Natural degradation of earthworks, trenches, walls and moats, Northern Thailand

We here investigate the geometry, age, and history of several enigmatic northern Thailand earthwork entrenchments that are mostly located on hills and could not have held water to form moats. The earthworks are either oval or rectangular in plan view and typically encircle 0.3 to 1 sq km areas with no potsherd debris that would indicate former towns. Most trenches are 3-5 m deep with inner walls 4.5-8 m high. Some encircling earthworks are concentric double trenches spaced approximately 10 m apart. Historians have suggested these earthworks enclosed defensible areas where people in outlying villages sought refuge when under attack by neighboring rulers, the Chinese Ho, or the Burmese. We argue that some encircling entrenchments may have been for the capture or containment of elephants. Nearly all of the once near-vertical original walls have degraded to slopes of 32-47 degrees. Fitting calculated curves of the diffusion-based scarp-degradation model to our height-slope data, and assuming most scarps have degraded since the end of La Na Kingdom time A. D. 1558, we derive a diffusion coefficient of 0.002 m(2) y(-1). Slopes of the rectangular earthwork at Souvannkhomkham, Laos, across the Mekong River from Chiang Saen Noi, are significantly more degraded (approximately 32 degrees), indicating an age of 800-1200 years. Locations of these earthworks are established in hope that they will be preserved as part of the Thai and Lao archaeological legacy.