Onlap of a rocky shoreline by marine beds of the Mississippian Windsor Group occurred at the southeastern margin of the composite upper Paleozoic Maritimes Basin in central Nova Scotia. The sedimentology of thin basal rudaceous deposits suggests that, although the transgression was rapid, there were times of prolonged shoreline stability. Boulder beach deposits indicate a high-energy coastline and a large marine basin. Clastic delta deposits were proximal to thick biohermal banks in shallow areas near the southeast margin of the sea while sulphate deposition was occurring basinward. According to their contained biota and stratigraphic relationships, the marginal banks evolved in a less saline environment than banks forming away from river inputs, on paleotopographic highs located farther offshore. Because diversity of the biota decreases with an increase in both water depth and distance from the paleoshore and because bank development in shallow or marginal areas was apparently synchronous with evaporitic deposition in deeper or less marginal areas, we conclude that both a vertical and a lateral gradient of salinity existed due to the thrusting of fluvial fresh water above salt water and its infiltration along the paleoshoreline. As salinity increased with time, bank building eventually aborted and sulphate deposition gave way to salt deposition in the deepest parts of the basin. Lastly, the onlap of evaporites above marginal clastic deposits indicates that evaporite deposition occurred in a transgressive context, although a gradual thinning of the water column may have simultaneously occurred due to basin infilling.