Raised Beach

Marine-terrace systems exhibit a characteristic landward-to-seaward ordering of features that records the transition from active shoreline to abandoned coastal benches: low‑tide ramp or cliff with associated deposition, the modern wave‑cut platform, a shoreline notch or inner edge, the modern sea cliff, one or more older wave‑cut platforms and their paleo‑shoreline angles and cliffs, and finally terrace cover deposits (marine sediments, colluvium, alluvial‑fan deposits) and decayed or buried shore features. These elements provide a template for interpreting both vertical stacking and lateral relationships among active and former coastal environments and for identifying discrete paleo‑sea levels.

Platform geometry is largely controlled by tidal range and wave action. Cross‑profiles of marine terraces are typically linear to concave and exhibit gentle gradients, commonly between 1° and 5°. Planform widths are highly variable—locally reaching up to about 1,000 m—and many sub‑horizontal platforms terminate seaward in a low‑tide cliff; thus the presence, form and width of platforms are strongly modulated by tidal amplitude and dynamics. Cliff faces bounding terraces record the relative importance of marine (wave abrasion, undercutting) versus subaerial (weathering, mass wasting) processes: steeper cliffs reflect dominant marine erosion, whereas more subdued slopes indicate stronger subaerial modification. The shoreline notch or preserved shoreline angle at the junction of platform and cliff marks the locus of maximum sea ingressions and serves as a primary paleo‑sea‑level indicator.

Terrace systems may persist as long, laterally continuous benches extending parallel to the coast for tens of kilometres, reflecting sustained erosional processes and former sea‑level positions. Preservation varies with age and post‑depositional processes: older terraces are often mantled by marine deposits, colluvial veneers or alluvial‑fan sediments, while the highest and youngest terrace levels are generally more exposed and prone to decay or burial. Soils and surficial covers on terraces are diverse and record complex pedogenic histories; common soil types include planosols and solonetz, and in sheltered localities terraces may contain allochthonous sandy materials (for example, tsunami‑derived deposits) that affect texture and stratigraphy.

Tectonic uplift rate strongly influences how well terraces resolve past sea‑level oscillations. Regions uplifted relatively rapidly (> ~1 mm yr‑1) tend to preserve terraces that can be tied to individual interglacial highstands or marine isotope stages, yielding high‑resolution sea‑level records. In slowly uplifting settings, terraces are more likely to be polycyclic—products of multiple episodes of sea‑level return interspersed with exposure, weathering and reworking—complicating correlation to specific events. Finally, broad-scale differences in terrace geometry between hemispheres and the clear modulation of platform attributes by local to regional tidal regimes indicate that climatic, oceanographic and tidal factors, together with the balance of wave energy, subaerial denudation and tectonic uplift, jointly control terrace form, preservation and interpretive resolution.

Formation of Raised Beaches (Marine Terraces)

Marine terraces, commonly termed raised beaches, are terraced coastal surfaces that preserve former shorelines as uplifted, near-horizontal benches mantled by beach or shallow marine sediments. They develop when waves cut a planar platform during a sea‑level highstand; subsequent lowering of relative sea level combined with tectonic or isostatic uplift isolates and elevates that platform above the modern coast, producing a discrete terrace surface. Individual terraces frequently correspond to separate interglacial highstands and can be correlated to the global marine isotope stage (MIS) sequence, so each preserved bench potentially furnishes a stratigraphic marker for a particular isotopic highstand. The elevation of any terrace therefore reflects the interaction of eustatic sea level at the time of formation with local and regional vertical land movements—including tectonic uplift or subsidence and glacio‑isostatic responses—so absolute heights require interpretation within this multi‑factorial context. The likelihood that a terrace is preserved, its shape and continuity alongshore, and the completeness of terrace sequences depend on coastal gradient, bedrock and sediment properties, sediment supply, wave regime, and subsequent erosion or burial; as a consequence, terrace records are often spatially discontinuous or diachronous and demand careful geomorphic mapping. Because terraces record former shoreline positions, they are valuable for reconstructing past sea‑level magnitudes, estimating long‑term uplift rates, and linking regional coastal histories to global paleoclimate signals, but reliable interpretation depends on detailed stratigraphy, geomorphic analysis and independent chronological control of terrace deposits.

