Facts: Area is 1584 square miles; established as a National Park in 1910; Contiguous with Waterton Lakes National Park, Alberta; Contains seven peaks above 10,000 feet, the tallest is Mount Cleveland (10,466 ft/3,190 m).

Description: Rugged landscape sculpted by alpine glaciers from the amazingly well-preserved Precambrian strata of the Belt Supergroup.  The park has over 700 miles of trails that are enjoyed by hikers and climbers. (Take a Virtual Tour) Fishing in the Parks deep lakes is also a popular sport.

Key Words and Themes

Introduction

Figure 2. A typical park landscape is illustrated here in this view looking east from Bishops Cap.  Note that the layered rocks of the Belt Supergroup are gently inclined.  Most strata shown here are ±1.45 billion-year-old argillites, limestones and dolostones.   Argillites are shales recrystallized  by low-grade burial metamorphism.  They are stronger and less likely to split apart.  The deep broad valleys  and sharpened landscape are the product of alpine glaciation.  Photo by Douglas Fowler, from Harris, Tuttle and Tuttle CD (2004).

Structure of Waterton-Glacier National Park

The Akamina Syncline (figs. 3-5) forms the backbone of Glacier National Park. The Livingston and Lewis ranges are parallel belts on the west and east side of the park (fig. 4 rollover).  They occupy the upturned edges of the eroded doubly plunging Akamina Syncline. Flattop mountain and Reynolds mountain rest in the axis of the syncline, where strata are nearly horizontal. The entire synclinal structure lies on the back of Lewis Thrust Fault. This major fault is responsible for transporting the Precambrian rocks of the Belt Supergroup 40-50 miles eastward, and placing them on top of younger Cretaceous strata during the Sevier-Laramide Orogeny.

Figure 3.  Thin-skin deformation resulting from Sevier-Laramide compression. Compressional thickening of the section is caused by imbrication of sedimentary layers along low-angle thrust faults.  The Precambrian Belt Supergroup is thrust over younger Mesozoic rocks.  The Lewis Thrust Fault is the principle thrust beneath Waterton-Glacier National Park.  Red rectangle marks location of cross-section in figure 5.  (Image source unknown. Please notify me if you know the source of the image.)

Imagine how baffled early geologists must have been when they encountered Cretaceous rocks overlain by rocks that predated them by a billion years.  This hardly conformed to the law of superposition!  To add to the confusion there is very little deformation along the fault.  High fluid pressure allowed the rocks to glided along a very narrow shear zone.  (Read about thrusts and the controversy they engendered from Talk Origins.)  The Flathead Fault (fig. 4 rollover) is a young normal fault that truncates the western limb of the Akamina Syncline. As are most of the normal faults throughout the western U.S., the Flathead Fault is a manifestation of the on-going regional uplift and extension that is tearing apart the Basin and Range, splintering the northwest margin of the Colorado Plateau (High Plateaus Region), and  rejuvenating the Southern Rocky Mountain uplifts.

Figure. 4a.  Geologic Structures of Glacier National Park (rollover).  The bright white patches  on the peaks around Flattop are the remaining active glacier.

Figure 4b. Interactive Google Map of Glacier National Park. Click to view in a larger map and explore photos an videos by choosing more in the menu bar.

Figure 5.  Shown here is the Akamina Syncline, which forms the backbone of Glacier National Park.  The syncline is piggybacked onto the Lewis Thrust. Note that the Precambrian Belt Group has been transported over younger Cretaceous rocks. The Flathead (normal) Fault underlie Lake McDonald on the far left of the diagram. Modified from 26.7 of Harris and Tuttle (Adapted from O.B. Raup et al., 1983)  Cross section is along the Going-to the-Sun road, south of Flathead Mountain.

Belt Supergroup

Mid-Proterozoic rocks are exposed in Waterton-Glacier National Park, just as they are in the gorge of the Grand Canyon,  the Black Mountains above Death Valley, and the peaks of Rocky Mountain National Park.   However, what makes these rocks stand out is their extremely low grade of metamorphism and deformation.  Unlike other the Precambrian sections, these rocks have not been intensely deformed, cooked, or recrystallized by earlier plate collisions.  They have been hardened and slightly recrystallized by deep burial, but that is all.  Therefore the Mid-Proterozoic rocks of the Belt Supergroup are some of the best preserved sedimentary rocks of that age in the world!   Deposited in a rift basin and passive margin(?) these rocks contain limestones, argillites, dolostones, and quartzites with remarkably preserved sedimentary features. Because they were deposited before the evolution of complex plant and animal life they record processes undisturbed by bioturbation, and erosion uninhibited by land plants.  The ancient lifeforms, which are well-preserved here, are blue-green algae (cyanobacteria)--single-cell, photosynthetic primitive organisms.  The algae formed mats (filamentous colonies) that trapped carbonate mud and built cabbage-shaped columns of rock know as stromatolites. Beautifully preserved stromatolites are found in the 1.45 billion-year-old Altyn Formation and in the Appekunny and Siyeh formations. Contemporary versions of these primitive colonies can still be found today in western Australia and the Gulf of California. (Read also Archaebacteria: A Life Form On Mars? to learn more about these ancient lifeforms and their significance.)

