July 2 – Halfway Up The Mountain

Today’s factimal: Chief Mountain was formed by erosion of a huge thrust fault.

The story of Chief Mountain near Glacier National Park starts more than 1,600,000,000 years ago when this part of the country wasn’t just flat, it was actually shaped like a huge, shallow bowl and called the Belt Sea. That bowl was full of water which was full of stromatolites; big, flat mats of blue-green goo that ate CO2 and put out oxygen. They put out so much oxygen that they caused the first mass extinction event in Earth’s history! As time went by, different things were washed down from the shores and into the Belt Sea, creating more than 18,000 ft of alternating layers of sand, limestone, siltstone, and shale. As these got buried deeper, they metamorphosed into quartzite, marble, siltite, and argillite. One section of that sediment is especially important. Known as the Appekunny Formation, this mud and siltstone layer has fossils from the oldest known animals on Earth; these critters lived here 1,400,000,000 years ago (or shortly after your dad was born).

A cross section of teh geology in Glacier National Park (Image courtesy Montana Earth Science)

A cross section of teh geology in Glacier National Park
(Image courtesy Montana Earth Science)

But the Belt Sea was too good to last. About 150,000,000 years ago, North America ran into the Farallon Plate. This started volcanoes and mountains forming on both sides of the Belt Sea and changed its name to the Western Interior Seaway, which divided North America into three parts: Appalachia, over on the East, Laramidia on the West and Canada to the North. Glacier National Park is on the edge of Laramidia, where the land met the sea. For the next few hundred million years, the water remained even as the critters in it changed. Where trilobites and hallucigenia had roamed, now there were fish, octopus-like critters known as ammonites, and sea-going reptiles known as plesiosaurs. On the shore were dinosaurs including Seismosaurus, Triceratops, and T. Rex. Over the years, more sediments washed into the sea along with the bodies of dead critters; their skeletons fell into the sediment and some were preserved as the fossils that are commonly found in the area.

And then something terrible happened. The massive forces of plate tectonics ripped an entire section of land away and created the Lewis Thrust Fault. This enormous fault pushed a slab of rock that was up to five miles thick and more than two hundred miles long nearly fifty miles into North America, creating part of the Rocky Mountain chain. Water and ice eroded the overthrust, leaving behind a small mountain known as Chief Mountain; geology geeks call it a klippe and the Blackfeet call it Nínaiistáko and consider it to be sacred.

But forming Chief Mountain and the rest of the area didn’t happen overnight. It took nearly 60,000,000 years of alternating glaciation and warm periods before the land took on the form we know today. When the glaciers came, they would fill the valleys with ice, leaving just the tippy top of the mountains peaking through (“peaking through” – get it?). And when the glaciers piled up in the mountains, they slowly flowed down to the seas. As they flowed downhill, they scraped the sides of the mountains smooth and ground the rock below into flour. That’s why the valleys in the region are shaped like a U instead of a V and why the valley floors have very fine soil called loess.

Terms for parts of a glacier (Image courtesy  Luis María Benítez)

Terms for parts of a glacier
(Image courtesy Luis María Benítez)

That’s also part of why the area is known as Glacier National Park. The other part is because the mountains here are so tall and so far north, they used to catch more snow each winter than could melt in one summer. That helped the glaciers build and fill the valleys. When the last Ice Age ended about 100,000 years ago, the glaciers started melting faster than the snow could accumulate; as a result, the glaciers shrank. This process has gotten faster recently, thanks in part to climate change. In 1910, there were 150 glaciers in the park. By 2000, that had dropped to just 37. Today, only 25 active glaciers remain. And some scientists estimate that there may be no glaciers left in the park by 2030.

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