BACKGROUND - Canadian Rockies: Including Banff and Jasper National Parks - page 496

al massive chunks (plates) that are always moving and occasionally bump into each oth-
er. 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.
The Ice Ages
No one knows why, but around one million years ago the world's climate cooled a few de-
grees. 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 Moun-
tains. 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 pre-
glacier V shape to the trademark, postglacial U shape. The retreat of this ice sheet, begin-
ning 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, creat-
ing 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.
FOOTHILLS ERRATICS TRAIN
During the last ice age, a sheet of ice up to 1,000 meters (3,280 feet) thick crept
southward across the area that is now Jasper National Park. A landslide deposited
hundreds of large quartzite boulders atop the ice, which continued moving south,