The southern, subsurface boundary of the crustal section typical of the Grenville Orogen can be traced from the St. Lawrence Estuary to western Newfoundland by means of a prominent gravity and magnetic gradient. North of the boundary, the gravity and magnetic data suggest that the upper surface of the Precambrian basement is irregular and is overlain by Paleozoic sedimentary rocks of the St. Lawrence Platform which outcrop in a regular undisturbed pattern on Ile d'Anticosti. The areas known to have a thick Paleozoic section, such as Ile d'Anticosti and offshore from Bay of Islands, Newfoundland, coincide with areas of relatively low gravity. East of Ile d'Anticosti, however, an area with similar low gravity values and bounded by steep magnetic gradients was previously thought to be underlain by a thin Paleozoic section on the basis of seismic refraction data obtained from nearby areas. If the Paleozoic section is indeed thin, the gravity and magnetic anomalies can be explained by the presence of an anorthosite intrusion with a minimum thickness of 4 km within the Precambrian basement. However, a more tenable interpretation, which is consistent with the known sedimentary thicknesses elsewhere in the Gulf and which does not contravene the refraction data, is that the Paleozoic section thickens locally to at least 5 km. This thick section is postulated to occur over a downfaulted portion of a salient on the early Paleozoic margin of North America.