Initial Publication Date: April 10, 2018

Landscapes of the Rocky Mountain Front

Author: Mariah Cannon, Department of Earth Sciences, Montana State University

"We came up over the crest and were walled to a stop. The western skyline before us was filled high with a steel-blue army of mountains, drawn in battalions of peaks and reefs and gorges and crags as far along the entire rim of the earth as could be seen. Summit after summit bladed up thousands of feet as if charging into the air to strike first at storm and lightning, valleys and clefts chasmed wide as if split and hollowed by thunderblast upon thunderblast." Ivan Doig, This House of Sky


General Overview of Surficial Geology

From plains to rugged mountains, Montana displays the greatest variations in landforms. Many of these structures can be seen in even a small area. The Augusta/Choteau area sits on the front range of the great Rocky Mountains, meeting with wide open plains. Many different processes lent their hand in creating this extraordinary landscape. This wasn't an overnight transformation, these processes took millions of years to create the structures and features we see today. Weathering and erosion have taken place and has worn down much of the land from its original state. Humans have also added their fair share of transformation of the landscape. By damming rivers, the discharge is much lower, and reservoirs form (i.e. Gibson Reservoir). This dramatically changes the way the land will continue to form in the future. Who knows what the land would have looked like if it had remained untouched?

Geomorphology

Geomorphology is the study of the way the land is sculpted in relation to underlying geologic structures. It views the current landforms and explains the way they were formed. Scottish geologist Charles Lyell is famous for his idea that 'the present is the key to the past.' This idea is a simplified version of Uniformitarianism, which is the idea that geologic processes happening today, are essentially the same geologic processes that formed the Earth and its landscapes as we know it. Here are some simple examples of geomorphology found in the area:

  • Most of the outstanding landscapes found on the Rocky Mountain Front are due to the intense history of deformation and uplift (see structural geology in the area.
  • The Sun River has incised through the many ridges formed by the numerous thrust faults in the area, causing the typical 'V-shaped' fluvial valley.
  • More eastward are the plains of Montana, which contain many clues to the glacial history of the area.
    • Continue below for more information on the glacial features found in the area!

Glacial Features and Geomorphology in the Area--What to Look For

Erosional Glacial Features

  • U-shaped valleys v. V-shaped valleys
    • U-shaped valleys are carved by glaciers whereas V-shaped valleys indicate erosion by fluvial river processes.
  • Glacial striations
    • Where the ice mass passed walls of rock with great friction and carves transverse scratches/gouges into the rock surface.
  • Glacial polish
    • After glacial ice has passed over a surface for a considerable amount of time, the amount of striations will accumulate and form a shiny, smooth surface.
  • Aretes and Horns
    • When glaciers carve the sides of peaks and form sharp jagged ridges. Aretes form when two sides are eroded, and horns form when 3 sides are eroded.

Depositional Glacial Features

  • Glacial erratics
    • Massive rocks that are not native to the area have been transported due to glacial movement
  • Glacial till
    • Material made up of unsorted sediments that have been transported by glacial movement and has been deposited by melting of receding glaciers. This material varies in size from clay sized particles all the way up to boulder sized erratics.
  • Moraines
    • Formed when glacial till is deposited in landforms in front of or adjacent to the ice mass. When the rate of melting equals the rate of downhill flow of the glacier, the depositional process acts as a conveyer belt dumping the till in the land forms around the glacier. Lateral moraines form along the sides of moving glaciers while terminal moraines form at the toe of a glacier, and a recessional moraine forms as the glacier is retreating and indicates period of "standstill" of the ice.
  • Glacial Outwash
    • Glaciers generate tremendous volumes of melt water. Streams that derive from a melting glacier erode, transport and re-deposit sediments in river deposits (e.g., channels, bars). As the melt water migrates, it creates an outwash plain. Melt waters can carry any size of sediment, depending on the amount of discharge. Glacial erratics can be moved in this way.


Glacial Story

  • Cordilleran Ice Sheet
    • The Cordilleran ice sheet was a massive continental glacier that covered the northwestern corner of North America, extending into the western edge of Montana. The world's last period of maximum glacial extent was during the Pleistocene epoch, which started approximately 2 million years ago and ended roughly 10,000 years ago. Over its lifespan, the Cordilleran ice sheet made multiple advances and retreats over the land. Eventually, the last glacial maximum came to an end around 12,500 years ago. Along the way, there were multiple glaciations that took place. Two that are of interest are the Bull Lake glaciation (~200,000 to 130,000 years before present) and the Pinedale glaciation (~30,000-10,000 years before present). There is evidence all around the area telling the tale of these two glacial periods. So be on the lookout when driving through the area!
      • Bull Lake Glaciation
        • Evidence suggests that Bull Lake mountain ice extended as far east as the little town of Choteau, MT where it met the Bull Lake continental ice, moving west. Mixed moraine materials, along with erratics, near Choteau suggest the location of meeting was here.
      • Pinedale Glaciation
        • There is clear evidence for this glaciation, which took place after the Bull Lake period. An eighty foot moraine stands tall over the flat plain that was formed from the Pinedale glacial outwash. This moraine marks the extent of the mountainous ice from the Pinedale period.
    • In the image up and to the right, a lobe of the Cordilleran ice sheet came down and covered northwestern Montana. Our area of interest falls under the smaller lobe called Sun River glacier.
  • Where are they now?
    • While the ice sheet has been long gone, you can still find glaciers throughout the area. If a glacier is no longer mobile, its title changes to snow field. While they are much smaller, and only continuing to decrease in size, you can find lots of evidence of Montana's glacial history by looking at the landscapes.

Heading North? Check Out Glacier National Park

  • The amazing landscapes of the Rocky Mountain front continue up into Glacier National Park, which boasts unbelievable views of glacial geomorphology, great structural geology, and of course glaciers!
  • Tons of hiking trails ranging in intensity from easy day hikes, to much longer multiple day hikes. No matter the length, they all have amazing and rewarding views at the end.