Geologically isolated, natural grazing refuges that have never been grazed by domestic livestock can be foci of diversity for rare species. This study compared refuges protected from grazing by ravines to adjacent grazed sites in an uncommon grassland type in eastern Colorado. We also tested whether differences between refuge and grazed sites were due to protection from grazing versus abiotic conditions, based on temporary caging of little bluestem plants (Schizachyrium scoparium [Michaux] Nash). Regionally rare species were not exclusive to refuges, but occurred also on adjacent grazed sites. However, refuges showed greater species richness, as well as greater abundance of several tall stature species that are uncommon in the surrounding shortgrass steppe. Temporary protection of little bluestem plants in grazed sites resulted in significant changes in growth and reproductive output, in the direction of refuge plants. Though this geological refuge did not manifest an exclusive set of rare species as reported from other systems, it does preserve strong source populations of uncommon species that are sensitive to grazing and thereby contributes to regional grassland diversity.