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Before 1.1 billion years ago, most of southern Ontario did not exist. It was a big deep ocean, with a bottom of cold oceanic crust. At that time, the cold oceanic crust began to sink, and the ocean started to converge, creating many subduction zones. Much like modern-day Japan, these subduction zones created volcanic island arcs, but still the ocean converged, and continents were starting to collide, much like Indian and Asia.
Eventually, the basement rock cooled so much that it went below sea level. Shallow seas made extensive deposits, as the basement deflated. The sediments made flat layers for the most part, except over the megathrusts, where they had some difficulty. The continued sedimentation solidified the rock, but the rock over the weak megathrusts continued to fracture and slide, forming faults.
At some point in the past, we Ontario residents were lucky that the deep Moho once again heated up, lifting Ontario into the position we now enjoy. This probably happened during the last mass convergence of the continents, just before the Atlantic separation.