The Mission Mountains or Mission Range are a range of the Rocky Mountains located in northwestern Montana in the United States. They lie chiefly in Lake County and Missoula County and are south and east of Flathead Lake and west of the Swan Range. On the east side of the range is the Swan River Valley and on the west side the Mission Valley.
The highest point in the Mission Mountains is McDonald Peak at . The range is named for its proximity to the Jesuit St. Ignatius Mission, established in the mid-19th century in what is today St. Ignatius, Montana.
Geology
The Mission Mountains are composed largely of what is called "Belt Rock" from the Belt Supergroup. The sedimentary rocks in this group formed between 1.47 and 1.4 billion years ago in the Belt Basin. The roughly circular basin collected sediments from surrounding areas for millions of years. The basin was eventually buried and later re-exposed through the collision of several tectonic plates around 80 million years ago.
Much of the Belt Rock found in the Mission Mountains is a crumbly sedimentary rock known as mudstone. The mudstone in the Belt supergroup is often characterized by mudcracks, which points to it being formed while wet, drying, cracking, and then being repeatedly flooded with new wet material that also dried and cracked.
Most of the rock in the Mission Mountains hails from the end of the Proterozoic Eon, towards the end of what is called Precambrian time. Because they are so old, the only evidence of life in the rocks is algae blooms and very basic plant fossils. These organisms played, however, the important role of converting carbon dioxide in the water into oxygen that was pumped into the acidic and poorly oxygenated atmosphere.
The color of the mudstone in the Missions has much to do with the presence of the mineral hematite during its formation. Hematite is formed by iron particles' reaction to oxygen in the atmosphere. Green and gray stones found in the Missions were most likely formed in deep water, the red in more shallow water. Ripple marks can be found in much of the rock; they would have formed mostly in shallow water with gentle waves.
Uplift
The features of the Mission Mountains strongly reflect work of the last few ice ages, the latest of those being just over 10,000 years ago. But the range is also the product of a much longer story, one that can be started with the breakup of the Pangaea super-continent. As the continents began to spread out, the North American Plate inched westward, grinding over and against the Pacific Plate as it went. This subduction of the Pacific Plate caused the rise of the Rocky Mountains and thereby the Missions.
About 66 million years ago, this process of uplift began to slow. This time, called the Cenozoic Era, is when the area began to look much like it does today. Looking at the globe at that time, the continents would have been about where they are today and plant and animal life would be recognizable. At that time, however, the deep valleys of western Montana would not yet have formed.
This development is believed to have come about 40 million years ago as the extensional forces that caused the uplift of the Rockies began to cause the crust to thin and crack. Near-vertical faults formed almost uniformly throughout the region, most trending northwest to southeast. The blocks then broke up, some dropping and creating valleys like the Flathead and the Swan. In all, the whole process took around 100 million years.
Pleistocene glaciation
Three million years ago, at the end of the Cenozoic Era, western Montana would have been full of tall mountains, but it was the next geologic process that made them what they are today. Large glaciers began to form in the area 2–3 million years ago; this was the beginning of the Pleistocene Epoch. Since then, ending just 10,000 years ago, the Mission Mountains and their surroundings have been shaped by water.
The formation of the Flathead Lobe of the Alberta Cordilleran ice sheet is what set this history into motion. At its thickest points, the Flathead Lobe glacier may have extended 4,000 feet above the valley floor. The glacier reached hundreds of miles down the Rocky Mountain Trench, ending as far south as St. Ignatius, Montana. At the northern end of the range, the glacier flow split, in part flowing slowly into the Swan Valley.
A view of the area at that time would have been majestic, with large glaciers flowing around both sides and partially over the range. Smaller glaciers would have also flowed out of the mountains and joined the larger one in the valley. This explains the southward pointed, hook-shaped ridges at the end of each canyon in the range.