30. Folds in Metasediments, Belcher Islands, Canada
What at first glance may look like swirls of paint on a blue canvas are in fact the Belcher Islands in Hudson’s Bay. These unusual low-lying islands extend over about 13,000 square kilometers (5000 square miles) but have a land area of only about 2800 square kilometers (1080 square miles). Their ribbony appearance is the result of the submergence of an eroded sequence of thinly-bedded, folded metasedimentary rocks, of which the harder, more resistant emerge above sea level. The rocks are of Aphebian age, 1640–2340 million years old.
The weight of the great continental ice sheets lying on northern Canada was sufficient to push the existing land below sea level by perhaps as much as 1000 meters (3280 feet) around Hudson’s Bay. Now that the ice has gone, the land is recovering isostatically, so the highest of the areas below sea level a few thousand years ago are now just emerging. The rate of uplift immediately after the Ice Age was about 12 centimeters (4.8 inches) per year; it has now slowed to one centimeter (0.4 inches) per year and this will continue for some time into the future. The rate of uplift may be slow enough that erosion is able to maintain the islands’ topography at a steady level.
STS-41G, October 1984. Picture #17-43-006.