finer grained tills of the west coast sit within Assemblage Zone 1 of (Phillips et al, 2010) whereas the sandy tills sit within Assemblage Zones 2 and 3 where the presence of elongated sub-glacial landforms indicate a faster ice stream down the Menai Strait. The composition of the matrix is likely controlled by local source material (bedrock and superficial deposits) and may influence ice stream velocity or even be a product of it. However, the quantity of data collected so far is insufficient in explore this relationship fully and further field and laboratory work is required.

Previous workers have assumed till deposits are dominated by local materials with erratic rich tills along the east coast (Lewis and Richards, 2005). This led Whittow and Ball (1970) to propose a late ‘Liverpool Bay Phase’ ice re-advance, but these erratic rich tills and underlying locally-sourced tills may well represent the same event. The tills at Beaumaris are more sand rich than elsewhere on the island and contain Carboniferous limestone boulders, and their presence supports Whittow and Ball’s proposition. Harris (1991) inspected the till at Wylfa Head in northern Anglesey and showed locally-derived material interdigitates with erratic rich till, suggesting formation in a single event. The XRFS data produced from this study (Cemlyn Bay) supports the notion of two till compositions or a vertical gradation and further work using this technique could potentially contribute to an improved understanding, till unit distribution, ice steam flow paths in the region, and mixing processes in the deforming bed of a glacier (for a review see Evans et al 2006).