PHOTO: The Three Sisters. The Three Sisters is composed of massive beds of sloping marine limestone, called the Palliser Formation. Photo by Chris Seashore

Author: Jon Turk

Most of the mountains around Fernie are upside down.
Oh, I know that they look like right side up mountains, with broad bases rooted firmly in the valley floor and pointy peaks reaching skyward. But to a geologist, they are upside down.

These mountains are composed of sedimentary rocks, which form as sediment collects in shallow seas or in inland basins. Sediment accumulates in horizontal layers; after one layer forms, younger material settles on top of the older layer. In the Grand Canyon of the Colorado River, for example, the oldest sedimentary rocks lie at the bottom of the canyon and are about 500 million years old, while the youngest rocks are at the top and are about 250 million years old.

But the rocks on the tops of the Fernie mountains are about 180 million years older than the rocks on the bottoms. The sequence is turned upside down.To understand how this happened, let’s look back about 360 million years. At that time, North America was south of where it is now and Fernie was close to the equator. Western British Columbia as we know it today didn’t exist, and the Pacific was only 80 kms. west of Fernie. The seas were full of shelled animals and fish, plants had migrated onto the continents, and the first amphibians had adapted to the desiccation of land. Over the course of about five million years, marine chemicals precipitated to form limestone on the sea floor; this rock now forms the craggy summit of the Three Sisters.

Today, geologists understand that the uppermost 100 kilometres of the Earth’s surface is fractured into seven large, and several smaller, segments called tectonic plates. These plates float on dense, slightly gooey or plastic rock beneath. A current in the deep plastic rock carried North America northward, speeding along about as fast as a fingernail grows. At the same time, the Pacific Ocean Plate was headed northeast, and smashed into North America. The collision buckled the continent just as a car fender buckles during a head-on collision. Segments of ocean floor sunk under North America. The sinking plate stirred up the plastic rock to create volcanic eruptions and mountains.

GRAPHIC: Ocean Plate. A schematic view of an ocean plate subducting beneath a continent. Courtesy of Jon Turk, Saunders College Publishing.

About 180 million years ago, when dinosaurs were dominating terrestrial ecosystems, the sinking conveyor belt of ocean floor carried a chain of Pacific islands eastward until they smashed into the northwest coast of North America. Land surfaces folded and massive chunks of sea floor slowly rose out of the ocean like a whale rising to breathe. This old ocean floor, now surrounded by land, became a long, narrow trench of low lying swamps and brackish inland seas. Mud eroded from the mountains and settled into the trench, giant ferns grew, died, and settled into the ooze. Gradually, this mixture of mud and vegetation compressed and turned to shale and coal.

More islands smashed into the west coast, squeezing this band of swamps and inland seas under inexorable pressure. Earthquakes shook the land and cohesive chunks of rock fractured along massive thrust faults or folded as the rock turned plastic under the pressure. Sections of the old limestone sea floor slid along the thrust faults to ride up and over the younger coal-bearing shale. Thus, the compression and thrust faulting forced older rocks over younger rocks, turning the mountains upside down.

PHOTO: Mt. Klaur. Mt. Klauer, northwest of the Three Sisters, is composed of limestone folded by compressive forces during the mountain building process. Photo by Jon Turk.

For reasons that geologists don’t well understand, the collision between plates relaxed about 45 million years ago. Massive mountains that had been held aloft partly by the compressive forces, started to ooze outward, like a mound of honey on a tabletop. Only the deeper core of the mountains was hot enough to flow plastically, however. The cooler surface cracked, like a layer of frosting covering the honey. One of these cracks is now called the Elk Valley.

Much later, glaciers descended, pushed megatons of rock around, ground down cliffs and thus put the finishing touches on the landscape. Finished for now, that is. As you drive up through this chasm in the Earth toward the upside down mountains, remember that all bets are off for the future and our present landscape is only a temporary signpost in the great sweep of geologic history.

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