Major transgressions and regressions are recognized on the basis of vertical sedimentary sequence between major unconformities in the stratigraphic record. The more laterally continuous an unconformity is, the more time significance it is interpreted to have. Studies of transgressions and regressions during the Holocene and Pleistocene provide new insights into the character of unconformities and the interpretation of depositional sequences. In addition, these studies indicate that extensive regressive deposits do not develop during falling sea level.

During transgressions, the base of the transgressive depositional sequence is marked by a subaerially eroded unconformity at the top of the pretransgression deposits. The deposits just above the basal surface are usually back-barrier lagoon or estuarine sediments. Three types of basal contacts can develop depending upon the material directly overlying the unconformity: fringing marsh, distal lagoon, or lagoonal beach sediment. The processes which develop these lithosomes also serve to make the contact lithologically indistinct. In fact, with lagoonal beach and distal lagoon the contact can become gradational owing to erosion and bioturbation, respectively. Considerable topographic relief is present on this surface, whereas paleosoil is rarely preserved. The most lithologically distinc contact developed during the transgression is the ravinement surface caused by shoreface retreat. This contact is also the most laterally continuous and has the least topographic relief; it is underlain by back-barrier lagoonal deposits and overlain by nearshore marine deposits.

During many transgressions and regressions, such as have occurred in the Quaternary, the sequence of back-barrier lagoon, truncated by the ravinement surface and overlain by nearshore marine deposits may be repeated several times in one vertical section. When the ravinement surface is mistaken for a major unconformity and the commonly obscured contact at the base of the lagoonal lithosome is taken as a gradational facies change, the vertical sequence is interpreted as prograding (regressive) shoreline deposits with nearshore marine overlain by lagoon. Each such sequence is interpreted to be separated by a transgressive surface. When the ravinement surface and the basal contact are recognized as such, the section is interpreted as a set of transgressive sequences with lagoon truncated y the ravinement surface, overlain by transgressive nearshore marine deposits.

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