About four hundred million years ago, eroded debris from the mountains of the Canadian Shield (which covered North America from Greenland to Guatemala) was washed west by rivers and deposited as mud, shale and limestone on a “continental slope”. Then, about two hundred million years ago, two strings of volcanic Pacific islands began to move eastward on the Pacific Continental 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. The lighter rock of the islands stayed above, detaching itself from the plate as they crashed into the continent. The orderly deposits on the continental slope were crumpled and uplifted, their layers 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 (in Yoho and Kootenay national parks), and then moving east, where 4km of uplift created the Eastern Main Ranges (the mountains on a line with Lake Louise).
Following the first islands, the second archipelago also crashed into the continent, striking the debris of the earlier collision and creating more folding, rupturing and uplifting of the earlier ranges. About sixty million years ago, the aftershock from this encounter created the Rockies’ easternmost Front Ranges (the line of mountains that rears up from the prairies). Three ice ages over the last 240,000 years applied the final touches, carving sharp profiles and dumping further debris.