#137 - Frozen in Time

Wild ice! . . .
I took this photo while skating with friends on Upper Holter Lake 20 miles north of Helena on January 27, 2026. Nordic skating (aka wild ice skating) has become very popular in the Helena area in recent years. Upper Holter Lake is part of the Missouri River, and on the day I took the photo the ice was 3-4 inches thick. The bubbles were most likely methane gas that formed as organic material decayed on the bottom of the lake.

Stacks of Bubbles . . .
Here is the scenario that probably caused the patch of bubbles to form: As night-time temperatures dropped below freezing, a layer of ice formed on the surface of the lake. Bubbles of methane rose but were trapped beneath the layer of ice. During the next cold night a second layer formed below the first, surrounding the first batch of bubbles. More bubbles rose and were trapped below the second layer of ice. This second batch of bubbles was embedded as the third layer of ice formed. This continued over several days, forming multiple layers of ice and bubbles, causing the stacking effect shown in the photo above.

For more about Nordic Skating in Montana . . .

1. CLICK HERE to see an album that includes all of the photos and reels from my skate on Upper Holter Lake.

2. If you're on Facebook, check out the MT Icebuds Facebook Page.

3. To access my blog post and photo tour from a day spent skating on Dead River near Harlem, Montana CLICK HERE.

4. Click on the image below to watch an animation of my time spent skating on Upper Holter Lake - includes a few short reels and several more photos:

Relive 'Skate on Upper Holter Lake 1/27/26'

#130 - Kelvin-Helmholtz Clouds Caused by Wind Shear

The right place, at the right time.
This photo of an amazing Kelvin-Helmholtz cloud was taken on January 26, 2019 by Hannah Martin, one of my freshman Earth Science students at Helena High School. She snapped the photo from the Helena Valley, looking west - Mount Helena can be seen on the left, and the distant horizon marks the Continental Divide. Also known as "fluctus" or "billow" clouds, they were named after Lord Kelvin (1824-1907) and Hermann Von Helmholtz (1821-1894) who identified the type of instability responsible for the unique waves. Such clouds are fairly rare, and may only last for a few minutes.

Like wind across water.
The waves form at the boundary between layers of air that have different densities and wind speeds (wind shear). Air in the layer above the cloud is moving faster than air in the layer containing the cloud. Development of waves on the cloudy layer is similar to what happens when waves form on the ocean as wind blows across the water. In the photo the wavy layer is more dense than the clear air flowing above it - just as water is more dense than air blowing over its surface.

Clouds provide a "visual".
The type of motion that causes the wave pattern is actually not that uncommon in the atmosphere, although we usually don't see it. In order for us to see it, clouds must be present in the lower layer (as they were when the photo was taken). We can't see clear air, but we can see clouds. One of the nice things about clouds is they provide clues about the type of motion currently happening in the atmosphere. Want to know more? - Watch the 4.5-minute video below, which includes a great demo.

Term for students to define: wind shear

1. Article - More about KH Clouds

2. Another good article about KH Clouds

Enter your address to see where you would have been millions of years ago.

This is a fantastic interactive visualization that was created (and is maintained) by Ian Webster. Once the site opens, enter your address in the box in the upper left, then select the time (in Earth's history) near the top of the web page. To see more of Ian's work, go to ianww.com.

Thanks to Rick Dees for showing me this!

#126 - Classic Chinook Arch over Helena, Montana

A Mountain Wave.
This photo was taken from the Helena High athletic fields, looking west toward the Continental Divide. It shows the classic Chinook arch that appeared on December 13, 2018. The clear area between the arch and the mountains exists because the air is down-sloping there. As air flows over the Rockies it may develop an up and down motion like water flowing over rocks in the rapids of a river. Although the air flows downward once it gets over the mountains, it may continue to oscillate up and down as it flows away from the mountains for several hundred miles. The upward flowing part of this "mountain wave" is what forms the long arch of clouds. (Click on the image to enlarge it or CLICK HERE to watch a 24-second video of the arch shown in the photo.)

Here's how it works.
As the air flows down-slope, it is warmed by compression. Then, as the wave action continues and the air begins to rise again, the air cools by expansion. If there is enough vapor in the air, the arch of clouds will form as vapor condenses to form cloud droplets (or cloud crystals). Typically, the long area of clouds will form near the crest (top) of the first wave and then get blown eastward by higher level winds. If the mountain wave continues, and another downward turn is taken, the arch (cloud) will evaporate farther downstream (east).

