Bedrock basins in the Sierra Nevada, Alta California

A 360-km-long belt of more than 1,400 meter-sized granitic bedrock basins occurs at 1,200 to 2,500 m elevation on the west flank of the Sierra Nevada. The circular, smooth basins are 0.7 to 1.7 min diameter and are commonly 50 to 1,000 liters in volume. They are man-made as shown by their restricted size and elevation range, uniform circular shape, distinct basin shapes in different cultural areas, and the presence of bedrock mortars at 80 percent of the basin sites. Moreover, the juxtaposition of a northern cluster of basins to the vicinity of a rare salt spring suggests that these basins were constructed to evaporate salt. Several basins contain an A.O. 1350 volcanic ash, indicating that some existed before the end of the Medieval Climatic Anomaly (MCA; A.O. 800-1350). The basin belt was more productive in terms of food sources during the MCA, and it is postulated that warmer, drier conditions promoted the construction of cisterns to contain fresh water in order to prolong the time of occupation of mountain camps in late summer. Construction of the granitic basins required enormous energy and produced one of the largest and better preserved sets of Native Californian features.