Wave resistance is used in many definitions as a criterion for "reef." It is therefore important to clarify what wave resistance means, whether it can be recognized in ancient reefs, and what effect the use of this criterion has on our thinking. For present purposes the noun "buildup" is used for all organically constructed mounds, including reefs, mudbanks, and algal stromatolites, irrespective of wave activity.

Wave resistance has several legitimate meanings, but none can be used as a reef criterion unless a definite degree of wave action is specified. Reefs would then be arbitrarily and undesirably separated from other buildups, whatever their biologic relations. Wave resistance as a criterion is therefore rejected, but as a variable characteristic, differing from one buildup to the next, it allows each to be placed in a unified wave-resistance hierarchy.

The degree of wave resistance required in a buildup depends on the wave environment, defined by the depth of water over the buildup, wave dimensions, and the relative frequency of waves of various dimensions. Wave energy increases rapidly upward, so that small depth changes involve large energy differences. Wave action (water-particle velocity) diminishes downward through the wave zone to 4% of its surface value at a depth equal to half the wavelength. Turbulence in the lower part of the wave zone requires no special adaptation by organisms; other forms of current may have more influence. Increase in wave size lowers the depth to which wave action is effective and increases turbulence at all depths in the wave zone. If large waves damage the buildup, their relative frequency becomes i portant, because wave resistance depends on a balance between wave damage and repair by organisms. The critical size of damaging waves depends on the "wave-resistance efficiency" of the constructing organisms, a function of growth form and strength. Organism growth rate contributes indirectly to the wave-resistant capacity of a buildup through its role in repairing damage.

Diagnosis of the wave-resistant capacity of ancient buildups is difficult. The geologic history of modern turbulence-indicative species (such as Acropora palmata) is short. Growth forms, distinctive in strong surf, become less diagnostic with decreasing wave action. Current action must be distinguished from wave action. Erosional debris, such as detached blocks, may be the product of slumping or boring organisms rather than turbulence. Storm damage may leave a record which gives a false impression of prevailing wave conditions.

Wave-resistance as a variable characteristic must, despite difficulties, become increasingly important in the study of buildups through geologic time. Organic evolution made possible the development of increasingly wave-resistant buildups and culminated in modern coral-algal reefs.

As a wider variety of constructing organisms became available and a correspondingly greater range of wave environments was colonized, the range of buildups that could exist at any one time increased. Thus, if one considers wave-resistance as a variable characteristic instead of an absolute value, arbitrary definitions are less important than environments and evolutionary relations.

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