Causes

The development of raised beaches and marine terraces reflects the interplay between global sea-level change and vertical motions of the crust, so that neither eustatic nor tectono-isostatic controls can be considered in isolation. Long-term reconstructions show very large multimillion-year sea-level swings, whereas the most recent glacial–interglacial amplitude is comparatively small; nonetheless, Pleistocene glacioeustasy was sufficient to lower global sea level by roughly 100 m at the last glacial maximum, exposing continental shelves and driving widespread shoreline regression. Eustatic drivers therefore set the background chronology of shoreline transgression and regression; the dominant mechanism in the Quaternary is glacioeustasy, with additional contributions from changes in basin voidage such as sedimento-eustasy and tectono-eustasy.

Superimposed on these global signals are local and regional vertical movements. Glacial isostatic adjustment (GIA) following ice-sheet loading produces significant crustal rebound — for example, present uplift rates in parts of Scandinavia approach 10 mm yr−1 — and such isostatic responses alter relative sea level at the coast. Where uplift is active, marine terraces commonly form during sea-level highstands and are preserved by subsequent emergence; in formerly glaciated areas, terraces may record periods of isostatic stillstand as much as eustatic highstands.

Because eustatic, isostatic and tectonic effects interact, relative sea-level histories can be complex: isostatic or tectonic movement may be partly compensated by global sea-level change, so terrace elevations do not always provide a straightforward index of past global sea level. Consequently, terrace sequences are most readily correlated with marine oxygen isotope stages and used for paleoclimatic inference where tectonic/isostatic influences are small or can be independently constrained. Finally, the geomorphic appearance of terrace staircases is sensitive to the tempo of change: episodic, stepwise uplift produces well-defined terrace risers, whereas gradual or smoothly varying relative sea level often fails to generate discrete, mappable marine terraces.

Processes

Marine terraces principally develop where persistent wave action and shore‑zone sediment transport attack rocky temperate coasts, truncating cliffs and producing a near‑horizontal or gently sloping wave‑cut (abrasion) platform as the shoreline retreats. Mechanical wave processes are augmented by subaerial weathering and cavitation, so platform genesis commonly reflects an interplay of marine abrasion and terrestrial weathering rather than a single mechanism.

Erosion rates that create these terraces vary widely according to shoreline lithology, nearshore bathymetry and bedrock structure: resistant granitic coasts may erode at only millimetres per year, whereas unconsolidated volcanic ejecta can retreat by metres annually. Relative sea‑level change—episodes of regression and transgression—sequentially exposes or drowns platforms, producing terrace staircases; small cliff notches preserved in cliff faces commonly mark short stillstands during terrace formation.

Terrace geometry is sensitive to tidal and lithologic controls. Terrace gradients tend to increase with tidal range and diminish with increasing rock resistance, while terrace widths are generally narrower on strong, coherent rocks and broader where rock strength is low. Tectonic and topographic context further modulates terrace multiplicity and preservation: elevated rates of vertical movement (uplift or subsidence) and steeper hinterland slopes favour the development and retention of multiple terrace levels within a given time span.

Origin pathways are mixed: some shore platforms form mainly by denudation (removal by marine processes) whereas marine‑built terraces accrue through deposition of material derived from shore erosion; individual terraces frequently record both erosional and depositional contributions. This mixed genesis underlies an active scientific debate over the relative importance of direct wave erosion versus subaerial and chemical weathering in shaping platforms.

In intertropical settings, reef flats and uplifted coral reefs constitute a significant class of marine terraces. These are biologically constructed surfaces produced by reef growth, shoreline progradation and accumulation of carbonate framework, subsequently exposed by relative sea‑level fall or tectonic uplift. Terrace sequences can preserve coastal histories extending over hundreds of thousands of years and therefore serve as chronostratigraphic records of sea‑level oscillation and tectonism; nevertheless, terraces are also vulnerable to relatively rapid physical degradation.

Degradation agents include deeper transgressions that force renewed cliff retreat and remove older terrace levels, burial or obscuration by colluvium and alluvial deposits, and fluvial processes such as incision and backwearing by active streams. The long‑term evolution and preservation of coastal terrace sequences thus reflects a complex coupling of marine, subaerial and fluvial processes acting in concert with lithologic and tectonic controls.

