Gielstra, D., Resler, L.M., Butler, D.R., Cerveny, N.V., & Gielstra, J. # Welcome to Glacier National Park! Established in 1910 by President William Taft, the park exhibits unparalleled beauty in alpine glaciers, rich wildlife, and a depth of cultural heritage. This field trip explores the geodiversity of three of the glaciers within the park. Grinnell Glacier, named for one of the early advocates of the area who called this place the “Crown of the Continent”, Sperry Glacier, and Sexton Glacier. You will experience a variety of geomorphic factors and processes that help define the geodiversity of alpine glacial environments and discover the unique processes and features that make each glacier special. ## Land Acknowledgment Before beginning the field trip, the valuable cultural heritage of this region is shaped by the Native communities that still reside there. We acknowledge that the land on which our data were collected in Glacier National Park, Montana is the home of the Blackfeet, Salish, Pend d’ Oreille, and Kootenai people. We take this opportunity to thank the original caretakers of this land. ![](/api/images/small_PXL_20210714_155917564_af169f9790.jpg) [Enter VR of the Sexton Glacier vantage point from Baring Falls Hike.](/scenes/glacier-hike) Sexton Glacier is located at (48.7018° N, 113.6348° W) and is ranked lowest in geodiversity of the three glaciers (a weighted value of 65/100). In order to maximize the experience of this field trip, let’s begin with a terminology refresher and introduction to glacial geomorphology. A **glacier** is a moving body of ice that formed on land from snow accumulation that transforms into dense glacial ice over a number of years. The transformation of snow into ice in the relatively warmer environment of Montana involves ice minerals at a pressure melting point (Martini et al, 2001). Glaciers build in stages: fresh snow with more surface area and crystals, old snow that is more rounded and compacted, and firn. Firn, a German term, translates to “last year snow”. **Firn** crystals are more rounded, compacted, and denser. The glacier moves downslope because of its weight, which increases with annual layer accumulation of ice. Glaciers interact with the surrounding geology to create glacial geology and landforms - collectively known as glacial geomorphology. In times of glacial recession, or melting, another set of geomorphological processes and features are developed - which we will explore in the glacial basins of Grinnell, Sexton, and Sperry Glaciers. **Table 1: The criteria, indicators, and parameters used to assess the Glacier National Park, Montana glacier basins adapted from Brilha (2016, p.127).** ![](/api/images/small_Brilha_scientificvalue_geodiversity_8e0bb1d5c8.png) ## Geodiversity Geodiversity describes the various rocks, minerals, soil, and fossils that are found on the landscape as well as describes the processes that form those patterns. In order to explore the differences between glacial basins, we have employed the Brilha (2016) geodiversity index to measure the geodiversity of individual glacial basins in Glacier National Park, Montana. Specifically we evaluate the scientific value of the basins of Grinnell, Sexton, and Sperry glaciers. A major adjustment to the Brilha geodiversity index for this study includes incorporation of geomorphological diversity. The resultant scores highlight the differences between these glaciers and why those variations occur within this geomorphic setting. [Click to enter VR of Grinnell Glacier Front](/scenes/grinnell-glacier) ## Why do we care? Glaciers are a valuable feature tied to Glacier National Park, and these features are critical drivers in geodiversity. It is important to study glacial geomorphology over time as Glacier National Park is warming at two times the global average, which is accelerating the loss of glacial ice (National Park Service, n.d.). The park is predicted to lose its glaciers by 2030. Grinnell Glacier is located at (48.7524° N , 113.7285° W ) is one of the best-known glaciers in the United States and is an important feature in scientific research and recreation. It is estimated that the Grinnell Glacier loses approximately 2 to 3 acres annually, with over 100+ acres lost since 1966 (NASA Earth Observatory, 2015; NPS, n.d.). Among the three glaciers