A major fault cutting through most of the crust can be identified and mapped on the Long Island platform using multichannel seismic reflection profiles and magnetic data. The fault, here called the Block Island fault (BIF), strikes north-northeast, dips westward at low angle, and does not resemble the thin-skinned thrust faulting observed in the foreland of the Appalachians. The BIF is located within the hinterland of the Appalachian mountain belt in the collision zone between Africa and North America. We present several interpretations but favor one in which the fault originated as an east-verging mid–late Paleozoic thrust fault, possibly related to the collision of Avalon or Meguma with North America. It was probably reactivated during early Mesozoic continental breakup and again in the Late Cretaceous and Tertiary, causing the steeply dipping postrift New Shoreham fault to form, either as an antithetic (normal) or splay (reverse) fault.