Land and sea-level history

The horizontal and vertical displacement of shorelines, when linked to the age of their parent interglacial stage, provides either a means to compute a coast’s mean uplift rate or, where uplift is independently constrained, to reconstruct past eustatic sea level. Such inferences, however, depend critically on accurate knowledge of paleo‑eustatic positions; uncertainty in the former sea‑level elevation relative to present directly limits confidence in uplift estimates. Chronological control on marine‑terrace sequences therefore typically combines relative geomorphologic ordering with absolute age assignment by correlating individual shoreline angles to dated global or regional sea‑level markers.

The best‑documented terrace worldwide is that associated with the last interglacial, Marine Isotope Stage 5e. Although MIS 5e is conventionally delimited at ~130–116 ka, field evidence from tectonically stable localities (e.g., Hawaii, Barbados) records a broader span (~134–113 ka) with a concentration of ages between ~128 and 116 ka. Older terrace levels commonly used in long‑term uplift and sea‑level reconstructions include those correlated with MIS 9 (~303–339 ka) and MIS 11 (~362–423 ka). Global syntheses indicate that MIS 5e, MIS 9 and MIS 11 sea levels were each on the order of +3 ± 3 m relative to present, whereas MIS 7 (~180–240 ka) has an estimated eustatic level close to present (~−1 ± 1 m). Because MIS 7’s eustatic signal is near modern sea level, terraces of that age tend to be weakly developed or absent compared with the clearer expressions of MIS 5e, MIS 9 and MIS 11.

Where measured terrace elevations exceed the stated uncertainties in paleo‑eustatic reconstructions for the Holocene and Late Pleistocene, those eustatic uncertainties do not materially change the geological interpretation of net coastal uplift. Nonetheless, marine‑terrace sequences are not purely eustatic records: glacial isostatic adjustment can produce apparent shoreward and vertical shifts of former beaches through progressive rebound following ice loading and melt, and coseismic uplift associated with earthquakes can generate raised shoreline angles and terraces that are asynchronous with global highstands (documented examples of coseismic terraces are presently confined to the Holocene). Recognition of these non‑eustatic mechanisms is essential when using terrace elevations to infer uplift histories or past sea levels.

Mapping and surveying

The lowest marine terrace at Tongue Point, New Zealand, was investigated to characterise terrace morphology, elevation and associated depositional and erosional features through an integrated programme that couples spatial form, stratigraphy and age constraints. Robust geomorphic interpretation requires extensive dating combined with systematic surveying and detailed mapping so that terrace geometry can be related to stratigraphic architecture and chronological control.

Stereoscopic aerial-photograph interpretation, using overlapping images at scales of ≈1:10,000–1:25,000, is employed to resolve three-dimensional landform geometry and to delineate terrace edges, scarps and palaeoshorelines. These remote-sensing results are verified by ground-truthing: field inspections conducted alongside topographic maps at ≈1:10,000 scale document on-surface evidence of erosion and accumulation and refine remotely derived boundaries. Detailed sediment and surface analyses of eroded and deposited material (composition, thickness and erosional features) provide process-level insight into terrace formation and subsequent modification.

Precise vertical control is essential: elevations should be determined with an aneroid barometer or, preferably, a tripod-mounted levelling instrument, with vertical accuracy targeted to 1 cm (0.39 in). Elevation and topographic measurements are typically recorded at intervals of 50–100 m (160–330 ft), with sampling density increased in areas of complex relief. In geographically inaccessible sectors, photogrammetry and tacheometry offer practical alternatives to conventional levelling, allowing acquisition of precise spatial and topographic control where dense field surveys are impractical.

Correlation and dating of marine terraces

Marine terraces preserve physical markers of former shorelines—bench surfaces, wave-cut notches and associated beach deposits—that serve as sea‑level index points for reconstructing past sea level, tectonic uplift and palaeoenvironments. Reliable terrace chronologies require both relative (correlative) and absolute (chronometric) approaches because each technique has distinct temporal resolution and vulnerability to post‑depositional alteration; therefore multi‑method strategies are standard.

