Montana Earth Science Picture of the Week

Lateral Moraines formed by Valley Glacier

Postcard Photo by Lawrence Dodge of Big Sky Magic Enterprises

Evidence of an Valley Glacier . . .
This photo, taken 30 miles north of Missoula, shows “lateral moraines” that were deposited along the sides of a valley glacier (a.k.a. "alpine glacier") during the last ice age. The moraines are the curved ridges along the sides of Mission Reservoir. These forested ridges consist of rock material that the glacier removed from the mountains in the upper part of the drainage basin, high above the lake. Over the thousands of years since the moraine was formed, soil has formed on top and trees have taken root.

A Conveyor Belt for Rocks . . .
Rock material that has been transported and deposited by glaciers is called "till". As the glacier formed, rocks became stuck to its bottom and sides. Then as the ice flowed toward the valley floor, these rocks scoured away even more of the mountain’s surface. The glacial ice flowed to the position marked by the location of lake where it melted and dropped the rocks. For thousands of years, snowfall continued to replace the ice as it flowed away from the mountain tops. This "conveyor belt" took much of the mountain with it, forming the moraines. Much of the till deposited at the end of the glacier was washed away as the ice melted, so the "end moraine” is missing. Since the end of the last ice age (10,000 years ago), soil has developed on the lateral moraines and trees have taken root.

Geologists describe till as “unsorted” because it is made up of all sizes of rocks. This characteristic helps geologists distinguish materials deposited by glaciers from those deposited by running water, which tends to deposit different sizes of rocks in different areas.

NOTE: The Mission Reservoir is about 3 miles in length. It can be found on the Flathead Indian Reservation, 5 miles east of St. Ignatius.

Terms: lateral, till


See past pictures of the week
*More aboout moraines
Cool photo of a glacier with moraines
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By Rod Benson
Earth Science Teacher at Helena High School

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