In April, 1991, a northwest-southeast trending 120-km-long seismic refraction/wide-angle reflection profile was recorded across the Atlantic Coastal Plain of South Carolina, the passive margin of the eastern United States formed by Mesozoic extension during the opening of the Atlantic and Gulf of Mexico. Two-dimensional ray tracing of first arrivals and reflections indicates large lateral velocity variations in the upper 5 km of the crust. From northwest to southeast along the profile, Coastal Plain sediments thicken from a few tens of meters to more than 1 km. P-wave velocities within the sediments range from 1.85 to 3.5 km s−1, while intercalated basalts have velocities of 5.2–5.5 km s−1. The top of the crystalline basement dips eastward and is characterized by velocities of 6.0–6.2 km s−1. High velocities of 6.2 km s−1 within the crystalline basement are locally restricted to a shallow 25-km-wide zone adjacent and east of the Dunbarton basin. Seismic, gravity and magnetic observations suggest that this anomaly represents a pre-Cretaceous mafic intrusion formed during Mesozoic rifting. Mesozoic rifting is also evident from observed eastward thinning of the crust from 37 to 32 km along the profile.