Abstract

Numerous vertical to subvertical, cylindrical shafts occur in the rugged exposures of the Middle Silurian Amabel Formation bedrock along the Eramosa River, approximately 10 km northeast of Guelph. These shafts vary from < 1 m to 10 m in diameter, with depths as great as 12 m. They can occur as isolated depressions but more commonly are clustered closely together, and many coalesce to form sinuous bedrock walls. These depressions may contain soil, fine sediment, rubble from the surrounding dolomite bedrock, and rounded clasts of Precambrian basement lithologies. Till is conspicuously absent. These shafts occur in close proximity to cavernous pores that were formed during pre-Wisconsinan to Early Wisconsinan time, suggesting a possible karstic origin. The cylindrical shafts, however, with their greater midpoint diameters, sculpted interiors, and inclined axes, and their lack of vertical flutes and till, are more likely to have originated from glaciofluvial discharge at the end of the Late Wisconsinan. The development of these shafts may have been assisted by cavitation, which enhanced the erosional capacity of sediment-laden meltwaters.

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