used for this field trip, Grinnell Glacier ranked second (with a weighted value of 70/100) for geodiversity using the Brilha (2016) geodiversity index. The complexity of Grinnell includes an extensive forefront which allows for glacial processes to act on the landscape promoting geomorphic diversity. For example, Grinnell Glacier has excellent examples of roche moutonnée, whereas Sexton Glacier has noticeably less development of this feature. The potential for geomorphic and **glacial hazards** at Grinnell are higher as a bulk of the glacier has melted, forming Upper Grinnell Lake. A **Glacier Lake Outburst Flood (GLOF)** could result from an icefall/rockfall. Should this type of mass wasting event be large enough to send a tsunami-like wave, it would degrade the Little Ice Age moraine that currently dams the lake on the downhill slope. The wave might travel down the valley impacting both ecological communities and park architecture or “parkitecture” of the Many Glacier Chalet Complex risking the historic structure of the famed Many Glacier Hotel. **Roche moutonnée** (ˈrȯsh-ˌmü-tə-ˈnā; French for “fleecy rock”), an elongate rounded ice-sculptured hillock of bedrock forms by both quarrying and abrasion processes (Merriam-Webster). As the glacial ice flows, it abrades the back face of the bedrock upstream of the bedrock slope (Martini et al., 2001). Embedded clasts on the stoss side (upslope) abrade the rock creating striations, and the downslope side, the lee side, becomes plucked as the glacier moves (Martini et al., 2001) (Photographs taken by David R. Butler). ![Grinnel labelled](/api/images/small_628_21837a98b8.jpg) ![](/api/images/small_roche_montenee_pope_fc6845cb15.png) ![](/api/images/small_617_8f460c48ca.jpg) Photograph showing striations from the abraded back side of the roche moutonnée (Photograph taken by David R. Butler). The upper, center of the photograph shows the beautiful blue color of the Siyeh sedimentary rock formation where a small area of the surface rock has been plucked out by the glacier. The buff and darker black-brown colors are from a **weathering rind** of iron skin rock coatings. Often when we are looking at a rock surface, we are looking at weathering at the air-rock interface. The natural color of the rock is actually quite different. The **sedimentary rocks** of the Belt Supergroup are ancient sedimentary layers that are 1.6 to 1.4 Ga with the Siyeh Formation deposited and lithified into limestone and dolomite 1.1 Ga ago. Sedimentary rocks are made up of bits and pieces of pre-existing rock that is weathered and eroded and then lithified into a unique layer. There is a notable magmatic diorite sill of igneous rock found just above Sexton Glacier and Grinnell Glacier and was formed in the Late Proterozoic by a deposited submarine lava flow. The sill is the dark layer of lava that intrudes between layers of limestone and is approximately 100 feet thick (30m). The presence of white metamorphic marble was caused by the heat of the magmatic intrusion recrystallizing the Siyeh limestone (National Park Service, n.d.) (Figure from National Park Service). ![](/api/images/diagram_2b90439c6e.jpg) **Ripple marks** are indicative of an ancient, shallow sea floor. The blue limestone weathers with a buff-colored rind as metals in the rocks oxidize at the air-rock interface (Photograph taken by Johan Gielstra). ![](/api/images/small_PXL_20210723_220052124_1ebcd0c912.jpg) ![](/api/images/small_PXL_20210723_215534203_aa0c912338.jpg) The layers of lithified sediments are evident in this image. **Mud cracks** are found in the same areas of Grinnell Glacier as the ripple marks (Photograph taken by Johan Gielstra). ![](/api/images/small_PXL_20210723_174352616_93fee36de5.jpg) Prehistoric microbial photosynthetic cyanobacteria mats, stromatolites, are fossils preserved in the sedimentary rock of the Helena (Siyeh) Formation (Photograph taken by Johan Gielstra). ![](/api/images/small_PXL_20210723_233750424_905303e0e4.jpg) Chatter marks, crescentic gouges form when the bedrock under the ice of the moving glacier fractures. The ice flow was from right to left. Used for scale, the hiking pole is set to 110 cm (Photograph taken by Dianna Gielstra). Please click the link to continue exploring a Lidar model of a roche moutonnée. The plucking from the front of the