WHY THE CANADIAN PRAIRIES ARE SO INCREDIBLY FLAT

Drive across Manitoba and the land goes flat as a table: the Red River drops about 1 metre for every 5 kilometres. Under those wheat fields lies the floor of a lost lake that, at its peak, was bigger than all 5 Great Lakes combined. Why are the Canadian Prairies so incredibly flat? The answer starts deep underground with Tyndall Stone, quarried at Garson, 37 km northeast of Winnipeg, since about 1895. It's the same creamy limestone in the walls of the Parliament Buildings in Ottawa, and it's full of sea creatures from a warm inland sea about 450 million years old. Underneath the Prairies sits a stack of sedimentary rock up to 5.5 km thick, lying almost perfectly level. Next comes the ice. At the height of the last ice age, the Laurentide Ice Sheet sat over the Prairies at a thickness of roughly 1,700 to 2,100 metres. We follow it as it smooths the land into a till plain, carves the Big Muddy and Qu'Appelle spillways in Saskatchewan, and traps its own meltwater against a wall of ice. Then there's Lake Agassiz, the giant that geologist Warren Upham spent 15 years mapping. You'll see the beach ridges it left high on the Manitoba Escarpment, hundreds of kilometres from any coast. You'll see why the old lake comes back to the Red River Valley in big flood years like 1997, when about 28,000 people had to leave their homes. And at the end: where all that water went, the ocean far away that may have felt it, why scientists are still debating what happened next, and what a strange reddish layer of mud at the bottom of Hudson Strait has to do with a wheat field south of Winnipeg. 00:00 A wheat field flat as a table 02:05 A sea floor in the walls of Parliament 05:00 5.5 km of flat-lying rock 06:35 The 3 steps of the Prairies 08:40 Ice 2 kilometres thick 10:40 Big Muddy, Lake Regina and the Qu'Appelle 12:50 Upham's lake and the beaches with no sea 16:15 Bigger than all 5 Great Lakes 17:30 The Red River and the 1997 "Red Sea" 19:40 The ice dam over Hudson Bay #CanadianPrairies #LakeAgassiz #Manitoba