Cuspate foreland

Did You Know?

• Cuspate forelands can be separated into three distinct areas: the central nose or apex, and two marginal wings.
• Located between the mainland and the foreland are often lagoons or marshy areas.
• In the United Kingdom, 11 taxa of invertebrates are found on shingle habitats.

Formation

Scientists still debate exactly how cuspate forelands form, but the most common explanation involves longshore drift. When longshore drift pushes sediment from two opposite directions, two spits can merge into a triangular shape that sticks out from a coastline or lakeshore. This process depends on dominant and prevailing winds blowing against each other. Wave diffraction around a barrier can also trigger formation. These landforms appear on both coasts and lake edges. Along a coast, they may develop sheltered behind an offshore island, on a stretch with no nearby islands, or at a river mouth where sediment piles up. In a strait or on a coast without islands or shoals, longshore drift, wind, and waves carry sediment from opposite directions. If waves hit the shore at a wide angle, the sediment converges, builds up, and forms beach ridges. Over time, steady accretion and progradation create the cuspate foreland. Dungeness, on southern Britain’s coast, formed when southwest waves from the English Channel met east waves from the Strait of Dover. Another example lies between the Awatere River and White Bluffs in Marlborough, New Zealand, where ridges on the eastern and northern sides face the dominant waves. In other cases, spits form when longshore drift moves beach material until the coastline makes a sharp turn, causing the material to spill over the corner, often across a river mouth. For a cuspate foreland, the prevailing wind and a strong opposing wind push shingle from both directions toward a change in the coastline, allowing the foreland to grow. Most cuspate forelands develop where the coastline juts out at enough of an angle for drifting material to spill over from longshore drift in both directions.

Movement

Once formed, cuspate forelands may remain in place and continue developing as sediment accumulates, or they may migrate down the coast as one side erodes and the other accretes. Migrating forelands are typical of open coastlines, and the direction of migration is often indicated by successive beach ridges on the advancing side where wave energy is lower. Longshore drift is commonly cited as the main process driving movement, but observed cases where two cuspate forelands on the same shoreline migrated in opposite directions show that longshore drift does not always provide a sufficient explanation. When an offshore sandbank is present, the foreland's position usually relates to the sandbank's position; changes in the sandbank's position typically cause the foreland to follow. The sandbank acts like an island, refracting waves, and also supplies sediment as it erodes and is pushed toward the coastline, often in the opposite direction to longshore drift. A cuspate foreland formed near an island may extend to the island, forming a tombolo. Depending on physical conditions such as storms, the feature can alternate between a cuspate foreland and a tombolo, as occurs at Gabo Island in South Australia.

Succession

After a cuspate foreland attains its triangular shape, pioneer species hardy enough to survive the environment colonize it. These species secure the foreland and allow more sediment to accumulate, further stabilizing it. Colonization and succession depend on several factors: if the shingle is too coarse, fine sediment between spaces is reduced, and seed germination is low; seeds also fail if fresh water retention is insufficient. Stable shingle forelands often have vegetation above the high tide line. As vegetation establishes, mites and collembolans break down plant matter, accumulating organic matter. Plants promote soil development and increase water retention, creating a habitat for more plants.

Impacts and management

Management issues for cuspate forelands vary with their formation. A depositional foreland may be vulnerable if human interference alters sediment transport, whereas a relic foreland from past erosion is less affected by such interference. For a foreland to be maintained, sediment input must exceed output. Coastal development and engineering must be regulated to allow sediment to continue moving toward the foreland for deposition. Development on forelands is risky due to erosion and vulnerability to storms and sea level rise; as sea levels rise, forelands may move inland. At Point Pelee, about 1,900 hectares of former agricultural land on the cuspate foreland is now underwater from wind erosion and compaction of organic soils. This foreland is especially vulnerable when high lake levels combine with spring and autumn cyclonic activity, and erosion also occurs when spring storms cause ice to scour the lake bottom at the foreland's edge. Uncertainty about its formation creates management uncertainty, though Parks Canada recognizes the importance of including Point Pelee National Park in management plans. Where an aquifer lies beneath a cuspate foreland, water removal must be regulated; at Dungeness, water restrictions maintain the aquifer level. Coastal management must account for natural processes on forelands, as many provide bird habitat. Alternative approaches such as soft defences are needed instead of high-impact defences like sea walls. Some forelands naturally lack vegetation due to high disturbance from wave action, but with increased storm frequency from climate change, the effects on forelands and their vegetation require effective management.