Glacial Striations on Snake Butte

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This photo, taken on Snake Butte in north-central Montana, shows scratches that were made as the Laurentide Ice Sheet (continental glacier) flowed across here during the last ice age. Rocks that were stuck to the bottom of the ice caused the gouges - called "striations." Snake Butte is about 10 miles south of Harlem, MT on the Ft. Belknap Indian Reservation. The Little Rockies can be seen in the distance.

Striations help determine which direction the glacier flowed as it grew across the land, and also help locate "spreading centers" where the ice grew FROM, before merging to form the single continental glacier that covered Canada. Striations found in various parts of Canada reveal that there were three places in northern Canada where snowfall accumulations contributed to the ice sheet that eventually reached the USA (Of course there was no USA at that time!). These striations on Snake Butte prove the glacier flowed toward the southeast as it moved over the butte - probably because the Bears Paw Mountains (several miles southwest of here) forced the ice in that direction.

Below: The black dashed line indicates how far south the ice advanced into Montana when the ice age peaked about 18,000 years ago.

In addition to striations, the Laurentide Ice Sheet left other clues in the Snake Butte area.

1. Erratics – Metamorphic rocks from the Hudson Bay area can be found on Snake Butte (and throughout northern Montana). These rocks, which were embedded in the ice, were dropped here when the ice melted.

2. The Snake Butte Boulder Train – Large pieces of Snake Butte (igneous rock) have been scattered in a straight line extending to the southeast of Snake Butte. As the glacier flowed across the butte, pieces of the butte were carried away, and eventually dropped as the ice melted. Although they are few and far between, the line extends for almost 50 miles. The direction of the boulder train matches the direction indicated by the striations.

For much more about Snake Butte, CLICK HERE (includes a photo tour).

Concretions Along the White Cliffs of the Missouri River

There are several places in and along the White Cliffs of the Missouri River in north-central Montana where you can see great examples of concretions. A concretion is a roughly spherical mass of sandstone embedded in less durable sandstone. Concretions form within layers of sand/sandstone that have already been deposited, usually before the rest of the sand has hardened into rock. Concretions form when a mineral precipitates and cements sediment around some sort of "nucleus", which is often organic - a leaf, tooth, piece of shell or fossil. Fossil collectors sometimes break open concretions in their search for fossil animal and plant specimens. The "concretionary cement" often makes the concretion harder and more resistant to weathering than the sandstone it is embedded in.

Below: Here are a couple more photos of concretions along the White Cliffs of the Missouri. This 47-mile stretch is one of the premier canoe/kayak trips in the USA. CLICK HERE for an account of the 3-day journey (lots of photos). Here is another link to more information about concretions, including a short video (does not open on many mobile devices).

#134 - The Big Flat sits on Flaxville Formation

Click on map to enlarge.

All that's left . . .
The Flaxville Formation is made of gravels and sands that were deposited by large braided streams flowing eastward from the Rocky Mountains millions of years ago (Miocene-Pliocene, 2.5-10 million years ago). The Flaxville Formation experienced significant erosion during the late Pliocene and Quaternary periods, a process that continues today. This erosion dissected a once-widespread plain of fluvial (river) gravels, leaving only the isolated higher-elevation plateaus seen today - shown as the pale green patches on the map above.

The Big Flat . . .
The small farming communities of Hogeland and Turner sit on the remnant called the Boundary Plateau, known to locals as "The Big Flat". Most of the gravel beneath the soils of the Big Flat is unconsolidated (not cemented together). However, along Cherry Ridge 7 miles west of the plateau, the Flaxville Formation outcrops as cliffs of conglomerate - an unusual type of sedimentary rock made of gravel held together by a natural cement (photos below). Cherry Ridge is 20 miles north of Zurich (Zurich is 36 miles east of Havre).

Source of the gravel? . . .
The gravel consists of well-rounded pebbles from less than an inch to a foot or more diameter, of quartzite and the familiar red and green argillite from Rocky Mountains (shown in photo below, click to enlarge). Limestone, also from the Rockies, may have been dissolved and redeposited as the cementing material holding the gravel together. The conglomerate that form the cliffs here on Cherry Ridge north of Zurich has been thoroughly cemented with calcite - the same mineral that limestone is made of.

Click on map to enlarge.

Just high enough . . .
The Laurentide Ice Sheet covered this area during the last two glacial periods, growing about as far south as the Missouri River. The second to last advance, known as the Illinoian Ice Age, peaked 190,000-130,000 years ago, and the last one, called the Wisconsin Ice Age, reached its maximun 25,000-20,000 years ago. Although the ice sheet grew over the Big Flat during the Illinoian Ice Age, the glacier flowed around it during the more recent Wisconsin Ice Age. As a result, soil on the Big Flat was given over 100,000 extra years to develop, compared to soils on the surrounding area. This, combined with the fact that the surface is flat, makes the boundary plateau a good place to grow crops. The image below shows the correlation - The Big Flat (outlined in yellow) is covered with farmland. In some places the Flaxville gravels beneath the Big Flat are saturated with an abundance of groundwater, making center-pivot irrigation possible - another plus for farmers.

Term: fluvial