The Post Clam-shell Expansion centred mainly around the opening up of the Atlantic
ocean-bed and its northern continuity, the Arctic Ocean-bed. Both of these tectonic
developments dramatically affected North America’s continental position and shape.
The Arctic Ocean-bed pushed Alaska (which was nested above Russia’s north west coast)
and North America along the entire length of the Asian continent in a strike/slip
motion. Meanwhile the North Atlantic Ocean-bed began pushing North America in a north-west
direction creating another strike/slip action in the Rocky Mountains area. In other
words, Alaska and the west side of America moved at a different rate to central and
eastern parts of North America.
North America’s northern crust suffered a great deal of stretching too and many basins
developed and these formed the numerous great lakes.