Glacial Striations on Snake Butte

Click on the image to see a larger view.

This photo, taken on Snake Butte in north-central Montana, shows scratches that were made as the Laurentide Ice Sheet (continental glacier) flowed across here during the last ice age. Rocks that were stuck to the bottom of the ice caused the gouges - called "striations." Snake Butte is about 10 miles south of Harlem, MT on the Ft. Belknap Indian Reservation. The Little Rockies can be seen in the distance.

Striations help determine which direction the glacier flowed as it grew across the land, and also help locate "spreading centers" where the ice grew FROM, before merging to form the single continental glacier that covered Canada. Striations found in various parts of Canada reveal that there were three places in northern Canada where snowfall accumulations contributed to the ice sheet that eventually reached the USA (Of course there was no USA at that time!). These striations on Snake Butte prove the glacier flowed toward the southeast as it moved over the butte - probably because the Bears Paw Mountains (several miles southwest of here) forced the ice in that direction.

Below: The black dashed line indicates how far south the ice advanced into Montana when the ice age peaked about 18,000 years ago.

In addition to striations, the Laurentide Ice Sheet left other clues in the Snake Butte area.

1. Erratics – Metamorphic rocks from the Hudson Bay area can be found on Snake Butte (and throughout northern Montana). These rocks, which were embedded in the ice, were dropped here when the ice melted.

2. The Snake Butte Boulder Train – Large pieces of Snake Butte (igneous rock) have been scattered in a straight line extending to the southeast of Snake Butte. As the glacier flowed across the butte, pieces of the butte were carried away, and eventually dropped as the ice melted. Although they are few and far between, the line extends for almost 50 miles. The direction of the boulder train matches the direction indicated by the striations.

For much more about Snake Butte, CLICK HERE (includes a photo tour).

Concretions Along the White Cliffs of the Missouri River

There are several places in and along the White Cliffs of the Missouri River in north-central Montana where you can see great examples of concretions. A concretion is a roughly spherical mass of sandstone embedded in less durable sandstone. Concretions form within layers of sand/sandstone that have already been deposited, usually before the rest of the sand has hardened into rock. Concretions form when a mineral precipitates and cements sediment around some sort of "nucleus", which is often organic - a leaf, tooth, piece of shell or fossil. Fossil collectors sometimes break open concretions in their search for fossil animal and plant specimens. The "concretionary cement" often makes the concretion harder and more resistant to weathering than the sandstone it is embedded in.

Below: Here are a couple more photos of concretions along the White Cliffs of the Missouri. This 47-mile stretch is one of the premier canoe/kayak trips in the USA. CLICK HERE for an account of the 3-day journey (lots of photos). Here is another link to more information about concretions, including a short video (does not open on many mobile devices).

Strange Fossils in Glacier Park

These are fossils of stromatolites that can be seen along one of the most popular trails in Glacier Park. CLICK HERE to see more photos taken along the Highline Trail.

Glacier Park is made almost entirely of rocks from the Belt Formation (aka "The Belt Supergroup") - layer upon layer of sandstones, shales, and carbonates from the late Precambrian Era. At that time there were no organisms with bones or shells, so stromatolites like those shown in the photo, are the only fossils that can be found in Glacier Park.

Stromatolites are mound-like, multi-layered colonies of algae (blue-green algae; aka cyanobacteria), and their formation has much to do with the way they change the chemistry of the shallow water they live in. The photosynthetic cyanobacteria remove carbon dioxide from the surrounding water, causing calcium carbonate to precipitate onto their slimy, mat-like colonies. Calcium carbonate, along with grains of sediment (silt, etc.), stick to the bio-film layer that covers the colonies. As the cyanobacteria continue to grow up through the sediment, a new layer forms. This process occurs over and over again, creating layered mounds, columns, or sheets.

Fossils of different species of stromatolites can be found in different areas of the park. Stromatolites that lived in the Precambrian played a major role in increasing the amount of oxygen in the atmosphere of the primeval Earth ("The Great Oxygenation Event"). Living stromatolites can be found today at Shark Bay in western Australia.

