The Creation Of The Canadian Rockies
About 600 million years ago the vast granite mountains of the Canadian Shield covered North America from Greenland to Guatemala (today the Shield's eroded remnants are restricted largely to northeast Canada). For the next 400 million years, eroded debris from the Shield - mud, sand and gravel - was washed westward by streams and rivers and deposited on the offshore "continental slope" (westward because the Shield had a very slight tilt). Heavier elements such as gravel accumulated close to the shore, lighter deposits like sand and mud were swept out to sea or left in lagoons. The enormous weight and pressure of the sediment, which built up to a depth of 20km, converted mud to shale, sand to sandstone and the natural debris of the reefs and sea bed - rich in lime-producing algae - into limestone. Two further stages were necessary before these deposits - now the strata so familar in the profile of the Rockies - could be lifted from the sea bed and left several thousand metres above sea level to produce the mountains we see today. The mountain-building stage of the Rockies took just 100 million years, with the collision of the North American and Pacific continental plates (gigantic 50-kilometre-thick floating platforms of the earth's crust). About 200 million years ago, two separate strings of volcanic Pacific islands, each half the size of British Columbia, began to move eastward on the Pacific Plate towards the North American coast. When the first string arrived off the coast, the heavier Pacific Plate slid beneath the edge of the North American Plate and into the earth's molten interior. The lighter, more buoyant rock of the islands stayed "afloat", detaching itself from the plate before crashing into the continent with spectacular effect. The thick orderly deposits on the continental slope were crumpled and uplifted, their layers breaking up and riding over each other to produce the coast's present-day interior and Columbia Mountains. Over the next 75 million years the aftershock of the collision moved inland, bulldozing the ancient sedimentary layers still further to create the Rockies' Western Main Ranges (roughly the mountain edge of Yoho and Kootenay national parks), and then moving further east, where some 4km of uplift created the Eastern Main Ranges (the mountains roughly on a line with Lake Louise). Finally the detached islands "bonded" and mingled with the new mainland mountains (their "exotic" rocks can be found in geological tangles as far east as Salmon Arm in BC). Behind the first string of islands the second archipelago had also now crashed into the continent, striking the debris of the earlier collision. The result was geological chaos, with more folding, rupturing and uplifting of the earlier ranges. About 60 million years ago, the aftershock from this encounter created the Rockies' easternmost Front Ranges (the distinct line of mountains that rears up so dramatically from the prairies), together with the foothills that spill around Kananaskis and Waterton lakes. The third stage of the Rockies' formation, erosion and glaciation, was relatively short-lived, at least three Ice Ages over the last 240,000 years turning the mountains into a region resembling present-day Antarctica. While only mountain summits peeked out from ice many kilometres thick, however, glaciers and the like were applying the final touches, carving sharp profiles and dumping further debris
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