New radiocarbon-dated cores obtained by Vibracorers in the western Gulf of Maine confirm that a short-lived, relative sea-level lowstand of ∼−55 m occurred at 11–10.5 ka. These cores and younger salt-marsh data also reveal that rates of transgression varied throughout the Holocene, probably due to local variations in glacial isostasy. The isostatic component is resolved by subtracting published approximations of eustatic sea level from our well-determined observations of local relative sea level. A large peak in the isostatic curve coincides with the lowstand and is interpreted as a forebulge 20–25 m in amplitude. Forebulge migration is estimated at 7–11 km/100 yr, based on the timing of lowstands across the region.