Clinker, rock baked and fused by in situ burning of underlying coal, posesses a thermoremanent and/or thermochemical magnetization that resides in magnetite, hematite, and goethite. Data from individual sites imply that clinker may provide a high resolution recording of the geomagnetic field (e.g., within-site virtual geomagnetic pole angular standard deviations are usually less than 10¿), but this is dependent on its thermal history and the time over which magnetic phases formed. Data from 17 sites in the Powder River Basin (13 normal polarity, 4 reverse) yield a mean of Decl.=348¿, Incl.=62¿, &agr;95=5¿, k=42 and a corresponding pole position of 81¿ lat., 58¿ long, with a virtual geomagnetic pole angular standard deviation of 17.3¿. The mechanism of clinker formation, the wide geographic distribution, and the range of available fission-track ages suggest that clinker may contain a nearly complete geomagnetic field record for much of the Quaternary.