Though many people assume the Rocky Mountains reach all the way to the Pacific, they are only 120 miles deep from east to west. The "Rockies" consist of four sets of ranges between Alberta's foothills and the Rocky Mountain Trench/Columbia Valley. Each of these ranges is separated by parallel valleys.
The Front Ranges are those east of the Bow, North Saskatchewan, and Athabasca Rivers (for those driving, east of the Bow Valley Parkway and Icefields Parkway). The eastern slopes of this range were worn down by multiple glacial periods and rise over a mile above the height of the foothills. The Endless Chain Ridge, runs with a knife edge for 20 miles on the east side of Highway 93 always a mile above the roadway. These ranges are relatively dry, sitting in the "rain shadow" of the more westerly (and often higher) mountains.
The Eastern Main Ranges form the Continental Divide, where water on one slope falls toward the Pacific Ocean 600 km away, and water on the other slope flows to the Arctic Ocean 1,300 km away or the Hudson's Bay 1,700 km away. This range contains the 20 tallest peaks in the Canadian Rockies, including Mount Robson (3,954 metres, 12,972 feet). The elevation results in massive amounts of snowfall, creating some of the largest glaciers, including the Columbia Icefield.
The Western Main Range includes the mountains west of the Kootenay River. Rivers cutting through this range are much steeper and deeper than those to the east, because they travel a much shorter distance to the ocean. The valley bottoms are at a much lower elevation and receive more rainfall, resulting in rich growths of tall, lush forests.
The Western Range lies between Golden and Radium Hot Springs in British Columbia. These mountains rose first and are more eroded and rounded than the ones to the east, are mostly of weak shale. The mountain valleys are more V-shaped than those in the eastern Rockies, because these mountains escaped much of the glacial impact of the past few ice ages.
The Rocky Mountain Trench forms the western boundary of the Rockies, with a broad valley that extends in a north-westerly direction for hundreds of miles, the home of the Columbia River, which begins near Radium Hot Springs. The weather warm and drier than to the east, with a 100 more frost-free days each year than in Banff. To the west of the Columbia River are the Buggaboo, Selkirk and Purcell ranges that are so popular with heli-skiers. Despite their proximity, these are not really part of the "Rockies".