Preliminary seismic reflection profiling of Bermuda's lagoons, using the Uniboom system, followed by reconnaissance drilling, has shown that the lowest horizon on the seismic profiles is a strongly reflecting layer, almost horizontal, and with surface roughness of 1 to 3 m. It lies at a depth of about 19 m below sea level near the center of the platform and slopes very gently to 32 m beneath the rim. It appears to be the foundation upon which the rim, reefs, and lagoons have developed, culminating in the present configuration.

Above this surface there are Pleistocene reefs, thick

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sand bodies, and large expanses of lagoonal sediment. The reefs have only a few meters of relief and are commonly draped with lagoonal sediment of unknown age relative to the reefs. The sand bodies are especially common adjacent to the northwest rim and in the southwest lagoon. Drilling has revealed one marine sand body in the northwest, and another, possibly eolian, in the southwest. Lagoonal deposits cover large areas between reefs and sand bodies, and in the northeast, almost the entire area inside the rim.

Recent reefs are founded on a variety of substrates. Close to shore they locally fringe the Pleistocene ridges. Over Pleistocene sand bodies in the northwest and southwest, there are prominent reef and sediment shoals. On the rim, reefs encrust whatever Pleistocene rim lies below. However, many lagoon reefs are founded not on topographic highs but on nearly level lagoonal-sediment surfaces, some of which are the lagoonal drapes over Pleistocene reefs. In a few places, recent reefs occur over broad shallow depressions in Pleistocene surfaces. All recent reefs have far greater relief (5 to 20 m) than the buried Pleistocene reefs.

Few of the associations and features predicted by the karst origin of atolls hypothesis are present. For example: (1) the rim seems to have built up from the basal reflecting surface, rather than being a solutional modification of it; (2) stacked lagoonal sequences, where slow sedimentation perpetuates a steep-sided solutional depression, are uncommon, and most are present in the enclosed sounds and harbors; (3) many reefs are founded on flat surfaces, not on solutional pinnacles; and (4) deep sink holes are very rare. In general, Bermuda lacks clear signs of being a reef-encrusted paleokarst feature.

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