Common absolute techniques applied to terrace materials include radiocarbon dating of organic remains (useful to ~50 ka but sensitive to contamination and reworking), uranium‑thorium dating of authigenic carbonates and corals (effective for many late Pleistocene corals when detrital contributions are accounted for), electron spin resonance on teeth and shells, and luminescence methods (OSL/TL) on quartz or feldspar sands that date last light exposure. Cosmogenic‑nuclide exposure dating (e.g., 10Be, 26Al) directly ages the timing of surface emergence of bedrock or clasts but depends critically on assessment of inheritance, burial, erosion and complex exposure histories. Tephrochronology can provide high‑precision correlation where unique geochemical fingerprints of ash layers can be matched to well‑dated regional tephra horizons; its success requires in situ, undisturbed ash and robust geochemical correlation.

Relative and correlative indicators remain essential for stratigraphic context: internal superposition, soil and weathering profiles, biostratigraphy (molluscan assemblages, microfossils, pollen) and palaeomagnetic signatures allow terraces to be placed within broader stratigraphic frameworks and to infer palaeoenvironmental conditions. Correlation to global or regional eustatic records (for example Marine Isotope Stage curves from oxygen‑isotope stacks) provides a means to assign terraces to known interglacial highstands, but this requires precise terrace elevation and selection of an appropriate local sea‑level indicator, together with corrections for tectonic uplift, glacio‑isostatic adjustment, sediment compaction and local tidal range.

Interpretation is strengthened by cross‑validation: concordant results from different methods (e.g., cosmogenic exposure ages corroborated by nearby OSL ages and consistent fossil assemblages, or U–Th coral ages used as anchor points linked by stratigraphy and tephra) reduce ambiguity and help identify outliers caused by reworking or diagenesis. Terrace‑based uplift rates are commonly estimated as (measured terrace elevation − inferred palaeo‑sea‑level) / terrace age, but such calculations must correct for faulting and tilting, flexure, glacio‑hydro‑isostatic effects, and sediment compaction to isolate tectonic uplift.

Field and analytical workflow should begin with comprehensive geomorphic mapping (field mapping integrated with GPS and LiDAR/DEM analysis), identification of shoreline indicators and terrace stratigraphy, and targeted sampling for multiple dating methods. Laboratory cross‑checks, age–elevation plots and explicit regional correlation schemes with quantified uncertainties are essential outputs. Reported uncertainties must explicitly consider reworking and bioturbation, diagenetic alteration (especially of carbonates and organics), detrital contamination in U–Th systems, cosmogenic inheritance, radiocarbon reservoir effects and any tectonic tilting or differential uplift across the study area.

When applied in combination and with transparent uncertainty reporting, multi‑proxy dating and correlation of marine terraces permit robust reconstructions of late Quaternary sea‑level changes, spatial and temporal patterns of coastal uplift or subsidence, seismic and tectonic hazard assessment, and palaeoenvironmental interpretation. Best practice emphasizes explicit multi‑method integration, documentation of assumptions and corrections, and regionally replicated sampling to validate correlations across spatial scales.

Correlational dating of raised beaches relies on integrating morphological, sedimentary and paleontological evidence to establish relative chronologies along coastlines. In settings where the shoreline has regressed—either through tectonic uplift or falling sea level—surface elevation is commonly taken as the primary ordering criterion: higher, more landward benches are interpreted as older. Morphostratigraphic correlations are strengthened by assessing lateral continuity and planform size of terraces, since coherent terrace surfaces of similar form are likely to belong to the same generation.

A suite of terrestrial signatures is routinely used as auxiliaries to the morphostratigraphic framework. Paleosols and associated fluvial, glacial, eolian and periglacial landforms and deposits record surface processes and episodes of subaerial exposure that help link and sequence terraces along a coast. Where terraces formed at or close to former glacier termini, geometric attributes such as terrace width can furnish a relative age ordering: progressively narrower bench deposits often reflect older positions of a slowly retreating glacier front and thus permit correlation of successive terminus-related terraces.

Lithostratigraphic criteria also provide a record of shoreline migration. Alternations between terrestrial and littoral or shallow-marine facies within a stratigraphic column record transgressive and regressive events and can be used to match terrace sequences across sites. However, the value of lithostratigraphy depends on the preservation of continuous, minimally disturbed sedimentary sequences; unconformities, erosional truncation and depositional hiatuses frequently interrupt the record and complicate correlation.

