In Cumberland County, on the southeast coast of Maine, two spits connect the mainland to two rock islands. A narrow channel separates the two headlands, kept open by strong currents generated as the bay enclosed by the double tombolo is flooded and drained by 2.5-m tides.

A shallow-marine (Presumpscot Formation) clay was deposited during postglacial time. Following this, crustal rebound in early Holocene time raised the area 65 m above sea level. With later (~5,000 years B.P.) subsidence and continued eustatic sea-level rise, the two spits prograded over the marine clay, forming tombolos to the offshore islands. With further rise of sea level, the two tombolos began migrating toward each other at a rate of at least 500 m in the past 1,000 to 2,000 years, leaving relict marsh exposed on the present beach face.

Subsurface studies of the resulting stratigraphy show five distinct environments: (1) nearshore, intertidal, washover and dune sands of the barriers; (2) tidal-pool muds, deposited in the relative quiet of the bay, currently being transgressed by the barriers; (3) a flood-tidal delta of organic-rich sands and muds; (4) tidal-channel sands; and (5) back-barrier marsh.

With the high energy conditions generated by northeast and southeast storms, and continuing stability of relative sea level, the spits will continue their migration until the bay is completely infilled, resulting in a highly complex stratigraphy created by the "merging" of two tombolos.

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