Usually the jokes are about Saskatchewan, but Manitoba is substantially flatter. Much of Manitoba was the lakebed of glacial lake Agassiz - the largest lake to have existed in north america, and about 50 times the size of Missoula. When the ice dams broke, Lake Agassiz dumped so much fresh water into the North Atlantic that it disrupted global ocean currents, triggering a period of rapid global cooling known as the Younger Dryas.

Fun fact, if look up Niverville MB on the map (just outside of Winnipeg) you can see iceberg scours still present on the landscape - this is where icebergs scraped the bottom of the lake.

"I've heard every joke

I've heard every word you say"