Date of this Version
Tree-ring analysis can be a valuable tool to date geomorphic events in regions lacking long historical records. In this study, the latest detectable movement of a section of a large landslide block in the Tower Falls area of Yellowstone National Park, Wyoming, is dated by tree-ring analysis of Douglas fir trees (Pseudotsuga menziesii var. glauca) damaged by the event. The movement tilted many of the trees and damaged their root systems. Thirteen old, tilted Douglas fir trees, at three sites, were sampled within the section of the landslide block that moved during the life of these trees. In addition, 10 young, upright, undisturbed Douglas firs were also sampled at the sites in order to establish a minimum age for the movement. The oldest of the 10 young, upright trees had an age of about 135 years, indicating that the latest movement of the landslide block occurred prior to 1865 A.D. The youngest of the 13 old, tilted trees dated to the early 1600s, providing a maximum age for this latest landslide movement. Analysis of the tree-ring record of the older, tilted Douglas firs revealed an abrupt reduction in annual-ring width beginning in 1694 A.D. As no other period in the tree-ring record between 1865 and 1600 A.D. revealed such an abrupt reduction in annual-ring width, the landslide movement is thought to have occurred sometime between the end of the 1693 A.D. growing season and before the end of the 1694 A.D. growing season. Because Yellowstone National Park is within the Intermountain seismic belt, a zone of pronounced seismic activity, movement of the landslide block may have been caused by an earthquake at that time.