Glaciers

Extending east and west from the upturned limbs of the syncline are glacially carved troughs, some are filled with water forming finger-like lakes, such as St. Mary's Lake and McDonald Lake, several miles long. Many of the lakes are dammed by broad, looping end moraines left behind by valley glaciers long since retreated. A few small cirque and bench glaciers still occupy the high basins and ledges around Flattop Mountain (fig. 4).  

Presently only 27 named glaciers dot the upper elevations of Glacier National Park, and all of these are less the 1.5  square miles in area.  The large glaciers that carved the U-shaped valleys and deposited the the looping moraines  along the mountain front were formed during the Pleistocene Epoch (1.8 ma -11ka).  Pleistocene glaciers reached their maximum around 20,000 years ago and were melted by 10,000 years ago.   The present glaciers are relicts of those formed during the Little Ice Age (Neoglacial period), a renewed cooling event that started around 6000 years ago and ended with the advance of the Industrial Revolution.  These glaciers were much larger in the 1800's when around 140 glaciers existed, but are now retreating in response to global warming.   Estimates from climate models indicate that the remaining glaciers will be gone by 2025. (View Glacier and Climate Change podcast.) In 1850 the Grinnell and Salamander Glaciers (fig.  7) were part of a much larger glacier that has retreated and split into to two much smaller glacial bodies.  (To learn more about glacial landforms at Glacier National Park go to Karen Lemke's Alpine Glacial Landforms.)

Unique Geologic Features

Chief Mountain, Montana

Chief Mountain, Montana (fig. 6) is the most noted example of a klippe, an isolated outlier of a thrust sheet.  This stranded block of Precambrian rock sits above younger Cretaceous gray shales.  The surrounding portion of the thrust sheet has be removed by erosion leaving behind this isolated block of Proterozoic rock.  In contrast, a structural window is an isolated region in a largely intact thrust sheet where erosion has exposed the underlying younger rock. These are more common in the Appalachians. (See Thrust sheets within the Grandfather Mountain Window, Southern Appalachians, by Bowling and Winberry)

Figure 6. Chief Mountain is an isolated outlier (klippe) of the Lewis Thrust sheet.   The Proterozoic rocks of the Belt Supergroup were transported approximately 50 miles eastward along the fault, and placed on top of younger Cretaceous rocks.  The fault trace lies at the base of the cliffs where the sturdy Altyn Formation of the Belt Supergroup rests on weak Cretaceous shales.   The Altyn Formation is composed of limestone and dolostone. (Image source: National Parks Service)

Purcell Sill and Volcanics

The Purcell sill (fig. 7) is a 130-300 ft-thick intrusion injected between layers of  limestone in the Helena Formation.  From a distance it appears as though a black Sanford Sharpie, which bled white, was drawn across the landscape.  Its dioritic composition is intermediate between silicic and mafic and its texture is finest (diabasic) along the contacts, and coarser (dioritic) towards the center where cooling was slower.  The limestone above and below the sill was metamorphosed to marble by heat emanating from the sill. In this case, metamorphism involved removal of organic material and recrystallization.  Metamorphism caused by intrusions that bake the rock around them is called contact metamorphism.  The region of affected rock next to the intrusion is called the contact aureole.  As you can see, the bleached marble zone does not extent much beyond the dike into the surrounding Helena Formation.  In some areas the dioritic magma broke through the surface forming volcanic deposits.  The Purcell lava flows form a 50-253-ft-thick marker horizon between the Snowslip and Shepard Formations.  Volcanic lavas and tuffs are significant because they can be radiometrically dated, thus enabling geologists to obtain absolute ages on the sections that contain them.  The Purcell volcanics have been dated at around 1.45 ga (Evans and others, 2000).

Figure 7. Grinnell Glacier and 50-ft thick Purcell sill.  In 1850 the Grinnell and Salamander Glacier were joined in a single much larger glacier.  The diorite sill and related volcanics give absolute ages around 1.45 ga. Photo by Robert K. Smith, from Harris, Tuttle and Tuttle CD (2004).

Going-to-the-Sun Road

The 53-mile Going-to-the-Sun-Road (flg. 8) takes travelers from St. Mary Lake (East) across the continental divide through Logan Pass to Lake McDonald (west).  The road is both a National Historic Landmark and Historic Civil Engineering Landmark.  Take the etour and learn about construction of the road. (Go to the bottom of the page and click PowerTour)

Figure 8. Going-to-the-Sun-Road is a feat of 20th century engineering and it winds it way along nearly vertical walls carved by alpine glaciers and across the continental divide.

Continental Divide and Triple Divide Peak

The Rockies form the Continental Divide, which separates drainages flowing east from those that flow west. Triple Divide Peak, located in Glacier National Park, is the unique point on the continent where water is split into the drainages of the Atlantic, Pacific and Arctic ocean.

References and additional sites to explore

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