The most important element in paleogeography, and perhaps the most difficult to locate, is the shoreline. Specific indicators are rare, faint, or easily destroyed. Nevertheless, we require a "key" which will permit us, tentatively at least, to identify ancient coasts.

River sands placed on modern beaches are modified in a systematic way. The distribution below a critical diameter is filtered to provide a new, distinctive, size curve. The result is an inflection so located in many samples that it does not appreciably affect the standard deviation. The inflection, or "break," which results from surf action may not be an absolute indicator, but it appears to be fairly good. This has been verified observationally (studying near-shore sands) and experimentally (placing fluvial sands in the breakers).

Under wave action, the "surf-break" starts in the "fines" and moves into the coarser sizes. The rate at which it moves is a measure of wave energy level; hence its position depends on both wave energy and duration-of-working. The "surf break" should be common in sands worked by low to moderate energy waves; along coasts having moderate to high energy breakers, the inflection may be missing due to an absence of material coarse enough to record it.

Shorelines of interior seas, such as ancient seaways, are generally marked by low to moderate wave energy levels. Hence the "surf break" may be a widely useful, although not foolproof, device.

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