Two common kinds of shoestring sands are deposits of stream channels and of shoreline barriers. The shapes of transverse sections through shoestring sands of channel origin are thought to be flat on top and concave up at the base. By contrast, sections through shoestring sands of barrier origin are regarded as flat on the bottom and convex up at the top.
Fragmentary evidence from the Holocene shelf sediments off Long Island, New York, suggests that a shoestring sand of barrier origin is present, but that its transverse section is flat on top and concave up at the base. This evidence consists of two continuous seismic-reflection profiles and six cores of Holocene bottom sediments. The seismic profiles, made nearly perpendicular to the shore, display a lenticular sedimentary body 600 to 800 m wide and a maximum of 10 m thick. The shape of this body is flat on top and concave up at the base. The cores of Holocene sand collected seaward of the lens consist of sediment that we interpret as diagnostic shoreface storm deposits. The Holocene cores collected landward of the lens consist of materials containing diagnostic evidence of origin in a back-barrier area.
In the absence of more complete field data, it is impossible to present a unique interpretation of the geologic relations among the lens-shaped sediment body, the cores seaward of it, and the cores landward of it. One possibility is that they are all of different ages and the lens originated by subaerial erosion into exposed shelf sediments of a channel parallel with shore and that this channel later was filled by sediments. Another possibility, preferred by us, is that the lens and the belts of unlike sediment flanking it, all formed at the same time near a barrier that existed 7 km offshore when sea level was 24 m lower. If so, the lens best can be interpreted as the product of inlet migration parallel with shore.