INTEGRATING THE DIRECT AND INDIRECT METHODS IN LICHENOMETRY FOR DATING BUDDHIST SACRED WALLS IN LANGTANG VALLEY, NEPAL HIMALAYA

Mani walls, Buddhist sacred walls composed of stones carved with Tibetan scriptures and images, are common throughout the Himalaya. Although standard tourist photographs, they have received little scholarly study. However, recent studies have argued that, based on the locations of the mani walls, they were constructed as landslide warnings. In 2009 and 2011, interviews were conducted with long-term residents and lay lamas in Langtang Valley, Nepal Himalaya, regarding the history, meaning and maintenance of the mani walls. The common tradition is that the mani walls were constructed when the valley was first settled 400-500 years ago. However, there was a wide range of opinions as to when, whether, why and how the mani walls are cleaned, especially of lichens. The time since last cleaning of each mani wall was determined by integrating the direct and indirect methods in lichenometry using the lichen Rhizocarpon geographicum. (The direct method is based on measured growth rates of individual lichens, while the indirect method is based on measuring lichen sizes on objects of known age.) The maximum lichen sizes on each of 21 mani walls between Langtang Village and Kyanjin Gompa were measured and photographed in May 2009 and then re-measured and re-photographed in May 2014. The indirect method was calibrated based on maximum sizes of lichens found on boulders on two glacial moraines and carved stones on one stupa (small temple) whose ages were provided by local residents. Additional calibration was provided by measurements on two debris flows that had been dated by Be-10 in another study. The direct method was calibrated by measuring 20 lichens (not found on mani walls) in both May 2009 and May 2014. The combination of five indirect calibration points (size as a function of time) and 20 direct calibration points (time derivative of size as a function of size) resulted in a robust calibration curve not overly influenced by a small number of non-representative lichens. Preliminary results show that, on average, the mani walls were last fully cleaned (20 ± 13) years ago. Moreover, the calibration curve has been used to show that landslides just upslope of mani walls are younger than the mani walls, which reinforces the argument that the mani walls were constructed as landslide warnings. Further results will be reported at the meeting.