Definitions

Johnson (1919) defined beach ridges as depositional features constructed by waves at successive shore positions. Reineck and Singh (1975) characterized beach ridges as having been formed at high-tide level of “rather coarse sediment” and related to storms or exceptionally high water stages. Bates and Jackson (1980) designated beach ridges as low mounds of beach and beach-and-dune material, heaped up by waves over the backshore beyond the present limit of storm waves or ordinary tides, but there is a risk of confusing wave-deposited and wind-deposited ridges. Referring to relict strandplain dunes as beach ridges, Carter (1986) used the term very broadly to cover “all large constructional forms of the upper beach, capable of preservation,” applying it also to landward-shifting offshore bars, already welded swash bars, and prograded berm ridges.

Davis et al. (1972), Fraser and Hester (1977), Carter (1986, Carter 1988) and others referred to onshore-migrating swash bars and/or to the stranded end products as “beach ridges.” For some time after welding to the shore, the prograding sandy berm ridges may continue to be impacted by daily beach processes. Most North American and Australian authors considered stabilized onshore beach ridges as either of predominantly wave-built or of wave and wind-constructed, composite origin (e.g., Price, 1982). Hesp (1984, Hesp 1985), Mason (1990), and Mason et al. (1997), thus distinguishing between low profile “smooth, terrestrial,” squat berm ridges and steep dune ridges that often overlie and bury them.

Beach ridge presently is defined as a relict shore ridge that is more or less parallel with the coastline and with other landward-adjacent ridges. It is built by wave swash (a berm ridge) that may be surmounted by wind-deposited sediment (a foredune). Once such a ridge becomes isolated from daily active beach processes by coastal progradation, which may lead to the construction of one or more new ridges to seaward, it becomes a beach ridge. On wide eolian backshore plains, shore-parallel dune ridges may also form behind active foredunes. Regardless of their dimensions, shapes, and origin, active beach/shore ridges impacted and modified almost daily by shore processes are excluded from the designation.

Associated landforms

A berm was originally defined as a narrow, scarp-backed, and wave-cut horizontal surface in the beach foreshore (e.g., Komar, 1976). Subsequently, it came to mean a wedge-shaped ridge, between an upper foreshore slope and a landward-inclined berm top surface (King, 1972). Its base is the horizontal plan that intersects the foreshore slope at the level of the backshore plain. Hine (1979) defined a berm as a shore-parallel linear body of triangular cross section with a horizontal to slightly landward-dipping surface (berm top) and a steeper seaward-dipping slope (beach face). Berms are short-lived and frequently reforming landforms, often absent from a beach.

Swash currents deposit sediment that builds the landward-sloping high-tidal berm above the level of the adjacent backshore. The ephemeral high-tidal sand berms are of aggradational origin, with secondary indications of erosional scarping. Increased onshore winds during falling tides briefly stabilize the water level. Intermediate-level berms with vertical scarplets may form during these stillstands.

Berm ridges, occasionally sizable and more permanent than berm surfaces are wave-built intertidal-supratidal landforms. The lithosomes are composed of intertidal and high tidal (swash-overwash) deposit, bounded by the backshore plane and the berm surface along its foreshore margin (Figure B45). After becoming isolated by progradation from the daily effects of beach processes, these inactive landforms attain the status of wave-constructed beach ridges. Formed on mainland or island beaches or on shore-parallel sand spits, berm ridges are bracketed between the foreshore and the landward (or lagoonward) margin of the backshore. On the landward side they may follow the shoreline of an elongated lagoon or beach pond (“cat’s eye pond” Coastal Research Group, 1969), enclosed by a sand spit (Figure B46). Shore-parallel inter-ridge swales bracket each ridge. Sets of prograding berm ridges form on beaches of limited sand supply. Short et al. (1989) reported on a nearly exclusively swash-built beach ridge plain in Australia, and in Egypt, Goodfriend and Stanley (1999) described a shelly sandridge plain, composed of 20–30 cm high, wave-built ridges without eolian cover.

