Raised beach
A raised beach, coastal terrace,[1] or perched coastline is a relatively flat, horizontal or gently inclined surface of marine origin,[2] mostly an old abrasion platform which has been lifted out of the sphere of wave activity (sometimes called "tread").
The platform of a marine terrace usually has a gradient between 1°–5° depending on the former tidal range with, commonly, a linear to concave profile.
[9] The cliff faces that delimit the platform can vary in steepness depending on the relative roles of marine and subaerial processes.
[12] While marine terraces in areas of relatively rapid uplift rates (> 1 mm/year) can often be correlated to individual interglacial periods or stages, those in areas of slower uplift rates may have a polycyclic origin with stages of returning sea levels following periods of exposure to weathering.
[14][15][16][17][18] The formation of marine terraces is controlled by changes in environmental conditions and by tectonic activity during recent geological times.
Processes of eustasy lead to glacioeustatic sea level fluctuations due to changes of the water volume in the oceans, and hence to regressions and transgressions of the shoreline.
[19] When eustasy was the main factor for the formation of marine terraces, derived sea level fluctuations can indicate former climate changes.
This conclusion has to be treated with care, as isostatic adjustments and tectonic activities can be extensively overcompensated by a eustatic sea level rise.
[24] Hence, most of today's marine terrace sequences were formed by a combination of tectonic coastal uplift and Quaternary sea level fluctuations.
[2] While a terrace sequence can date back hundreds of thousands of years, its degradation is a rather fast process.
[3] Erosion and backwearing of slopes caused by incisive streams play another important role in this degradation process.
In order to estimate vertical uplift, the eustatic position of the considered paleo sea levels relative to the present one must be known as precisely as possible.
Current chronology relies principally on relative dating based on geomorphologic criteria, but in all cases the shoreline angle of the marine terraces is associated with numerical ages.
For exact interpretations of the morphology, extensive datings, surveying and mapping of marine terraces is applied.
The morphostratigraphic approach focuses especially in regions of marine regression on the altitude as the most important criterion to distinguish coastlines of different ages.
Also, paleo-soils as well as glacial, fluvial, eolian and periglacial landforms and sediments may be used to find correlations between terraces.
[24] On New Zealand's North Island, for instance, tephra and loess were used to date and correlate marine terraces.
[10][50] When analyzing the morphology of marine terraces, it must be considered, that both eustasy and isostasy can have an influence on the formation process.
Other important sites include various coasts of New Zealand, e.g. Turakirae Head near Wellington being one of the world's best and most thoroughly studied examples.
[47][48][52] Also along the Cook Strait in New Zealand, there is a well-defined sequence of uplifted marine terraces from the late Quaternary at Tongue Point.
[40][53] Hans Jenny famously researched the pygmy forests of the Mendocino and Sonoma county marine terraces.
The marine terrace's "ecological staircase" of Salt Point State Park is also bound by the San Andreas Fault.
[55] The coral marine terraces at Huon Peninsula, New Guinea, which extend over 80 km (50 mi) and rise over 600 m (2,000 ft) above present sea level[56] are currently on UNESCO's tentative list for world heritage sites under the name Houn Terraces - Stairway to the Past.
