A thermal infrared aerial image revealed evidence of human alteration of Sunset Crater basaltic ash over a 2 sq km area in north-central Arizona. Field investigation confirmed the presence of 19 linear ash ridges, reaching areal dimensions of 20 x 150 m, and 25 circular ash mounds ranging from 10 to 24 m in diameter. Independent lines of evidence (archaeological, chemical, and palynological) support the interpretation of these features as previously-unrecognized prehistoric agricultural plots. Associated ceramics place the use of these fields sometime between A.C. 1150 and A.C. 1250, a time when local Sinagua populations experienced significant changes in their social and natural environment. Pollen remains suggest that the fields were most likely used for growing Chenopodium or Amaranthus---annual herbs with edible greens and seeds---rather than corn and squash. this unusual ridge-and-mound field system provides another example of the extent to which the Sinagua invested labor in agricultural production during the later half of the 12th century and first half of the 13th. This study provides a clear example of how remote sensing benefits archaeology, and should alert archaeologists to the possibility of finding similar agricultural features in other semi-arid regions of the globe covered with basaltic ash.