The oldest sea ice in the Arctic is thousands of years old, and in places it’s a hundred meters (330 feet) thick, locked in ice shelves which cling to the farthest northern coasts. The most extensive Arctic ice shelves are in Canada along the coast of Ellesmere Island. In fact they define the coastline there, forming part of the permanent geography of that land.
At least, they used to.
The coastline of the Canadian Arctic is changing as Arctic ice shelves disappear — also reported here.