Abstract

Glacial ice covered the Chalk River area through most of the Wisconsin Stage. About 11 300 years ago, an ice retreat was followed immediately by a short incursion of Champlain Sea waters, which deposited at least 2.5 m of clay, silt, and sand. A local readvance, probably associated with the St. Narcisse event, deposited till on the marine sediments. Following the final retreat of the ice from the area, lacustrine and aeolian deposition occurred locally for a short time.About 10 500 years ago, the North Bay drainage route opened, greatly increasing the discharge of the Ottawa River. A faint terrace at a present elevation of 209 m probably formed at this time. Changes in the drainage routes of proglacial lakes and in the rate of ice retreat caused a general decrease in discharge rates, and resulted in the formation of pronounced terraces, now at 180, 160, and 129 m, and fainter terraces at 170, 141, and 137 m. By about 5000 years BP, the North Bay outlet closed, and the river fell to approximately 111 m, its present elevation at Chalk River.During the whole period of terrace formation, alluvial sands were being deposited and, as river levels fell, exposed sands were reworked by the wind until anchored by vegetation. Charcoal horizons within the aeolian sequences indicate that forest fires occasionally destroyed the vegetation cover, re-initiating aeolian activity. Locally, active dunes are present near Chalk River, but most of the area has been stabilized by vegetation.

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