Grain size distributions in eolian (wind-blown) deposits encode information about the atmospheric conditions that enabled their transport and deposition, and grain shape encodes information about provenance and postdepositional processes. Gale crater is a ~3.7 Ga impact crater with a vast diversity of exposed sedimentary strata indicative of a varied depositional history during the ear ly Hesperian epochs (Banham et al., 2018). The Stimson sandstone is a ~3 Ga unit of eolian sandstone in Gale crater that appears to have a coarser grain size distribution than the nearby Bagnold Dunes, an active dune field (Banham et al., 2018; Weitz et al., 2018). In this work, I hypothesize that the Martian paleoatmosphere had a different density than Mars’ current atmosphere, and that the source of the Stimson sandstone may have been more coarse than the source of the Bagnold dunes, leading to ancient sandstones with different grain size distributions than modern active sand dunes. To approach this question, I analyze images from the Mars Science Laboratory (MSL) Curiosity’s Mars Hand Lens Imager (MAHLI) and Remote Micro Imager cameras to determine the grain size distribution of targets. Additionally, I qualitatively describe grain shape and appearance, including roundedness, sphericity, pitting, and color, along with chemical composition where possible, to determine whether differences in grain size distributions can be attributed to provenance. From these results, I explore the role of paleoclimate and provenance in generating the grain size distribution seen in the Stimson. These results will help determine the provenance of the Stimson, as well as ancient Martian atmospheric conditions, in turn providing insights into the wet-to-dry transition and, potentially, the habitability of ancient Mars.