Figure B45
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Beach ridge-associated depositional facies and landforms on a prograding strandplain. Depositional facies: (1) subtidal; (2) wave-built, intertidal-to-supra-high tidal; (3) eolian. Landforms: b, berm; bsh, backshore plain; s, swale; A, foredune; B, accreting embryonic dunes on backshore plain in the early foreshore stage; C, single embryonic dune on berm ridge (backshore-berm) surface; pond: spit-growth-enclosed beach pond; D, pond-isolated berm ridge (intertidal-supratidal sand spit interval), without embryonic dunes. HT, high-tide level; LT, low-tide level. (Otvos, 2000) Reprinted with permission from Elsevier Science.

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Figure B46
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Prograding strandplain, southeastern Horn Island, MS. (A) Inter-swale ponds between old beach ridge sets in wooded interior. Elongated shore-parallel ponds of different orientation isolated from Gulf of Mexico (south) by barren narrow strip of backshore and berm ridges (USDA aerial photo, January, 1958. Width of image: 4.56 km). (B) Eleven years later: western half of previous image. Shore-parallel westward spit and spit-platform growth about to form new cat’s eye pond (bottom). Already isolated beach pond to east. Eroding (white) and forested (dark) old interior beach ridge sets, with swale ponds in the island interior (USCGS aerial photo, October, 1969. Width of image: 2.3 km).

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Several authors have regarded high-tidal sand berms as incipient (wave-built) beach ridges during the Australian “berm debate” (Davies, 1957; Bird, 1960; Hails, 1969). Later, Bird and Jones (1988) proposed that if a berm survived a 15-day tidal cycle, it becomes a beach ridge. Berms have also been credited with providing a foundation for the development of embryonic foredunes that develop into full-sized ones (Davies, 1957; Bird and Jones, 1988). However, foredunes often form along the seaward margin of the backshore plain as well. The presence of berms, often absent from beaches, especially from dissipative and high-energy beaches (e.g., Short, 1984), is not an indispensable precondition for foredune development (Hesp, 1984, Hesp, 1985).

Berm formation by wave action on the Tabasco shore of the Gulf of Mexico has been attributed to alternating “cut-and-fill” cycles of erosion and aggradation (Psuty, 1966). This berm-shaping process, however, appears to be a localized and ephemeral phenomenon without bearing on long-term strandplain development. The adjacent, 20-km wide Tabasco strandplain, for example, has been receiving abundant eolian sand supply and represents a foredune ridge plain, underlain by backshore and berm ridge deposits.

Truncation lines that separate mainland and island beach ridge sets

Where beach ridge progradation has been abruptly terminated by shore erosion, then followed by renewed beach growth, there are cross-cutting truncation lines that separate generations of beach ridges in mainland and island barriers of Quaternary age. This process was historically documented on several Mississippi coast strandplain islands (Otvos, 1981). The St. oseph Bay-area barrier spit and small mainland strandplains in NW Florida provide good illustrations (Figure B47).

Figure B47
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(A) Generations of narrow Late Holocene strandplain fans, northern St. Joseph Bay, NW Florida. Left: north tip of St. Joseph Peninsula (barrier spit). Right: small Holocene Palm Point mainland strandplain. Upper right corner: wide Late Pleistocene (Sangamon) beach ridges and partially filled swales represent erosion-impacted strandplain sectors. (B) St. Joseph Bay, NW Florida. Quaternary strandplains. Wide beach ridges of N-W-trending Late Pleistocene (Sangamon) strandplain (center field of photo) again contrast with narrow, crisply outlined Late Holocene strandplain ridges (lower right corner) (USGS aerial photo, October, 1978. Width of image: ca. 15 km). (Otvos, in press).

