THE ABOVE IMAGE WAS TAKEN FROM THE SELKIRK CREST ABOVE LITTLE HARRISON LAKE

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Myrtle’s Turtle is a huge dome of exposed granitic rock.

Lets discuss the formation of granitic rocks. Going deep beneath the surface reveals the variations of the granitic rocks in the Selkirk Mountains. Igneous rock forms by the crystallization of molten rock. If the molten rock is below the Earth’s surface it is called magma and if it reaches the surface it is called lava. Igneous rock forms from the crystallization or “freezing” of magma. As hard as it is to imagine, hot magma freezing it does but at temperatures way hotter than water. Magma typically freezes at temperatures ranging from 650 C to 1,100 C (1,202 F to 2,012 F).

Slow cooling created the large crystals

The variation in crystallization temperatures results from the minerals present. Magma infused with silica-rich minerals (like quartz) crystallizes at lower temperatures whereas mafic-rich minerals (like hornblende) crystallize at higher temperatures.

In the case of the Selkirk Mountains, the exposed rock is part of a pluton. Plutons are blob-like intrusions of magma that cooled underground in sizes upwards of tens of kilometers.

Not all magma within a pluton is of the same composition. As the magma moves upward it melts surrounding rock which can vary in composition along the way. This disparity in composition contributes to the variations seen across the Selkirk Mountains.

As a body of magma cools, not all areas cool at the same rate. The differences in cooling results in different crystal sizes. When the magma cools slowly, the minerals have a chance to form large crystals. If the magma cools quickly, the minerals don’t have a chance to grow big before all the space is filled.
Granite and granodiorite are both coarse-grained rocks which indicates the body of magma cooled slowly.

Some rocks have mineral grains that are two sizes, usually large crystals within a matrix of smaller crystals. These rocks are called porphyritic and are a result of the magma cooling in two stages. The large mineral grains, called phenocrysts, crystallized first while the magma cooled very slowly allowing for sizable growth. Then the magma, for one reason or another, cooled more quickly and the remaining minerals crystallized. Phenocrysts are commonly plagioclase (a mineral) because it is one of the first minerals to crystallize.

A porphyritic rock

A possible xenolith because the dark rock is unlike anything surrounding it.

On a larger scale are chunks of rock that don’t match the surrounding rock at all, like the really dark-colored rock I found along the Hidden Lake trail. One possibility for these “odd” rocks is they are xenoliths. Xenoliths are rock fragments that are foreign to the body of igneous rock in which they occur. They may have fallen from the edge of the magma chamber and didn’t melt completely before the magma solidified.

Another possibility is that it is part of a vein that cut through the igneous rock after it solidified. Fractures, sometimes from fault movement, fill with magma of varying compositions, cool and crystallize into veins. Sometimes the veins are mainly quartz.

The possibilities are really quite numerous on how igneous rocks form and I’ve only covered a few. Finding the different types of granite, granodiorite, mineral veins and xenoliths throughout the Selkirk Mountains is like a scavenger hunt without a list because you never know what variation you’re going to stumble upon next.

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The Kootenai Valley and the Selkirk, Purcell and Cabinet Mountains are the main geographic features in the county. One geologic feature is responsible for how we see them today–the Purcell Trench.

From atop Clifty Mountain (in the Cabinets), the Purcell Trench is visible into Canada.

The Purcell Trench is the valley structure between the Selkirk Mountains and the Cabinet and Purcell Mountains. The Purcell Trench extends beyond Boundary County. The southern edge is in Rathdrum Prairie, though it is harder to distinguish south of Sandpoint. The Purcell Trench also extends north into British Columbia where it eventually merges with the Rocky Mountain Trench.

Valleys can be made by erosion (water or glaciers) or by an underlying geologic structure. The Purcell Trench is too long, wide and straight to have been formed by erosion and, therefore, it is a result of a major bedrock structure.

Purcell Trench south as viewed from Tungsten Mountain in the Purcells

One bedrock structure responsible for creating valleys is a fault. The basin and range mountains in Nevada formed because of faulting. A fault is a fracture in rock where sliding occurs (both sides don’t have to move, often it is one piece moving against the other stationary side). The San Andreas fault in California is one example.

