Dating Stone Alignments by Luminescence

Abstract

AbstractStone alignments, including tipi rings and drive lines, are abundant on the northern Plains and adjacent Rocky Mountains, but they have been notoriously difficult to date. This paper applies luminescence dating to sediments directly underneath the rocks to estimate the age of placement of the rock. This is based on the assumption that before the rock was emplaced, turbation processes brought sufficient grains to the surface, where sunlight reset the signal. Single-grain dating of potassium feldspars allowed isolation of these original well-bleached grains, which by now have built up a signal because the rock prevents transfer to the surface. Plotting the number of original well-bleached grains with depth showed that these grains were concentrated just under the rock and decreased with depth. This is what would be predicted if the assumption is true. Dates were derived from several samples from Kutoyis in north central Montana, from Whitewater in eastern Montana, and from several sites in northwestern Wyoming. Many samples from Kutoyis and Wyoming dated to the last 600 years, but some samples from both places were more than 2,000 years old. The Whitewater features also dated to around 2,000 years ago. The ages are consistent with the cultural history of the areas.