Aerial thermograms of an area in north-central Arizona immediately to the north of Merriam Crater have revealed the existence of parallel arrays of alternating ridge and swale linear features in the ashfall zone of Sunset Crater. The patterns are not easily identified on simultaneously acquired panchromatic photographs. Pollen and soil analyses confirm the highly geometric pattern to be a previously unrecognized prehistoric agricultural field. Recovery of Sinagua sherds of known age found at nearby living sites and in the field indicates that the farming activity occurred between A.D. 1065 and 1250. After 700 years of abandonment, local plant succession for the field has not yet fully re-established the probable former shrub community, apparently due to differences in physical and chemical properties existing between field and nonfield soil areas, related perhaps to prehistoric agricultural practices.