The Prichard Formation (along with most of the rocks of that underlie the Flathead Reservation) are part of what's called the Belt Supergroup. The Prichard Formation, which underlies most of the southwestern part of reservation is very old. The rocks formed about 1.4 billion years ago, when the earth was around 3 billion years old. For perspective, the earliest fossils of land plants appear in rocks that are about 450 million years old. They are a billion years younger than the Belt rocks. Dinosaurs show up in the fossil record 200 million years after land plants first appear, and humans come on the scene some 240 million years after that. To see how these events fit together, make a timeline.

The Prichard as a whole is about 4 miles deep. Unit D is just one section of the Prichard. It is about 1,500 to 2,400 feet thick and is made up of gray, rust-colored weathering, silty argillite containing abundant chlorite crystals (formed when iron combines with silica compounds) and sparse garnet (garnet are silicate minerals that are used as gemstones and abrasives).

Most of the rocks on the Reservation, including those in the Perma area, are sedimentary rocks, which formed from layers of sediment like silt and clay and sand that accumulated on the bottom of an ancient inland sea. Over hundreds of millions of years, heat and pressure converted those sediments to rock. But there are also areas of igneous rocks—rocks that form from magma when it cools. That’s what the Paradise Sill is. Like other sills in the Perma area, it formed when magma rose up from below and forced itself between weak, easily penetrated, layers of rock or sediment. It is thought that the Paradise Sill magma intruded when the sediments were partly wet. The Paradise Sill, like other Perma sills, is a mafic rock, meaning it is high in magnesium and iron (in fact, the word mafic comes from combining the words magnesium (ma) and ferric (fic) (ferric means iron). When the rock is exposed, the iron oxidizes and gives the rock a reddish color.

You can tell the difference between igneous rocks and sedimentary rocks in the Perma area because igneous rocks are uniformly made up of crystals (see photo top right) whereas sedimentary rocks are layered and composed of much finer particles of clay and silt and occasionally sand. The images (lower right) illustrate the difference.

The Prichard Formation (along with most of the rocks that underlie the Flathead Reservation) are part of what's called the Belt Supergroup. The Prichard Formation, which underlies most of the southwestern part of the reservation is very old.

The rocks formed about 1.4 billion years ago, when the earth was around 3 billion years old. For perspective, the earliest fossils of land plants appear in rocks that are about 450 million years old. They are a billion years younger than the Belt rocks. Dinosaurs show up in the fossil record 200 million years after land plants first appear, and humans come on the scene some 240 million years after that. To see how these events fit together, make a timeline. The Prichard as a whole is about 4 miles deep.

Unit B is just one section of the Prichard. It is about 1,500 to 3,000 feet thick and is made up of dark-gray siltite and light-gray argillite but it rusts when exposed to the weather so it has a rusty-redish color. (Argillite is a clay-rich rock. It is a hardened siltstone or shale (shale is simply hardened mud)).

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What geologists call surfical deposits refers to what lies on the surface, above the bedrock. In the Perma area, these deposits are mostly found in the valley bottom and are cut by the river. They are alluvium and lake deposits. Alluvim (top right) is gravel, sand, and minor amounts of silt that have been deposited over the past few thousand years by the river (see the top photo at right). The thickness of these deposits, ranges from a few inches to about 40 feet.

The other kind of surficial deposit, the lake or lacustrine deposits, are the thick layers of silts and clays that settled out of Glacial Lake Missoula during the last ice age (lower right and bottom photo). That ice age (known as the Pleistocene Ice Age) started about 2 million years ago and ended about 10,000 years ago. The sediments are tan colored and made up of many thin layers of silt and clay. They can be from 1 foot to more than 300 feet thick. In places farther upstream, such as near Sloans Bridge, it forms high cliffs. Near Perma, the deposits, cut by streams or roads might be 10 to 20 feet high. Cavity nesting birds often excavate holes in these banks. Where the deposits are lower, coyotes, foxes, and badgers and other mammals find good places for dens because the deposits make for easy digging, and the dens are relatively stable.

