Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 208. Rock Creek Park is a special place for studying geology. Founded as the city park in 1890, Rock Creek Park serves as an oasis in the urban environment. Rock Creek Valley is a steep craggy Piedmont eroded zone where one can observe the bedrock, follow the evolution of the Fall Zone,and measure the topographically isolated Coastal Plain gravels. Figure 1 is the map of the stops to observe the general geology. The Rock Creek Valley is a textbook for research on the early Paleozoic history. Metasedimentary and igneous rocks reveal the marine and tectonic stories of a proto-Atlantic Ocean. Metamorphic mineralalogy is set in folds, faults, and fracture systems. Paleozoic tectonics is covered on the bluffs unconformable by gravels of the Mesozoic and Cenozoic. The Cretaceous ancestral Potomac sediments are also reworked into the lower elevation sediment deposits of late Cenozoic age (Miocene ? - Holocene). Figure 2 is a general geology map of the District. Rock Creek served as a field site for the 5th International Congress for Geologists in 1891. We will revisit some of those same stops almost a century later and follow in the footsteps of the early American pioneers: W. J. McGee, N. H. Darton, A. Keith, G. Williams, and B. Willis.