A bed of marine algae (seaweed) in a sand pit in Champlain Sea sediments southwest of Ottawa, Ontario, was dated by the radiocarbon method and gave an age of 10 800 ± 150 years (GSC-570). Marine shells from above and below the algal bed gave radiocarbon ages of 10 620 ± 200 years (GSC-587) and 10 880 ± 160 years (GSC-588) respectively.The algae and underlying shells have been excellently preserved as a result of rapid burial following deposition and remaining beneath a high water table. The similarity of the radiocarbon dates on the algae and on the shells above and below, and with previous dates on shells from other Champlain Sea deposits indicates the reliability of these shell dates.

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