Medium quartz sand derived from the Medina and Oriskany Sandstones was tumbled in the laboratory. These sands, as well as the fine sand and coarse silt produced by tumbling, were characterized using petrographic, SEM, Fourier, and oxygen isotopic composition analyses. The original Medina and Oriskany medium sand had delta 18 O values of 13.6 and 14.0 per mil, respectively. This sand was irregular in shape, and overgrowths with crystal faces were common. Tumbling medium Medina and Oriskany sand produced fine sand that was rounded and had lower delta 18 O values (12.3 and 12.8 per mil, respectively) and silt that was angular, commonly displayed crystal faces, and had higher delta 18 O values (14.3 and 14.9 per mil, respectively). These results indicate that the silt was produced by spalling quartz overgrowths. This process was more efficient for the Medina sand, in which "dust" coatings were well developed.