The upper surfaces of two patch reefs on the northern edge of the Bermuda platform were observed to consist of coral-coralline algal rock mounds amid a complex system of low-lying channels containing coarse- to fine-grained calcareous detritus. The rock mounds on the larger of the two reefs are noticeably concentrated toward the periphery. Detailed mapping of a part of one of these reefs indicates that the distribution of Spondylus valves and other coarse detritus bears a strong relation to the shape and width of the intermound channels. Oddly, 79% of the shells lay with the concave side up. The apparent preference for this generally unstable orientation is attributed to the lofting of shells during transport and the preferential burial of shells lying concave-side-downwa d.

The mode of the fine fractions of samples taken from shelly areas consistently is toward the right of the mode of samples taken from adjacent nonshelly areas. It thus appears that the shells act as baffles to permit the local deposition of fine sediment.

It is concluded that shell-bed sedimentology reflects the existence of two wave-energy regimes throughout the year. In winter, powerful storm waves determine the distribution of shell beds; calmer seas prevailing in summer months promote the preferential deposition of fines among the shelly material. Further, the rock-sand configuration on larger patch reefs suggests that the reefs may concentrate unimodal sand toward the center and bimodal sediments in the more narrow, circuitous, shell-rich channels toward the periphery.

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