Biostratigraphic indicators—fossil molluscs, foraminifera and pollen—add palaeoenvironmental and relative-age constraints. Molluscan assemblages are particularly informative because many species have depth-dependent ecologies and taphonomic signatures that permit reconstruction of former water depths and shoreline positions; microfossils and palynological records further refine environmental interpretation and can help distinguish contemporaneous from reworked deposits.

Robust chronologies derive from integrating these approaches with independent marker horizons and global frameworks. Datable tephra and loess beds have proven effective as stratigraphic tie‑points (for example, correlating North Island, New Zealand, terraces), and terraces are frequently placed within Quaternary glacio‑eustatic cycles by correlation with marine oxygen isotope stages (MIS). Best practice therefore combines elevation and morphological ordering with lithostratigraphic and biostratigraphic evidence plus any available absolute or marker-dated horizons to bracket ages and reduce ambiguity arising from local tectonics, erosion and depositional discontinuities.

Direct dating of marine terraces

Direct geochronology of marine terraces integrates multiple dating techniques applied to terrace deposits and surface materials to constrain the timing of terrace formation and abandonment. Radiocarbon (14C) dating of terrestrial biogenic remains preserved in coastal sediments (e.g., mollusc shells) remains widely used where suitable organic material is available. Where radiocarbon is compromised by contamination or material scarcity, uranium-series methods based on the 230Th/234U ratio can yield useful ages provided sufficient uranium and low detrital input. Paleomagnetic stratigraphy records remanent magnetization directions and polarity reversals that can be correlated with the geomagnetic polarity timescale and has been employed for terrace sequences in regions such as southern Italy.

Luminescence techniques, notably optically stimulated luminescence (OSL), date the time mineral grains were last exposed to sunlight and therefore suit fluvial, aeolian or fault-related sediments in tectonically active settings (for example, studies in the San Andreas Fault region and the Quaternary Eupcheon Fault of South Korea). Over the past decade, terrestrial cosmogenic nuclide (TCN) methods have markedly advanced terrace chronology: in situ cosmogenic isotopes (principally 10Be and 26Al) measured in quartz and related minerals accumulate while surfaces are exposed to cosmic rays, so their concentrations provide exposure ages that commonly mark the time a terrace was abandoned by the sea.

Each method carries specific sample requirements and limitations—14C requires preserved terrestrial biogenic material, U-series needs adequate uranium and low detrital contamination, OSL depends on effective sunlight bleaching prior to burial, and TCN dating assumes well-constrained exposure and burial histories or continuous exposure. Translating terrace ages into estimates of past eustatic sea level further depends on two central assumptions: that the eustatic sea-level position represented by at least one dated terrace is known, and that the coastal uplift rate has remained essentially constant over the interval of interest. Empirical applications across diverse tectonic settings (e.g., North Island, New Zealand for 14C; southern Italy for paleomagnetism; San Andreas and Eupcheon Fault studies for OSL) illustrate the complementary use of these methods in reconstructing Quaternary sea-level and tectonic histories.

Marine terraces—for example the bench-like coastal surfaces south of the Choapa River in Chile extensively studied by Roland Paskoff—serve as preserved records of former sea levels and as concrete archives of vertical crustal motion. Their absolute elevations and lateral distribution provide empirical constraints on patterns and rates of tectonic uplift: mapped terrace levels, together with the vertical offsets between successive raised shorelines and abandoned beach deposits, can be attributed to coseismic or cumulative uplift where independent historical or stratigraphic evidence allows correlation with earthquakes. A well-documented instance is the 1855 Wairarapa earthquake (New Zealand), whose ~2.7 m of coastal uplift is inferred from measured offsets of raised shorelines. Interpreting terrace histories requires combining terrace elevations with independently derived eustatic sea‑level curves so that the global sea‑level signal can be separated from local isostatic and tectonic movement; this partitioning is essential to obtain reliable uplift rates and to reconstruct relative sea level through time. Beyond tectonic applications, terraces constrain past shoreline positions and thus provide palaeoenvironmental and palaeoclimatic data that improve understanding of regional sea‑level change and inform projections of future coastal response to climate forcing.

