All About Argillite

by Skip Via

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.

It’s argillite, the colorful rocks and tiered strata that were formed during the Precambrian period 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 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 called Pangaea. Sediments–fine particles of clay and sand–built up over time, several 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.

Argillite strata (and some limestone) 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 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.

Even though argillite exhibits many different colors, the composition of all forms is the same. The color comes from chemicals that were in the water when the rocks formed. For example, red argillite indicates the presence of iron, green or blue indicates copper.

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.
Yellow argillite
Some argillite stones in a parking lot mud puddle, displaying an astonishing variety of colors and composition.

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