Why does North America have such an irregular coastline in its northern regions?

The irregular coastline of northern North America is primarily the result of extensive glaciation during the Quaternary Period.1

During the last glacial maximum, massive ice sheets covered much of the continent, scouring the landscape and carving deep, U-shaped valleys into the terrain. As the climate warmed and these glaciers retreated at the end of the Pleistocene Epoch, these deep valleys were submerged by rising sea levels, creating the characteristic fjords and jagged inlets that define much of the northern coast, including areas along the east coast of Canada and the Arctic islands.12

Additionally, the immense weight of the continental ice sheets depressed the Earth's crust. As the ice melted, the land began a slow process of isostatic recovery, or uplift. This ongoing adjustment, combined with the complex history of sea-level fluctuations and the presence of ancient crystalline rocks in the Canadian Shield, has resulted in a highly fragmented and irregular coastal geography.134