Abstract

The occurrence of abandoned shoreline bedrock erosional features at the edge of the Niagara Escarpment at Cabot Head indicates the existence of a group of islands in glacial Lake Algonquin during early postglacial time, referred to herein as the Cabot Head Archipelago. The abandoned shoreline features are situated as much as 80 m above the level of contemporary Georgian Bay. The range of abandoned shoreline bedrock erosional features, including shoreline cliffs, adjacent wave-cut platforms, wave-cut notches, shore stacks, shore caves, and other features, are described. The occurrence of these features is thought to be the result of the interaction between wave action in glacial Lake Algonquin and two distinct lithological facies representing the Wiarton – Colpoy Bay and Lions Head members of the Amabel Formation. The exceptional development of the abandoned shoreline bedrock erosional features in massive reefal dolostone between elevations of ∼250 and 255 m above sea level (asl) is interpreted as representing the relatively long-lived and stable Main stage of glacial Lake Algonquin (∼11 000–10 200 years BP). Shoreline erosional forms at elevations between ∼240 and 250 m asl may be indicative of declining lake levels partially controlled by bedrock structural factors. The final abandonment of the glacial Lake Algonquin shoreline in this area occurred when the eastern outlets of the lake became ice-free and its level dropped rapidly some 10 200 years BP. The Cabot Head Archipelago and the associated suite of raised and abandoned shoreline bedrock erosional features represent a rare assemblage of landforms within the Great Lakes basin, and possibly within Canada.

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