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Gravel-boulder (“storm”) ridges. Storm-associated high tides and waves build gravel ramparts as high as 6 m (Clapperton, 1990). Gravel-boulder ridges, associated with storm surges therefore rise well over their associated sea (lake) levels. Coarse clastic sediments, including shelly material resist backwash erosion and become stranded on these shore ridges. Permanent shingle emplacement at superelevated tide levels is aided by backwash percolation into the permeable gravelly substrate (Carter, 1988). For a given still-water level, the height of wave-built ridges built during winter storms may vary by as much as 2–2.6 m (Adams and Wesnousky, 1998). These “storm” ridges are common on glaciated and bedrock shores; also on tectonically or isostatically raised marine and lacustrine terraces. Examples abound in Canada’s Maritime Provinces, New England, and on high Pacific shores between Alaska and Mexico.

Coarse clastic beach ridges or bedrock terrace veneers accompany raised strandlines of pluvial and glacial lakes in the North American interior basins (Fulton, 1989; Morrison, 1991). Coarse clastic sediments were delivered by high-gradient streams, alluvial fans, mass wasting, occasionally even fluvioglacial processes. Adjacent bedrock areas that served as sediment sources have undergone intensive physical weathering under periglacial and cold-temperate conditions. Pluvial Lake Bonneville and its successor, early stage Great Salt Lake in Utah and Nevada; as well as Lake Lahontan in Nevada and California provide the best examples (Morrison, 1965, Morrison, 1991; Currey, 1980; Adams and Wesnousky, 1998, Adams and Wesnousky, 1999). Gravel-boulder ridges, deposited on wave-cut bedrock terraces form discontinuous tabular and tabular cross-stratified bodies, several meters thick (Adams and Wesnousky, 1998). Playa beach ridges that contain carbonate nodules, include paleosols and incorporate secondarily calcreted and gypsum-creted grit in the arid Lake Eyre basin, Australia, one of the world’s largest internally drained regions (Nanson et al., 1998).

Bouldery-gravelly coarse clastic sediments that often dominate ice-dammed glacial lake shorelines along the fluctuating glacial margin in North America have originated from reworked moraine till, fluvioglacial delta, periglacial colluvium, and in particular ice-contact (esker, kame, outwash delta) deposits. Major examples include relict shore features on glacial Lakes Agassiz, Algonquin, and Ojibway (Fulton, 1989, pp. 144–145, 257, 343, 362–364). Due to sand scarcity, the short life span of a given strandline, and erosive wave regimes, instead of regular beach ridges gravelly-bouldery shore zone lags frequently veneer wave-cut lake terrace surfaces.

Sediment types and ridge forms of sandy beach ridges. Depending on the wide range of wave and current conditions and source sediments on a given marine or lake shore sector, the upper beach deposits (intertidal-supratidal intervals) may be represented by fine-to-coarse, even gravelly sands in the berm ridges. Whereas, sorting tends to be good in uniformly sandy beaches, it becomes moderately sorted when coarser clastic fractions are also included (e.g., Thompson, 1992). Chappell and Grindrod (1984) described a rare transition between sandy ridges of a regular beach ridge plain and a small adjacent chenier plain, composed of shell-rich ridges. Reflecting the low relief and gentle seaward and landward slopes of the berm ridges, basinward-dipping parallel laminae and low-angle (3–5°) cross lamination tend to characterize the upper foreshore slope.

Subhorizontal or gently landward-dipping laminations occur on the landward beach ridge surfaces. The highly variable beach ridge dimensions, the height above still water level and slope angles depend on wave conditions, local tidal, or lake level ranges, including wind-induced rise in sea and lake-levels along given shore sectors.

“Pebble-armored ridges.” Pebble sheets plastered onto sand dunes during storms were designated as gravelly ramp barriers (Orford and Carter, 1982; Mason, 1990). The hydraulic ratios and shapes of shell bioclasts result in higher transport and dispersal rates than with regard to larger, denser silicate rock clasts. Whereas, gravel/boulder ridges accumulate during direct storms, their impact tends to flatten and disperse already existing ridges, composed of sand and lighter, platy shell clasts (Greensmith and Tucker, 1969; Rhodes, 1982, p. 17). Higher-energy events enable accumulation even of bioclastic rudites (Woodroffe et al., 1983; Meldahl, 1993).

