When the giant ice dams of the last ice age let go in north america, very similar things happened (without creating any islands) including washing away 1/2 of a mountain range in the pacific north west, and carving the St Lawrence river valley
Lake Missoula
One can see the various shoreline levels still on the sides of Mt. Jumbo and Mt. Sentinel; even weirder, and confusing to five year olds, fossils of ancient seabeds on same.