Environmental programs at the Savannah River Site (SRS) located in South Carolina rely on an interdisciplinary team of scientists - geologists, engineers, chemists, mathematicians, and others. The solutions developed by the team are based on focused environmental characterization followed by selecting and deploying clean-up technologies that are matched to the problem. Geology plays a very important role in the process. The stratigraphy and hydrostratigraphy are essential in understanding the migration of contaminant plumes and the performance of cleanup technologies. The SRS is underlain by sediments of the Atlantic Coastal Plain ranging in age from Late Cretaceous to Miocene in age which rest unconformably on crystalline and sedimentary base rock. These coastal plain deposits consist of deltaic and near-shore marine to marine with variable amounts of fluvial influence. Subsequently the sediments exhibit abrupt facies changes within very short distances. Core samples are important in delineating where the transmissive (aquifer zones) and non-transmissive (aquitards) facies are located. Additionally, core samples can provide a baseline for other types of characterization tools e.g. cone penetrometer, geophysical logs, seismic data, etc. Cleanup at several of the SRS waste sites exemplifies both the interdisciplinary nature of the work, as well as the pivotal role of geology. Several examples of core will be presented to illustrate the importance of understanding the stratigraphy and hydrostratigraphy for environmental characterization and environmental clean-up at the Savannah River Site.