The Empire Formation is gray to green and weathers to light green. The rock in the Empire is mostly argillite. Near Perma, the Empire Formation has grayish to very light green color. But elsewhere, like in Glacier National Park, it is more greenish. The greenness comes from a chemical process called reduction, the exact opposite of what occurs when iron rusts. The sediments—silt and clay—that would become the Empire Formation, settled out in the bottom of a large inland sea. They contained iron oxide, but because they were underwater and not exposed to oxygen, the iron did not rust or oxidize. Instead, it combined with silica compounds. Then heat and pressure converted the iron-silicate minerals to chlorite, the green compound that colored the rock.

Like the Prichard, the Empire Formation is old, between 1 billion and 700 million years old.

The Prichard Formation (along with most of the rocks that underlie the Flathead Reservation) are part of what's called the Belt Supergroup. The Prichard Formation, which underlies most of the southwestern part of reservation is very old. The rocks formed about 1.4 billion years ago, when the earth was around 3 billion years old. For perspective, the earliest fossils of land plants appear in rocks that are about 450 million years old. They are a billion years younger than the Belt rocks. Dinosaurs show up in the fossil record 200 million years after land plants first appear, and humans come on the scene some 240 million years after that. To see how these events fit together, make a timeline.

The Prichard as a whole is about 4 miles deep. Unit F is just one section of the Prichard. It is about 3,000 to 3,600 feet thick and is made up of fine-grained, dark-gray-colored siltite and gray argillite (siltite and argillite are hardened siltstone or shale—shale is simply hardened mud). When the rock splits and weathers, the iron it contains oxidizes or rusts, giving the rock a redish, tan color. Unit F also includes pyrite, also known as “fool’s gold”. This unit of the Prichard is layered and splits easily.

The Prichard Formation (along with most of the rocks of that underlie the Flathead Reservation) are part of what's called the Belt Supergroup. The Prichard Formation, which underlies most of the southwestern part of reservation is very old. The rocks formed about 1.4 billion years ago, when the earth was around 3 billion years old. For perspective, the earliest fossils of land plants appear in rocks that are about 450 million years old. They are a billion years younger than the Belt rocks. Dinosaurs show up in the fossil record 200 million years after land plants first appear, and humans come on the scene some 240 million years after that. To see how these events fit together, make a timeline.

The Prichard as a whole is about 4 miles deep. Unit G is just one section of the Prichard. It is about 900 to 1500 feet thick and is made up of:

• Fine grained quartzite (which is just hardened sandstone) that contains mica

• Gray-colored siltite and gray argillite. Argillite is a clay-rich rock. It is a hardened siltstone or shale (shale is simply hardened mud).

Unit G of the Prichard argillite is finely layered and splits or breaks apart easily. The layers are so fine, in fact, that they are hard to see unless you inspect it closely. Such smooth layers of clay and silt tell us this unit of the Prichard, Unit G, formed from layers of mud laid down 1.4 billion years ago in still waters, well below the reach of waves.

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Most of the rocks on the Reservation, including those in the Perma area, are sedimentary rocks, which formed from layers of sediment like silt and clay and sand that accumulated on the bottom of an ancient inland sea. Over hundreds of millions of years, heat and pressure converted those sediments to rock. But there are also areas of igneous rocks—rocks that form from magma when it cools. That’s what the Whiskey Gulch Sill is. Like other sills in the Perma area, it formed when magma rose up from below and forced itself between weak, easily penetrated, layers of rock or sediment. It is thought that the Whiskey Gulch Sill magma intruded when the sediments were partly wet. The Whiskey Gulch Sill, like other Perma sills, is a mafic rock, meaning it is high in magnesium and iron (in fact, the word mafic comes from combining the words magnesium (ma) and ferric (fic) (ferric means iron). When the rock is exposed, the iron oxidizes and gives the rock a reddish color.

You can tell the difference between igneous rocks and sedimentary rocks in the Perma area because igneous rocks are uniformly made up of crystals whereas sedimentary rocks are layered and composed of much finer particles of clay and silt and occasionally sand. The images below illustrate the difference (igneous on the left side, Belt sedimentary on the right).