Monday, October 5, 2009

Formation of the Bitterroot Mountains

The Bitterroot Mountains run north and south along the border of Montana and Idaho in southwestern Montana.

These mountains, like the rest of the Rockies, were formed by a combination of geologic forces, starting with plate tectonics. About 100 million years ago, the west coast of North America ran through western Idaho. The North American plate moved west, colliding with the plate holding the floor of the Pacific Ocean, causing the heavier Pacific plate to sink under the North American plate. The compression crinkled (not a geologic term) and thickened the western edge of the North American plate.  It also drove the oceanic plate down into the hot mantle, where the granite melted and created a large mass of magma.  The magma rose into the exisitng rock near the Earth's surface and formed the Idaho batholith, a huge mass of granite that covers about 10,000 square miles in central Idaho and includes the southern half of the Bitterroot Mountains.

With so many forces at work, the Earth's surface is thought to have bulged and heaved, becoming unstable. In some parts of Montana, the sedimentary layer on top to the bulged sections slid off in great slabs. In southwestern Montana, a large piece of the Idaho batholith broke off and moved east, forming the Sapphire Mountains. The gap created between the Sapphire Mountains and the Idaho batholith is the Bitterroot Valley.

Then the glaciers came. A glacier is a huge, flowing mass of ice that forms when, over time, snow falls faster than it melts, creating a mass of ice that moves as a result of its own weight and pressure. As they flow slowly downhill, glaciers pick up rocks and grit, and those act as abrasives, carving and gouging the landscape. Glaciers sculpted the distinctive shapes of the Bitterroot Mountains and their valleys.

The last period of glaciation in Montana ended about 10,000 years ago.  The Sapphire Mountains, which are not as high as the Bitterroots, were not affected by the glaciation, as evidenced by their smooth, unsculpted tops.  (This is the only pictue I have showing the Sapphire Mountains. I took it in mid-September 2009, when the Gird End Fire was sending up a plume of smoke neat Skalkaho Peak.  The view is looking east down Main Street in Hamilton.)

There were other forces at work shaping the landscape in the Bitterroot Mountains and the surrounding area, such as volcanic activity and erosion, coupled with wet and dry periods in the region. For a complete account of the geology of Montana, see Roadside Geology of Montana, by David Alt. It's a great, very readable book.

1 comment: