Montana Earth Science Picture of the Week

Evidence of Fire and Ice on the Hi-Line

This photo was taken a few miles northwest of Harlem. Harlem is located in north-central Montana, about 45 miles east of Havre. The photo features 5 different topics . . . two related to the last ice age, and three that involve volcanic activity.

1. Hardened magma . . . Fire! (Move your cursor over the image to see the numbers.)
The boulder perched on this hilltop is made of granite, a type of igneous rock formed as magma cools slowly beneath the surface. Once the magma becomes rock, millions of years of erosion can expose it and break it into smaller pieces.

2. Not from around these parts . . . Ice!
The strange thing about this piece of granite is that it was not formed in this area. In the hills north of Harlem, sedimentary rock is the norm. Although the Little Rockies and Bear Paw Mountains to the south of here are made out of igneous rock, that rock is much different from the granite shown in the photo. In fact, this granite formed several hundreds of miles to the north of here, somewhere in central Canada. The rock was carried to this location by the continental glacier that grew southward from northern Canada during the last ice age. Such rocks, called "erratics", are a common site on the Hi-Line.

3. Hills of "bentonite" . . . More fire!
The hill beneath the boulder is made of a clay-like material called "bentonite". During the middle Cretaceous time, about 115 million years ago, this part of Montana was at the bottom of an inland sea. Ash from volcanic eruptions in the western part of the state drifted eastward, and much of it settled into the waters of the shallow sea. If you look at fresh volcanic ash under a microscope it looks like shards of glass. But ash that is deposited in seawater undergoes some changes . . . It loses its sharp edges and is also changed chemically. The result is the gray bentonite clay that makes up the hill shown in the photo.

4. Too big for this little river . . . More ice!
The Milk River Valley is shown in the background. Geologists recognized that from Havre to Glasgow, the Milk River Valley is much too big to have been formed by the Milk River. As it turns out, this used to be the valley of the Missouri River. During the last ice age, the same glacier that brought the granite boulder here forced the Missouri to take a more southerly route through central Montana.

5. Snake Butte . . . More fire!
The flat-topped landform in the background is called Snake Butte. It formed about 50 million years ago as magma cooled beneath the surface to become a very hard rock called shonkinite. The magma originated at a volcanic center several miles to the west (The Bear Paw Mountains). After the magma solidified, less durable sedimentary rock above and around it eroded away, forming the lonely butte. Many of the flat-topped buttes in the Great Falls area were formed in a similar way.

Below: Here is another Hi-Line erratic. This one is located at the Bear Paw Battlefield south of Chinook. It is a metamorphic rock called "gneiss" (pronounced "nice").

Terms: erratic, bentonite


*More about that inland sea
*Ancient path of the Missouri River
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By Rod Benson
Earth Science Teacher at Helena High School

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