Beach ridges, strandplain versus terrace development

Strandplain formation may be a continuous process with grain-by-grain addition of sand to the widening foreshore. Continuous progradation of the neap berm at mid-to-high level results in a gently undulating, almost level beach plain. On mesotidal foreshores where neap high tide remains below the highest foreshore levels, continuously accreting neap berms are uninterrupted by inter-berm swales (Hine, 1979, Figure 17(A)). Increased sand supply along the low-microtidal Gulf of Mexico beaches leads to steady outbuilding of the foreshore, accompanied by consequent progradation of narrow, closely spaced beach ridges. Discontinuous beach ridge progradation involved either the stranding or remolding of landward-migrated mesotidal swash bars on the foreshore (Hine, 1979; Carter, 1986) or spit growth from and downdrift reattachment (Figure B46) to the beach in microtidal settings (Otvos, 1981). Both processes isolate elongated ponds. Fronted seaward (lakeward) by newly formed active foredunes, and slowly filled by eolian and washover sands, such ponds may gradually become wide supratidal inter-ridge swales (Figure B46).

A comparison of Late Pleistocene strandplain ridges with the sharply outlined, narrow Late Holocene beach ridges illustrates the fact that prolonged infilling and erosional modification of Pleistocene strandplain ridges result in more subdued, more gently sloping beach ridges, separated by wider swales (Figure B47).

Instead of strandplains, gently undulating, nearly level eolian sand terraces form when sand supply and beach progradation does not keep pace with rapidly growing and sand-trapping beach vegetation. Beach progradation and/or eolian sand supply rates under these conditions are relatively low (e.g., Ruz and Allard, 1994).

Rates of beach ridge development

Depending on ridge dimensions, sediment supply rates, hydrodynamic and vegetative conditions, beach ridge development may proceed slowly or quickly. Thus, Nanson et al. (1998) report on a beach ridge along an Australian playa lake that during a high lake stage, formed in less than one year. Development rates in a number of other calculated examples from worldwide locations ranged between 1.8 and 3.3 yr/ridge, at other sites the rates were as low as 30–150 yr ridge (in: Otvos, 2000, pp. 90–91).

Beach ridges as ancient sea/lake levels markers

Former sea (lake) levels may be identified when a hortizontal interface is recognizable between the wave-built foreshore and the overlying eolian lithosome in a given ridge. This was the case in Lake Michigan strandplain ridges (Fraser and Hester, 1977; Thompson, 1992) where low-angle sand and gravel cross beds and trough-cross-bedded lacustrine sands of wave-built origin underlie land snail-bearing, crossbedded, in part massive, structureless dune sands. On pure sand beaches that lack granule and pebble clasts due to the very short transport distance from the immediately adjacent source of sediment, distinctions between wave- and wind-deposited lithosomes often are difficult or impossible to make on granulometric grounds alone.

At times of superelevated lake and sea levels, associated with storm-related temporary rise of the water levels wave-built sandy, shelly, or gravelly beach ridges may aggrade significantly above normal high-tide levels (e.g., Mason, 1990; Mason and Jordan, 1993). Precise reconstruction of former sea levels from beach ridges therefore may be problematical.

Similar to beach ridge summits, coarse clasts are useful markers of reference surfaces to document postdepositional tectonic and isostatic changes, including vertical displacement, tilting, and warping of the land surface. In their absence, wave-cut bedrock terraces and lag clast-veneered strandlines may also serve this purpose. Strandline stairsteps were documented on isostatically uplifted marine and glacial lake shores in subarctic North America and Scandinavia (e.g., Hudson Bay, Tyrell Sea; Fulton, 1989). Flights of raised beaches characterize former pluvial/playa lake shores in western North America, Australia, and other presently arid and semiarid regions. Bounding surfaces of lower foreshore Lepidopthalmus (formerly, Callianassa) ghost shrimp-burrowed barrier ridge deposits and correlative adjacent lagoonal-saltmarsh surfaces in six Late Pliocene-Pleistocene coastal terrace sequences were similarly utilized on the Georgia-northeast Florida seaboard (Hoyt, 1969).