Prominent examples

Raised beaches and marine terraces are distributed across a wide range of coastal and geodynamic environments, from subduction and collisional margins to passive continental shores. Their occurrence on Pacific subduction fronts, Atlantic passive margins, collision zones (e.g., Kamchatka, Papua New Guinea, New Zealand, Japan) and temperate west‑facing Atlantic coasts demonstrates that terraces record the interplay between global sea‑level fluctuations and diverse modes of vertical crustal movement.

Classic north‑Atlantic and north‑eastern Atlantic sequences illustrate terrace formation on both continental margins and island coasts in temperate latitudes. Well‑developed raised beaches occur throughout the British Isles (Donegal Bay; County Cork and Kerry; numerous Cornish localities such as Bude and Perranporth; Welsh coasts including the Vale of Glamorgan, Gower, Pembrokeshire and Cardigan Bay; islands such as Jura and Arran), in Brittany (Finistère), Galicia (northern Spain) and at Squally Point in Eatonville, Nova Scotia (Cape Chignecto Provincial Park). These examples typify terrace geomorphology produced under modest uplift rates and repeated Quaternary sea‑level oscillations.

New Zealand provides some of the most intensively studied marine‑terrace sequences, where rapid tectonic uplift has produced well‑preserved staircase morphologies. Turakirae Head near Wellington is regarded as a globally important reference site, and along Cook Strait the Tongue Point profile preserves a stratified sequence in which a lower terrace corresponds to the last interglacial, a more eroded middle terrace to the penultimate interglacial, and an almost vanished highest shore platform illustrates progressive decay of older interglacial shorelines. In the Bay of Plenty on the North Island, a sequence of seven terraces records multiple episodes of relative sea‑level stand and/or uplift through the Quaternary.

On the Californian Pacific margin, marine terraces north of Santa Cruz (Davenport–Santa Cruz area) are closely related to seismic deformation on the San Andreas Fault; terraces there are interpreted as uplifted by repeated slip events, and modern infrastructure (Highway 1) follows lower terrace surfaces visible in aerial imagery. Terrace staircases along the San Andreas also control local ecology: Salt Point State Park exemplifies an “ecological staircase” in which terrace morphology, fault structure and distinctive plant communities (echoing Hans Jenny’s work on pygmy forests on Mendocino and Sonoma terraces) are spatially coupled.

Elsewhere, terrace elevation and extent reflect strong tectonic forcing. Along South America’s Pacific margin the highest terrace levels coincide with places where the subducting plate carries oceanic ridges, producing locally accelerated uplift. In the Indo‑Pacific some coral reef terraces reach extraordinary elevations: Cape Laundi on Sumba hosts a patch reef at about 475 m within a multi‑terrace reef sequence (with numerous wide benches), while the Huon Peninsula of New Guinea preserves reef terraces extending tens of kilometres and exceeding 600 m above present sea level (registered on UNESCO’s tentative list). Comparable high emergent reef and terrace levels are reported on some Philippine islands (to ~360 m) and along parts of the North African Mediterranean coast (notably Tunisia, to ~400 m), indicating either rapid uplift or long spans of emergent preservation.

Across these settings, marine terraces function as preserved shoreline markers that constrain relative sea level, timing of interglacial highstands, and rates and styles of tectonic uplift. Where terraces align with active faults they provide evidence of coseismic or cumulative uplift; where they support differentiated biotic assemblages they also document ecological succession on newly emergent coastal surfaces. Consequently, terraces are essential archives for reconstructing Quaternary sea‑level history, quantifying uplift rates, and interpreting coastal landscape evolution.

Tidal notch sequences are widely used as geomorphic markers of relative coastal uplift because successive notches preserve a record of past shoreline positions and sea-level changes. However, the common schematic that portrays tidal notches as horizontal features fixed at present sea level is an oversimplification: notch development spans an elevation continuum controlled by wave energy and local shoreline conditions.

At one end of this continuum are wave notches, which form in comparatively low-energy settings and generally align with the contemporary sea surface; at the other are surf notches, generated in high-energy, turbulent surf and capable of developing above mean sea level. Surf notches have been observed up to about 2 m (6.6 ft) above sea level, so an elevated notch cannot be taken automatically as evidence of tectonic uplift. Because Holocene sea-level history includes at least one interval of higher sea level, some raised tidal notches may record former eustatic stands rather than vertical land movement.