feature is evident. Striations and crescentic gouges are found on the abraded back of the feature. What other glacier features do you see? (Lidar Model captured by Dianna Gielstra) [LiDAR scan chatter marks Grinnell Glacier](https://sketchfab.com/3d-models/chatter-marks-striations-878ab71442614f728ff52e009a069d5e) ![](/api/images/small_PXL_20210723_210951584_dce911b5fd.jpg) **Bergschrunds** are crevasses located at the top of a mountain glacier adjacent to the head wall. The feature has a wedge shape caused by a gap created as the body of the glacier breaks away from the mountain side. Rock erosion beneath the bergschrund contributes to the process that forms the **cirque** (Martini et al., 2001) (Photograph taken by Johan Gielstra). [Click to enter VR of Sexton Glacier Front](/scenes/sexton-glacier-front) [Click to enter VR of Sexton Glacier Back](/scenes/sexton-glacier-back) Sexton Glacier ranked 3/3 (with a weighted value of 65/100) using the adapted Brilha (2016) geodiversity index examining scientific value. Sexton Glacier is a clear example of an alpine **cirque glacier** situated at the cliff-edge and with a shallow forefront. A cirque (French for circus) is a deep steep-walled basin formed by an alpine glacier, often circular or bowl-shaped. The less extensive area results in the development of fewer geomorphic features. The geodiversity of this site is lower than that at Grinnell and does not have a strong presence of roche moutonnée, and these examples are only partially formed (right of the image). There were fewer geodiversity features evident at this location (striations, ripple marks, stromatolites, etc.). ![](/api/images/small_IMG_0003_0d4cf2c7da.jpg) The steeper, cliff edge results in increased geomorphic hazard potential like rock fall (Butler, 1986) (Photograph taken by Dianna Gielstra). A rock fall can add darker colored material to the surface of the snow, changing the albedo from more reflectance of solar radiation to more absorption of solar radiation. The extra heat from thermal absorption can accelerate ice melt. ![](/api/images/small_8_4_01_sexton_glacier_moraine2_14be893ff2.jpg) The **firn line** is evident in this image and is tenuous in 2001. The firn line represents the transition between glacial ice and snow. The snow line is found above the firn line and in this image appears a brighter white in color. The shade from Matahpi Peak reduces melting where snow accumulates; however, the snow line transitions higher in the right side of the image where the southeast facing section of the glacier is more exposed. The **terminal moraines** are situated on the cliff edge. (Photograph taken by David R. Butler). Terminal moraines are composed of unsorted till. The image was taken while straightening a transect for sampling, and a meter stick is found in the foreground for scale. The image is oriented from the front of the glacier’s snowfield edge laterally across the terminal moraines toward the Siyeh Pass Trail (Photograph taken by Dianna Gielstra). ![](/api/images/small_PXL_20210712_221127817_bc658645f9.jpg) ![](/api/images/small_PXL_20210712_205335720_82669f6cc1.jpg) The unsorted till reflects the **dolomite** and limestone rocks of the Helena Formation. Material from the **diorite sill** is evident in the foreground, which is a darker rock with approximately two-thirds **plagioclase feldspar** (lighter color) (Photographs taken by Dianna Gielstra). [Enter the Sperry Glacier video](https://youtu.be/G2hTZWkWvA8) ![](/api/images/small_IMG_1205_370b13ae6f.JPG) Sperry Glacier is located at 48.6231° N, 113.7622° W and has a higher geodiversity index value than both Sexton Glacier and Grinnell Glacier (71.25 weighted score) due to its extensive **forefront** that allows for ample opportunity for glaciers to work the landscape geomorphically. Enter VR Sperry Glacier Back The area has thinly bedded **argillite** and massively bedded Siyeh limestone. [Lidar scan of geology near Sperry Glacier](https://sketchfab.com/3d-models/layering-sperry-a0a68a2e36e747ba831a3e922f0f1dbd) ![](/api/images/small_IMG_1211_8b3964964a.JPG) The combination of rock types (lithology) at Sperry Glacier creates a different kind of plucking and abrasion pattern than found at Grinnell Glacier, resulting in a more complex, undulating terrain (Photograph taken by Lynn Resler).
