Influence of rock falls, rock strength, and joint orientation on landscape in the Teton Range

Landslides, rock falls and other processes of mass wasting can significantly influence the shape of mountain landscapes. In addition to contributing to topographic evolution, rock falls can also pose hazards to climbers and visitors to areas where these events are common. Several rock fall events in Yosemite National Park have highlighted the importance of understanding rock fall processes (Yosemite National Park website).

The Teton Mountains in Grand Teton National Park, Wyoming, are also heavily influenced by rock falls. A hike into Garnet, Cascade, or Paintbrush Canyons takes you across fields of large boulders (Figure 1). These boulders were deposited when blocks of rock fell from steep and unstable walls above. Rock falls result from a number of processes. Large rainfall events may cause water to flow over surfaces and transport rock from its original location. Water filling cracks or joints in the rock may repeatedly freeze and thaw, wedging a block of rock away from the wall through the process of frost wedging. Shaking caused by earthquakes can cause unstable blocks of rock to fall from their position on a steep wall. The cone-shaped deposits of rock debris that accumulate on canyon floors are called talus fans (Figure 2). In the Teton Mountains these deposits have been accumulating debris since glaciers melted from the canyons beginning ~14,000 years ago (Licciardi et al., 2008).

The effects of rock falls on topography can be quantified in the Teton Range by measuring the volume of debris that has accumulated in talus fans since glaciers retreated. The volume and surface area of talus fans in Glacier Gulch, Garnet Canyon, and Avalanche Canyon were measured and mapped in the field (Figure 3). The surface area of exposed bedrock that contributed material to talus deposits was determined based on observations of aerial photographs, topographic maps, and features in the field (Figure 4). An erosion rate was calculated using the volume of talus, contributing bedrock area and time since glaciers retreated as shown in the following equations:

Material removed from walls (m) = talus volume (m³)/bedrock surface area (m²)

Erosion rate (m/yr) = material removed (m)/time since last glacial maximum (yr)

The average erosion rate for the three catchments shows that the walls and ridges are eroding at a rate of 1 x 10^-3 m/yr (1 mm/yr). The average talus fan volume is ~220,000 m³ and the average bedrock surface area above a talus fan is ~15,000 m².

In addition to understanding the impact of mass wasting on erosion rates in the mountain range, it is also possible to identify possible sources of weakness that can enhance the probability for a rock fall. Field observations and measurements on rock surfaces can provide rock strength values. One major influence on potential for rockfalls is the character of joints in the bedrock. Rock strength is based on the spacing, width, and orientation of joints. Joint spacing will influence the size of the debris that will fall from the wall. Larger boulders fall less frequently, require more force to be moved, and tend to occur when joints are widely spaced. Small spaces between joints create smaller particles which are easier to move and more readily displaced. Additionally, if the width of a joint is large enough for small sediment particles, soil, or other debris to accumulate in the cracks, they have an increased potential to break apart because the debris wedges between blocks of rock and pushes them away from each other. Another consideration is the presence of water in the joints. If water frequently flows through gaps in the rock, it will repeatedly freeze and thaw where temperatures are cool in high latitudes or altitudes. This freeze-thaw action pushes the blocks of rock apart until unstable blocks break away and fall to the catchment floor (Selby 1980).

In the Teton Range many joint systems potentially form weak surfaces in the bedrock which are susceptible to failure. Large scale joints with tens of meters spacing can be observed from a distance in the range and appear to contribute to the shape of some of the highest peaks, including the Grand Teton (Figure 5). Locally, smaller joints often dip into the walls of the catchment. The inward dipping surfaces may enhance the strength of walls and ridges by preventing blocks from slipping away from their source. Very few small joints allow water flow. Overall, the rock in the Teton Range is very strong and talus most likely forms as a result of the large scale planes of weakness.

Rock falls in the Teton Range effectively contribute to erosion of canyon walls since deglaciation. The orientation and spacing of joints has some control on where rock falls will occur. These surface features play a strong role in shaping the impressive landscape of the Teton Range.

• Licciardi, J. M., and Pierce, K. L., 2008, Cosmogenic exposure-age chronologies of Pinedale and Bull Lake glaciations in greater Yellowstone and the Teton Range, USA: Quaternary Science Reviews, v. 27, p. 814-831.

• Selby, M.J., 1980, A rock mass strength classification for geomorphic purposes: with tests from Antarctica and New Zealand: Zeitschrift fuer Geomorphologie, v. 24, no. 1, p. 31-51.

• Yosemite National Park Website on Rock Falls, http://www.nps.gov/yose/naturescience/rockfall.htm