The Rocky Mountains began rising 75 million years ago, making them relatively young compared to the world’s major mountain ranges. But to fully appreciate the geology of the Rockies, you must look back many hundreds of millions of years, to the Precambrian era. At this time, about 700 million years ago, the Pacific Ocean covered most of the western provinces and states.
The ocean advanced, then receded, several times over the next half billion years. Each time the ocean flooded eastward it deposited layers of silt and sand on its bed—layers that built up with each successive inundation.
Starting approximately 550 million years ago, the oceans began to come alive with marine invertebrates and the first crustaceans. As these creatures died and sank to the ocean floor, they added to the layers of sediment. Over time, the ever-increasing sediment load compressed the underlying layers into sandstone, shale, and quartzite.
Some 200 million years ago, the stability of the ocean floor began wavering along the West Coast of North America, culminating 75 million years ago as two plates of the earth’s crust collided. According to plate tectonics theory, the earth’s crust is broken into several massive chunks (plates) that are always moving and occasionally bump into each other. This isn’t something that happens overnight; a plate may move only a few centimeters over thousands of years. In the case of the Rockies, the Pacific Plate butted into the North American Plate and was forced beneath it.
The land at this subduction zone was crumpled and thrust upward, creating the Rocky Mountains. Layers of sediment laid down on the ocean floor over the course of hundreds of millions of years were folded, twisted, and squeezed; great slabs of rock broke away, and in places older strata were pushed on top of younger. By the beginning of the Tertiary period, around 65 million years ago, the present form of mountain contours was established and the geological framework of the mountains was in place.
No one knows why, but around one million years ago the world’s climate cooled a few degrees. Ice caps formed in Arctic regions and slowly moved south over North America and Eurasia. These advances, followed by retreats, occurred four times.
The final major glaciation began moving south 35,000 years ago. A sheet of ice up to 2,000 meters (6,560 feet) deep covered all but the highest peaks of the Rocky Mountains. The ice scoured the terrain, destroying all vegetation as it crept slowly forward. In the mountains, these rivers of ice carved hollows, known as cirques, into the slopes of the higher peaks. They rounded off lower peaks and reamed out valleys from their preglacier V shape to a trademark, postglacial U shape.
The retreat of this ice sheet, beginning around 12,000 years ago, also radically altered the landscape. Rock and debris that had been picked up by the ice on its march forward melted out during the retreat, creating high ridges known as lateral and terminal moraines. Many of these moraines blocked natural drainages, resulting in the formation of lakes. Meltwater drained into rivers and streams, incising deep channels into the sedimentary rock of the plains. Today, the only remnants of this ice age are the scattered ice fields along the Continental Divide, including the 325-square-kilometer (125-square-mile) Columbia Icefield .
Water in its various forms has had a profound effect on the appearance of the Canadian Rockies . In addition to the scouring action of the glaciers, flowing water in rivers and streams has, over the millennia, deeply etched the landscape. The process continues today.
The flow of water is directly related to divides, or high points of land that dictate the direction of water flow. The dominant divide in the Canadian Rockies, and indeed North America, is the Continental Divide. The natural boundary created by this divide forms the Alberta –British Columbia  border, while other, less obvious divides form borders of many parks of the Canadian Rockies. The five national parks are classic examples of this scenario.
The divides forming the boundaries of Banff National Park  encompass the entire upper watershed of the Bow River. The Bow flows southward through the park, then heads east out of the mountains and into the Saskatchewan River system, whose waters continue east to Hudson Bay and the Atlantic Ocean. The Bow River is fed by many lakes famed for their beauty, including the Bow, Louise, and Moraine. To the south, the rivers of Kananaskis Country  and Waterton Lakes National Park also drain into the Saskatchewan River system.
The boundary between Jasper and Banff National Parks is an important north–south divide. The Columbia Icefield , a remnant of the last ice age, lies on either side of this divide. Runoff from the south side of the ice field flows south into the Saskatchewan River system, while runoff from the north side forms the upper headwaters of the Athabasca River system. The Athabasca flows north through Jasper National Park  and into the Mackenzie River system, which continues north to the Arctic Ocean.
All water draining off the western slopes of the Continental Divide ends up in the Pacific Ocean via two major river systems: the Columbia and the Fraser. The mighty Columbia makes a wide northern loop before heading south into the U.S. state of Washington  and draining into the Pacific Ocean. Along the way it picks up the waters of the Kootenay River, which begins high in Kootenay National Park  and makes a lazy loop south through Montana  and Idaho before joining the Columbia at Castlegar, British Columbia , and the Kicking Horse River, which flows down from the divides that form the borders of Yoho National Park . The Fraser River, the longest river entirely within British Columbia, begins in the high reaches of Mount Robson Provincial Park.