Same arch, different vantage point.
The G.O.E.S. East satellite image below shows what the same Chinook arch looked like from space at 9:47 am MST. It is called an "arch" because an observer standing below it sees a curved patch of clear sky between the band of clouds and the mountains below. In the satellite image, the Chinook arch is the distinct eastern edge of the bright white cloud that extends from north to south through western Montana.

Term to define: GOES East Satellite

#6 - Mazama Ash in Montana

Right: This photo was taken 12 miles northeast of Helena, Montana. The white streak is a layer of ash from the explosive eruption of Mt. Mazama (Oregon) 7,700 years ago. Geologists can determine where the ash originated by comparing its chemical composition with the compositions of volcanoes found in the region. .

The volume of ash produced by Mazama was forty-two times greater than the amount produced by St. Helens in 1980. Prevailing winds caused the ash to spread eastward. Initially the ash covered much of the ground in the Northwest. But in the months following the eruption, wind and runoff transported the ash to low places (lakes, valleys), where it was eventually buried beneath layers of sediment. Today it can be seen in places where it has been exposed in road-cuts, cut banks, archeological digs, etc. The sediment above the ash layer in the photo to the right was deposited in the centuries after the ash settled here, then the exposed when road construction cut into the slope.

The Mazama ash layer has been found at many other places in the Northwest as well. In fact, Mazama ash serves as a good "key bed" for the region. Key beds help determine relative age - For example, several years ago archeologists came upon Mazama ash while excavating a Paleo-Indian campsite near Helmville, Montana. Mazama ash was exposed at the dig site ABOVE the evidence, indicating that the Indians used the site before the big eruption (at least 7,700 years ago).

The Mazama eruption also emptied significant amounts of magma from the chamber beneath the volcano. As a result, after the eruption the remaining cone collapsed into the chamber, forming a huge crater known as a "caldera". Today, Crater Lake (Oregon) fills the caldera.

For much more about Mazama Ash, CLICK HERE

Also, check out my Montana hiking blog at www.bigskywalker.com - Lots of geology!

Term: key bed

Below: Mazama ash in this cut bank on the Clark Fork River near Drummond.

Check out this Slick Crossword Puzzle-Maker

I wanted to make a crossword puzzle to help my students review some terms that were included in our recent hydrology unit. I found this site: crosswordhobbyest.com - Although the puzzle did not include all of the terms that I entered*, I'm satisfied with the end product. To access the puzzle CLICK HERE, or (to see a different look) click on the "start puzzling" link below.

*Unfortunately the puzzle-maker was not able to include these words: watershed, septic system, recharge, aeration.

#127 - Marble formed by Contact Metamorphism

The Elkhorn Mountain Volcanics.
The Elkhorn Mountains south of Helena, Montana are the remnants of volcanoes that were active in this area 74 to 81 million years ago. During that period, a tectonic plate was subducting beneath western North America, allowing magma to rise to the surface. As a result, the Elkhorns are made up primarily of extrusive igneous rocks, but are related to plutonic rocks of the nearby Boulder Batholith. The volcanic rocks that make up the Elkhorns (lots of andesite) formed when lava poured onto the surface and cooled, whereas the plutonic rocks (granite, etc.) of the batholith formed as magma beneath the volcanoes cooled underground.

Contact Metamorphism.
Despite the volcanic origin of the Elkhorns, the outcropping shown in the photo is made of marble - a metamorphic rock formed as limestone was changed by heat and/or pressure. Sometime during the late Cretaceous, magma melted its way into the area, coming close enough its heat to change the limestone into marble - a process known as “contact metamorphism”. Evidence for this is the presence of granite (formed as that magma cooled), located not far below the marble.

Ancient hot springs?
Limestone is usually formed by sediment deposited in a shallow tropical sea, so how did limestone form in a center of volcanic activity? One possibility is that hot springs existed here when the area was volcanically active. An unusual variety of limestone called "travertine" can form on the surface around hot springs by the rapid precipitation of calcium carbonate. This is what is happening today at Mammoth Hot Springs in Yellowstone Park where thick terraces of travertine continue to form as hot water comes to the surface.

Finishing touches.
In the millions of years that followed, the igneous rocks and marble were deformed (folded, faulted) by tectonic forces that built the Rocky Mountains (80 to 55 mya). As the formations were pushed up, they were also shaped by erosion, including glaciation. In fact, the marble in the photo marks the top of a cirque formed by a glacier that once (or multiple times) flowed from Elkhorn Peak toward the present-day location of the town of Elkhorn. More significant cirques lie on the northeast sides of Elkhorn Peak and Crow Peak - the two high points in the range. (Map)

Term to define: contact metamorphism

To access a blog a photo tour of the hike to the summits of Elkhorn Peak and Crow Peak, go to www.BigSkyWalker.com.