All About Argillite

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by Skip Via
skip@westvalleynaturalists.org

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You see it everywhere in the valley–lining the bottom of creek beds, along hiking trails, layered throughout road cuts, on the shores surrounding Tally Lake, in the cliffs around Flathead Lake, piled at the corners of plowed fields, covering fireplace hearths and floors, in the pavement of many area roads and parking lots, and often in your yard when you are trying to dig a new garden bed (especially if you live on a glacial drumlin, as I do). It’s the predominant rock in Glacier National Park a few miles to our east, as shown in this Google Earth image, below:

It’s argillite, the colorful rocks and tiered strata that were formed during the Precambrian period well over 500 million years ago–and western Montana is one of the best places in the world to see it.

What is Argillite?

Argillite began forming over half a billion and a half years ago when much of western Montana comprised the floor of the shallow Belt Sea. The continents as we know them now were part of a large land mass. Sediments–fine particles of clay and sand–built up over time, up to three miles thick, on the floor of the Belt Sea. Over many millions of years, these sediments sank and became compressed, forming the rock we know as argillite.

About 170 million years ago, during the Jurassic period, two tectonic plates collided, creating a lot of folding and deformation and forcing the much older Belt rocks over top of the younger rocks of the North American plate. This formed what we know as the Lewis Overthrust–a thick layer of older rocks overlaying younger rocks. Glacier National Park sits at the boundary between the younger rocks below and the much older rocks–more than a billion years older–on top. The boundary between those layers is clearly visible from Marias Pass and at Chief Mountain, the easternmost edge of the Lewis Overthrust. (For more on this process, see Glaciers, Faults, and Tectonics on this web site.)

Argillite strata on the cliffs around Flathead Lake.

How is it Formed?

In a word, pressure. Argillite begins as layers of sediment. The fine grains of silt, clay and sand get pressed together into an amorphous mass (lithified mud) that has no lines of cleavage. It’s impossible to split argillite into layers in the same way that one might split mudstone or slate. It will break apart where it wants to, not where you want it to. Even samples with many visible layers have this lack of fissility.

Argillite can metamorphose into slate given enough time and pressure. Some geologists describe argillite as a “low-grade metamorphosed” rock because of the structural changes caused by pressure.

In this sample from Glacier National Park you can see ripples created on the floor of the Belt Sea over 500 million years ago.

Why So Many Colors?

The most common colors of argillite in the valley are red and blue, but argillite can also be green, yellow, purple, turquoise, white, and even black. A sample may be a single color, or it may show many fine layers of different colors depending on the circumstances under which it was formed and/or compressed.

Multi-colored banding (layering) in some argillite samples.

Argillite’s color is the result of a variety of factors–the nature of the sediments that were eroded from the surrounding mountains and deposited on the Belt Sea floor, the relative amounts of sediments present, and the chemistry of the sea water when the rocks formed, among others. For example, red argillite indicates the presence of iron in the rock, which oxidized when exposed to oxygen. Blue and green samples indicate a lack of oxygen as the layers of sediment were compressed under the sea.

Can You Find Fossils in Argillite?

Yes, but not in the sense that we usually think of fossils. The sediments that formed these rocks accumulated before most living organisms appeared on Earth. The Belt Sea was home to several species of blue-green algae–the organisms that were responsible for putting oxygen in the young Earth’s atmosphere. Under microscopic examination, you can find fossils of these algae in argillite samples. But you won’t find any bones or leaves. Those came along many millions of years later.

Some Local Samples

Red argillite
Blue argillite with many compressed layers
Tightly striated sample, probably indicating some metamorphic changes. This sample has been rounded and polished from spending a lot of time in stream beds.
Green argillite
The astonishing variety of colors and compositions of argillite, enhanced here by a recent wave washing over it.
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jon
jon
4 years ago

well done

Andrew
Andrew
1 year ago

Thank you – this is just the information I was seeking. We have some of your argillite here on the lower Columbia River, deposited by the Missoula Floods during the last ice age.

Anthony
Anthony
1 year ago

Can this argillite be carved? Thank you.

Kal
Kal
4 months ago
Reply to  skip

There is a pre-columbian quarry site of red argillite in Yavapai County, Arizona. Native American examples of pendants, beads and labrets are on display at the Museum of Indigenous Peoples located in Prescott.
Would this be similar to the more well known Pipestone MN material?

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[…] stones come in gray, green and red, with some banded argillites being brown as well. A web page on argillite in part of the United States notes that argillite’s colour is the result of a variety of factors, including the nature of the […]

Roy
Roy
4 months ago

Might be too late to the party…this stone came from the N Fork Flathead River. Is it argalite? It is greenish and has maroonish bands through and around it .

Thanks

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Richard
Richard
8 months ago

Black is found in Haida gwaii BC Canada

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[…] Post) can also be found on Gemstone Beach. For examples of banded argillite from Montana, USA, see West Valley Naturalists – this shows that the banding on many of the Gemstone Beach stones is by comparison clear and […]

Jan
Jan
1 hour ago

What is Argillite?Argillite began forming over half a billion and a half years ago when much of western Montana……………………this part makes no sense