Check out my Montana Hiking Blog - lots of geology!

Source: T.N. & E.L. Taylor. 1993. The Biology and Evolution of Fossil Plants. Prentice Hall, New Jersey.

Great explanation of "Rain Shadows"!

This video is fron a YouTube channel called "2 Minute Geology", which focuses primarily on the Northwest USA (especially Washington). It is produced by Tom Foster and Nick Zentner (Central Washington University).

Laccoliths, Dikes, and Bears - Oh my!

I recently explored several plutonic formations associated with an ancient volcanic field in central Montana. The volcano(es), which was active during the late Cretaceous period, left its mark on the landscape in the form of several laccoliths, dikes, and sills. Centuries ago the sill shown in the photo below was used as a buffalo jump by Montana's first people. That is my shadow on the cliff. CLICK HERE to learn more about the trip and access the photo tour, which includes some nice shots of the bear that inspired me to climb a tree.

Hiking into the Crater of Mt. St. Helens

This photo was taken during a hike that I did a few years ago into the crater of Mt. St. Helens. The landscape was rocky and gray with hardly any vegetation - but VERY interesting. Plus, we were treated to an "up close and personal look" at geology in action - glaciation, volcanism, and a canyon being carved by melt-water from Crater Glacier. As an Earth Science teacher, I felt like a kid in a candy store. For more about this trip (many downloadable photos), CLICK HERE.

The photo was taken below the crater on the north slope of the volcano. Loowit Creek originates from melting snow and ice in the crater. Much of the water comes from Crater Glacier, which has formed and grown around the lava domes over the past several decades. Loowit Creek has cut down through the volcano, exposing the alternating layers that give composite volcanoes their name. Also known as stratovolcanoes, volcanoes like St. Helens consist of alternating layers of explosively erupted pyroclastics (ash, cinders, etc.) and lava flows.

Fascinating Geology of Ear Mountain in Montana

Above: Drone photo of my daughter and I on top Ear Mountain

The front of what?
Ear Mountain stands along the boundary between the mountains and the prairie 70 miles northwest of Great Falls (map); an area Montanans refer to as “The Front”. The name probably originates from the view enjoyed by those approaching from the east - That of a long wall marking the abrupt end of the great plains and the beginning of (front of) the mountains. The area, which extends northward to Glacier Park and beyond is known for its scenic beauty, grizzly bears, Chinook winds, and fascinating geology.

Sea creatures in Montana?
Ear Mountain along with many of its neighboring ridges, peaks, and cliffs consist of the Madison formation; layers of limestone and other carbonates, ranging from 275-520 meters thick, and made of sediment laid down 330-340 mya when much of the western USA was the floor of a shallow, tropical ocean (map). The Madison forms many of the state’s most iconic landforms, including the Gates of the Mountains, Mission Canyon, the Lewis and Clark Caverns, and the Bighorn Canyon. The Grand Canyon’s “Redwall limestone” and the limestone that surrounds the Black Hills of South Dakota are equivalent strata. In places the Madison is especially fossiliferous. Those adventurous enough to climb Ear Mountain will be treated to an abundance of horn coral, as they walk the perimeter of the plateau.

Not your ordinary faults.
Perhaps the most fascinating aspect the Ear Mountain and the surrounding region is the way the Madison formation and other layers were deformed as these mountains were built. In response to the collision of the Pacific Plate and the North American Plate 115-50 mya, immense slabs of rock broke, and then slid up and over younger layers to their west. Mountains formed by this over-thrust faulting extend from Helena northward through Glacier Park into Alberta (called the Lewis Thrust Belt). Although this provided some of the most scenic mountains in the state, the orientation of the slabs in the 60-mile stretch between Augusta and Heart Butte (map) is especially unique. In this area thinner slabs of rock slid eastward over younger rock layers like shingles on a roof to form distinct high ridges and deep valleys that run parallel to each other as shown in this photo.

Term: Mississippian Period

Related links:

Hiking on Top of Ear Mountain

More About the Thrust-Faulting Along "The Front"

More About the Madison Limestone

More About the Sevier Orogeny

The Madison Limestone in Montana