AV俱乐部

The Hartlen Till is an extensive, variably thick (> 20 m), highly compacted, grey silty diamicton that cores many of the drumlins exposed along the eastern shore of central Nova Scotia. Due to its apparent homogeneity, many of the observations made at specific locations are transferrable to other outcrops. Based on ice flow measurements, pebble provenance, and offshore stratigraphy, it has been previously determined that the till was deposited during the Caledonian glacial phase. As it commonly occurs at the base of the terrestrial stacks of tills, it may comprise material from the Meguma and associated terrains that had been previously exposed to cosmic rays for a prolonged period of time. Based on previous measurements of ¹⁰Be in till, it is assumed that the Hartlen quartz sand contains inherited 10Be from exposure as regolith prior to its deposition. The till therefore provides an ideal means of demonstrating the plausibility of a deformable bed in a drumlin environment using cosmogenic isotopes.

Although the concept of deformable beds accounting for a significant portion of the movement beneath glaciers is generally accepted, the thickness and contribution of a deforming bed at a given time is less predictable, varying with the material properties of the bed, flow velocity, and subglacial hydrology conditions. The thickness of a deforming bed has only been observed under modern glaciers. Our experiment uses a vertical sequence of eight samples of quartz sand from the Hartlen Till matrix. It was found that there was little variation in the concentrations of ¹⁰Be within the Hartlen Till, but the ¹⁰Be concentration in the base of the Lawrencetown Till was four to five times smaller. We can infer that the interglacial period that occurred before the Caledonia Phase (early Wisconsinan) was a long enough period of exposure to produce concentrations on the order of ~7.0x10⁴ atoms g^-1 in the Hartlen Till. This also suggests that the period between the deposition of the Hartlen and Lawrencetown tills was significantly shorter, producing concentrations of ~1.5x10⁴ atoms g^-1 in the Lawrencetown Till. The vertical trend in the Hartlen till allows us to assume that the deformable bed can reach a thickness of at least 5m, which corresponds well with previous studies of the deformable bed.

Keywords: deformable bed, cosmogenic nuclides, till genesis, interglacial, Caledonia Phase.
Pages: 79
Supervisor: John Gosse