As the last ice age waned, a great lake was born

During the final millennia of the last ice age, what was briefly the world’s largest lake sat along the southwestern edge of the ice sheet that smothered eastern Canada. Researchers have now determined when that lake first formed.

At one point, Canada’s Lake Agassiz contained about 30 percent more water than all freshwater bodies of the world hold today (SN: 11/2/02, p. 283). Numerous studies have suggested that the lake disappeared about 8,400 years ago, when the center of the Laurentide Ice Sheet collapsed into Hudson Bay, allowing the lake to drain into the ocean. Despite widespread agreement on the timing of Lake Agassiz’ demise, researchers hadn’t pinned down when the lake originated, says Kenneth Lepper, a geoscientist at North Dakota State University in Fargo.

He and his colleagues carbon-dated samples of wood retrieved from sediments in lakes that had formed atop the rocky debris, or moraine, that had been pushed south by the advancing ice sheet. Those dates are clustered around 13,950 years ago, the team reports in the July Geology. That’s the time when the Laurentide Ice Sheet began to melt and retreat from northern portions of the Great Plains, the team suggests. The nascent Lake Agassiz accumulated between the retreating ice sheet and the moraine, says Lepper.

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