578 SEDIMENTARY ENVIRONMENTS/Shoreline and Shoreface Deposits during large storms, some of the sediment is washed into the lagoons and is deposited as sheets of planar bedded and rippled deposits; these are termed ‘washover fans’ (Figure 9) Because sediment is removed from the seaward side of the barrier and is deposited on the landward side, the shoreline moves landward This process, termed barrier ‘rollover’, can be relatively rapid Parts of the modern-day Galveston Island in Texas have migrated 80 m in the past 100 years Generally, the migrating barrier system leaves very little sediment behind and the geological record of the transgression is an erosive surface, often associated with a lag of pebble or shell debris, concentrating the coarsest material from the underlying unit The barrier island will be preserved at the point at which it stops migrating landward and the system starts to prograde If the sea-level rise is very rapid, the entire barrier complex may be preserved by drowning Barrier island systems that occur in mesotidal settings are cut by tidal inlets through which tidal flows connect the lagoon with the open marine realm In such cases, the barrier is also locally reworked by tidal currents passing through inlets Deposition associated with the tidal inlet will include small subaqueous deltas on the landward and seaward sides, termed ‘flood’ and ‘ebb’ tidal deltas, respectively These deltas are composed of rippled and planar cross-stratified sandstones that interfinger with the shoreface deposits on the seaward side and with the lagoonal muds and washover deposits on the landward side Deposition of trough and tabular cross-bedding, often showing tidal draping, may also occur within the inlet channel This will frequently be reworked but can be preserved if the position of the inlet migrates laterally Although numerous authors have discussed progradational barrier systems, especially in modern settings, geologically such features are very short lived; they represent the transition from transgressive barrier systems to land-attached shorelines because, if the barrier progrades, the lagoon will rapidly (over geological time) be infilled Progradational systems with a strong component of longshore sediment transport can also develop beach-attached spits and even barrier systems that also shelter a lagoon on their landward side; however, these lagoons may be quickly filled by sediment that is introduced from the landward side by runoff and is washed over by storms from the seaward side High-Relief Transgressive Coastlines High-relief coastlines may occur when the system is in net erosion and the wave action produces a cliff The dominant processes are wave action and mass wasting Fluvial and tidal processes are negligible Wave action undercuts the cliff, which eventually collapses onto the foreshore; the cliff sediment is then reworked by wave processes and is transported either offshore or along the coastline by longshore drift In its simplest form, a rocky coastline typically comprises a cliff and a wave-cut platform (Figure 10) The nature of the bedrock geology, rates of relative sea-level fall, and the wave climate will characterize any particular example Variations in the bedrock geology due to lithological variations and structural geology (presence of faults and fractures) will result in differential erosion, leading to a variety of landforms, including stumps, stacks, arches, and caves Figure Facies model for a transgressive wave dominated shoreline system The barrier island migrates landward through erosion on the seaward side and by deposition in washover fans and flood tidal deltas SWB, Storm wave base