There has to be a reason for rock to move and in the case of the Purcell Trench  it was because this area was being stretched. As one can imagine, rock isn’t a likely candidate for stretching, so what happens is faulting. Faulting releases the tension built up by stretching.

Exposed granite in the Selkirks cooled several miles below the earth’s surface

What caused the stretching? Intruding magma. Long before the ice ages, a large mass of granitic magma was rising in the Earth’s crust under the present day Selkirk Mountains. As the giant bubble of magma neared the surface, it weakened the crust by stretching it. The giant bubble of granitic magma cooled before it reached the surface, creating a giant granite batholith. But the tension was still there from the stretching. Consider that the granite cooled miles below the surface and now is exposed in the Selkirk Mountains. Something had to move to make the granite visible, it wasn’t eroded away.

Quite a bit of imagination is necessary to envision what happened. The rock that is now the Cabinet and Purcell Mountains was once on top of where the Selkirk Mountains are located. When the magma intruded, it bulged up the overlying rock, much like a bubble forming in pizza crust. To release the tension, a fault formed along the current eastern front of the Selkirk Mountains. Over millions of years the rock on top (the current Purcell and Cabinet Mountains) slowly slid down the fault in an easterly direction into their current position, leaving an open trench behind. That open trench is now called the Purcell Trench.

Looking east across the Purcell Trench at the Purcell Mountains

As the overlying rock slide eastwards, the granite batholith was exposed.

The rocks around the fault zone are under tremendous pressure as the faulting occurs, which results in folding and metamorphosing the rocks. Rocks along the eastern front of the Selkirk Mountains may have a shiny look to them, with all the crystals aligned in one direction–these rocks were altered by the heat and pressure in the fault zone.

Have you taken a close look at the rock cut where the Myrtle Creek Road takes off from the West Side Road? This rock was folded deep in the crust along the fault zone as the Purcell Trench was being formed.

The Purcell Trench doesn’t look the same today as it did after it was initially created. Glaciers, lakes and rivers have scoured, eroded and deposited sediment to create the valley we know today as the Kootenai Valley.

In 1987, the Idaho Panhandle National Forest designated the Selkirk Crest to be a 26,700 acre wilderness study area. They never moved forward with idea of creating the Selkirk Crest into a wilderness.

Below are many of the favorite hikes and scrambles in the American Selkirks.

One thing you will notice while exploring the Selkirks, is the quality of the granite. There are few places in our region that you can walk up a 45°+ slope and not be nervous. But because of the events described above, we can enjoy the Selkirk Granite on these hikes.

When you see “ Peak 6514’” or other numbers, it means the peak is not named, and is designated by it’s elevation number.

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Links to Route Descriptions

TWO MOUTH LAKES 5785' TRAIL #286 KENT LAKE RIDGE 7243'
​HUNT LAKE 5813' GUNSIGHT PEAK 7352'
FAULT LAKE 5980' & HUNT PEAK 7058' TRAIL # 59
BURTON PEAK 6844' TRAIL #9
COOKS LAKE & PEAK 5993' TRAIL #236
​ HARRISON LAKE & PEAK 7292' TRAIL # 217
​​LITTLE HARRISON LAKE 6271' & PEAK 7292'
​BEEHIVE LAKE 6457'
PARKER PEAK 7670'
RUSSELL PEAK TRAIL #12 6618' & RUSSELL RIDGE #92​
IRON MOUNTAIN 6426' Trails #180 & 176
LONG CANYON TRAIL #16
LONG MOUNTAIN 7265' AND LAKE
​MYRTLE LAKE 5950' & MYRTLE PEAK 7122' TRAIL #286
​PYRAMID PEAK 7355' TRAIL #13
PYRAMID AND BALL LAKES TRAIL #43
RED TOP MOUNTAIN 6266' TRAIL #102
MOUNT ROOTHAAN 7326' AND CHIMNEY ROCK 7124' TRAIL #256
ROMAN NOSE LAKES & PEAK 7260' TRAILS # 165 & 160

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