Salt marshes are valuable yet fragile ecosystems, disappearing globally at an alarming rate. Facing this crisis, it becomes increasingly important to understand what forces drive their formation. Previous studies of marsh ontogeny relied on stratigraphy and physical monitoring, depending on inferences from multi-century and daily time scales, respectively. In this study, vertical accretion rates are evaluated at the same time resolution as a marsh’s lateral expansion, providing the first comprehensive view of a laterally expanding marsh’s sedimentary trajectory. 210Pb-derived (half-life, t1/2, of 22.3 yr) accretion rates are examined in a marsh at the Newport River (North Carolina, United States), a location experiencing ongoing emergence of new marshland over the past century. Accretion rates at all marsh sampling sites begin with slow sedimentation characteristic of the bay bottom, then shift to rapid, persistent sedimentation, eventually progressing from submerged mudflat to marsh table. Acceleration of vertical accretion occurs asynchronously across the marsh and prior to vegetative colonization, indicating a physical mechanism. We hypothesize that extant marsh tables act as promontories, effectively shielding adjacent mudflats from erosive forces, dictating the trajectory of marsh emergence, and yielding the pattern of alongshore marsh emergence at the Newport River.