![](/api/images/small_Sperry_b10b4e3d2b.jpg)
Striations are also evident as found on these argillite layers. Ripple marks indicate the presence of shallow seas from the Late Proterozoic during the Precambrian age recorded in the layers (Photographs taken by Lynn Resler). [Lidar scan of striations on Sperry Glacier area geology](https://sketchfab.com/3d-models/sperry-striations-becd2dc57c224be692be6c957b88b0fc) [Lidar scan of ripple marks on Sperry Glacier area geology](https://sketchfab.com/3d-models/ripple-marks-sperry-glacier-final-e18af11ed69e4a94b1226864eb5342c5) ![](/api/images/small_IMG_1233_4389a8b910.JPG)
![](/api/images/small_IMG_1234_f1636410e0.JPG) [Click to Enter VR of Sperry Glacier Back](/scenes/sperry-glacier-back-view) [Click to Enter VR of Sperry Glacier Lateral](/scenes/sperry-glacier-lateral-view) [Click to Enter VR of Sperry Glacier Front](/scenes/sperry-glacier-forefront) **References** Brilha, J. (2016). Inventory and Quantitative Assessment of Geosites and Geodiversity Sites: A Review. Geoheritage, 8:119–134. DOI 10.1007/s12371-014-0139-3 Butler, D.R. (1986). Snow-avalanche hazards in Glacier National Park, Montana: meteorologic and climatologic aspects. Physical Geography, 7(1): 72-87. Martini, P.I., Brookfield, M.E., & Sadura, S. (2001). Principles of Glacial Geomorphology and Geology. Prentice Hall. NASA Earth Observatory (2004, June 22) Sizing Up the Earth’s Glaciers. https://earthobservatory.nasa.gov/features/GLIMS National Park Service. (n.d.).Climate change-Glacier National Park.https://www.nps.gov/glac/learn/nature/climate-change.htm ## GLOSSARY **Argillite** Sedimentary rock made of consolidated clay. **Bergschrund** A crevasse located at the top of a mountain glacier adjacent to the head wall. **Chatter Marks** Crescentic gouges of fractured material removed from the bedrock because of rock fragments dragged under the ice of the moving glacier. **Cirque** A deep, steep-walled basin formed by an alpine glacier, often circular or bowl-shaped **Diorite Sill** Intrusion of magmatic rock between overlying rock layers **Dolomite** Sedimentary rock called dolostone. This is a highly absorbent rock that is formed as water evaporates and minerals recrystallize from solution. Firn, a German term, translates to “last year snow”, are more rounded ice crystals, compacted, and denser. **Firn Line** Represents the transition between glacial ice and snow. **Glacier** is a moving body of ice that formed on land from snow accumulation that transforms into dense glacial ice over a number of years. **Glacial Forefront** The leading position on the landscape of glacier movement. **Glacial Hazards** Hazards like mass wasting (rock fall), glacial flooding, etc. associated with glacier earth system processes. **Glacial Till** Rock debris formed from the process of plucking and grinding of the bedrock beneath a moving glacier **Glacier Lake Outburst Flood (GLOF)** Flooding from a glacial lake created by melted glacial ice and associated with a breach of a glacier moraine. **Head Wall** The steep cliff directly above a glacial cirque valley floor. **Igneous (Magmatic) Rock** A type of rock formed by cooling magma/lava, includes Diorite sill. **Lithology** The study of rock types. **Lee Side** The downslope side **Moraine** Accumulation of glacial sediment and rock moved and deposited by glaciers. **Mud Cracks** Indicator of desiccation in ancient mud layers that cause soils to crack and curl, material deposits in the cracks and are then transformed (lithified) into stone. **Ripple Marks** The structure of a rock that is an indicator of ancient seas. **Plagioclase Feldspar** Feldspar minerals that are light colored, translucent, and have a glassy appearance. These minerals tend to be translucent/transparent and are somewhat brittle. **Roche moutonnée** (ˈrȯsh-ˌmü-tə-ˈnā; French for “fleecy rock”), an elongate rounded ice-sculptured hillock of bedrock forms by both quarrying and abrasion processes. **Sandstone** Quartz and sand grains bound together to form rock. **Sedimentary Rock** Layer accumulations of pre-existing materials. The Siyeh Limestone is a chemical sedimentary rock found in thick, massive layers of lithified sediments (used in image); includes sandstone and dolomite. **Stoss Side** The upslope side Glacial Striations Linear abrasions of the rock caused by the movement of rocks trapped in the underside of glacial ice when glaciers move over bedrock. **Stromatolites** Prehistoric microbial cyanobacteria mats that secreted lime and are fossilized. Terminal Moraine Composed of unsorted glacial till found at the glacier forefront at the maximum extent of the glacier. **Weathering Rind** The immediate surface of the rock found at the air-rock interface in which weathering is occurring. This surface may grow rock coatings which can help protect the underlying rock. Rock coatings can also help to destroy the surface.