The rocks of Alberta range in age from ancient to almost “brand-new,” in geologic time. The 70-million-year-old Canadian Rockies, Alberta’s most distinctive natural feature, are relatively young compared to the world’s other major mountain ranges. By contrast, the Precambrian rock of the Canadian Shield, which is exposed in the province’s northeast corner and underlying parts of the rest, was the progenitor of North America. It was the first land on the continent to remain permanently above sea level and is among the oldest rock on earth, formed more than 2.5 billion years ago.
Approximately 700 million years ago, in the Precambrian era, forces beneath the earth caused uplift, pushing the coastline of the Pacific Ocean—which then covered most of the province—westward. The ocean advanced and receded several times over the next half billion years. Each time the ocean flooded eastward, it deposited layers of sediment on its bed, and the layers built up with each successive inundation. This sediment is now a layer of sedimentary rock covering most of Alberta, up to seven kilometers (4.3 miles) deep in the southwest.
Oil pools are created when oil globules—converted from decayed organic matter by the forces of heat and pressure beneath the earth’s surface—are trapped in porous rock capped by nonporous rock. During the Middle Devonian period, 375 million years ago, with the Pacific Ocean once again covering Alberta, coral reefs formed. Over time, this coral would be transformed into the porous rock that would hold the pools of oil.
Also during this period, as well as in later years of the Paleozoic era, trillions of microscopic organisms in the sea died and sank to the bottom of the ocean, creating mass quantities of decaying organic matter in and around the coral. This organic matter would be transformed over time into the oil itself.
Finally, approximately 300 million years ago, in the Carboniferous period, the Pacific Ocean extended as far as the foothills, and many rivers flowed into it from the east, carrying with them sediment that covered the porous reefs in nonporous layers. All of the elements necessary to eventually create and contain reserves of oil were then in place. A few hundred million years of “cooking” later, the primordial goo that fuels our modern internal-combustion society is pumped nonstop from beneath Alberta.
One other huge source of oil in Alberta isn’t pumped, but mined. In the early Cretaceous period, 130 million years ago, the Arctic Ocean flooded Alberta from the north, laying down the Athabasca Oil Sands along the Arctic seaway. These sands hold 1.7 trillion barrels of heavy oil—more than all of the known reserves of conventional crude oil on the planet.
Birth of the Rockies
Also during the Cretaceous period, the Mackenzie Mountains began to rise, cutting off the Arctic seaway and forming an inland sea where marinelife such as ammonites, fish, and large marine reptiles flourished. Dinosaurs roamed the coastal areas, feeding on the lush vegetation as well as on each other.
Then, approximately 70 million years ago, two plates of the earth’s crust collided. According to plate tectonics theory, 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 some places, older strata were pushed on top of younger.
By the beginning of the Tertiary period, approximately 65 million years ago, the present form of mountain contours was established and the geological framework of Alberta was in place. Then the forces of erosion went to work. The plains of the Late Tertiary period were at a higher altitude than those of today. The flat-topped Caribou Mountains, Buffalo Head Hills, Cypress Hills, Clear Hills, and Porcupine Hills are remnants of those higher plains.
The Ice Ages
No one knows why, but approximately one million years ago the world’s climate cooled and ice caps formed in Arctic regions, slowly moving south over North America and Eurasia. These advances, followed by retreats, occurred four times.
The final major glaciation began moving southward 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 and Cypress Hills. 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 preglacial V shape to a trademark postglacial U shape. The retreat of this ice sheet, beginning approximately 12,000 years ago, was just as destructive. 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 moraines. Many of these moraines blocked natural drainages, resulting in thousands of lakes across the north. And meltwater drained into rivers and streams, incising deep channels into the sedimentary rock of the plains.
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. But wind and water erosion continues, uncovering dinosaur bones hidden among layers of sediment and carving an eerie landscape of badlands along the sides of many prairie river valleys.
Alberta has three major watersheds draining 245 named rivers and 315 named creeks. More than half of the province drains into the Mackenzie River System, which flows north into the Arctic Ocean. The Peace River, which originates in the interior of British Columbia and flows northeast through Alberta, and the Athabasca River, whose initial source is the Columbia Icefield, are the province’s two major tributaries in this system. They eventually meet to form the Slave River, which flows into the Mackenzie at Great Slave Lake in the Northwest Territories.
Central Alberta is drained mainly by the Saskatchewan River System, which is the major source of water for Alberta’s farmers. This river system, which eventually flows into Hudson Bay, has three main tributaries: the North Saskatchewan River, originating from the Columbia Icefield; the Red Deer River, originating in the heart of Banff National Park and flowing through Dinosaur Valley on its way east; and the South Saskatchewan River. The latter is fed largely by the Bow River, flowing down from Banff, and the Oldman River, which cascades out of the Rockies south of Kananaskis Country.
A small area in the south of the province is drained by the Milk River, which flows southeast into the Mississippi River System, ending up in the Gulf of Mexico.
The amount of water flowing into any one of Alberta’s rivers depends on that particular river’s source. Rivers originating from melt-out of the winter snowpack reach peak flow in midsummer and often run dry by late summer. Those that originate from glaciers run light in spring and reach a peak in midsummer but continue a light flow until winter. Those that rise in the foothills and higher areas of the plains have highly variable flows, depending entirely on precipitation.
© Andrew Hempstead, from Moon Western Canada, 3rd Edition