| Summary: | The Dummer Moraine has been traditionally regarded as a terminal moraine. More recent work has indicated that the moraine may be related to the Precambrian-Palaeozoic contact (Gadd, 1980) and that the "Dummer till" is a lithological variant of the basal tills further south. Field observations in the Tweed area, Southern Ontario, support these conclusions and a conceptual model of till emplacement based on the observed presence of karstic limestone is proposed. This model invokes drainage of water from the base of the ice sheet as it advanced on to karstified Palaeozoic limestones from the Precambrian shield. The glacier became temporarily grounded and frozen to its bed. Limestone blocks plucked from the bed were deposited as a series of chaotic knolls and ridges. Locally, where the bed was less karstified, or a till drape built up, fluidization of the drift resulted in the emplacement of lodgement tills that interdigitate with the basal till. As the sediment load increased in the basal units of the ice, these beds stagnated and were finally sheared from, and overriden by, cleaner ice. This material remained in situ until the main ice body stagnated, when it was deposited as the chaotic Dummer Moraine. A supraglacial melt-out till caps the basal tills in several locations. It is expected that the Dummer will reform with each advance, but will migrate southward, in tandem with the erosion of the Palaeozoic/Precambrian boundary. The model is not proposed as a confirmed solution to the problem of the origin of the Dummer Moraine. It is intended to provide an hypothesis consistent with the available evidence and to stimulate further research in this area.
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