2B.3 - Lithology, recession and coastal stability
Learn why rock and sediment properties, water movement and vegetation produce different cliff profiles and recession rates, and how to measure and interpret coastal change.
Materials have different resistance
Lithology influences recession because grain size, mineral composition, cementation and fractures determine how easily material is detached or dissolved.
Lithology and recession
Lithology is the physical character of a rock or deposit. Coastal recession is the landward retreat of the coast, commonly measured as cliff-edge displacement over a stated period.
Igneous rocks, such as granite, form from cooling magma and often contain interlocking crystals. Intact crystalline rock can resist abrasion, but joints and mineral weathering weaken it. Sedimentary rocks form from deposited material or chemical/biological accumulation. Cemented sandstone may be strong, whereas mudstone can soften and break down when wet; limestone resists some mechanical attack but is vulnerable to dissolution. Metamorphic rocks have been changed by heat and pressure. Some are strong and crystalline, but foliation in rocks such as slate can introduce splitting surfaces.
Unconsolidated glacial till, traditionally called boulder clay, contains mixed particles deposited by ice without the cementation of bedrock. Waves can remove its fine matrix, while saturation can reduce slope strength. Boulder clay describes a heterogeneous deposit, not a uniform rock with one erosion rate.
Avoid ranking all igneous rock above all sedimentary rock. Actual resistance depends on the particular material and its structure, while exposure, beach protection and water conditions determine the forces acting on it. A fractured granite cliff may retreat faster locally than a sheltered, well-cemented sandstone cliff. A rate comparison is meaningful only when these other conditions and the measuring interval are considered.
Alternating layers create complex profiles
Contrasting layers interact because erosion of one bed can remove support from another, and permeability determines where groundwater concentrates.
Differential erosion is unequal removal of materials with different resistance. A weaker mudstone bed can be recessed while a stronger limestone or sandstone bed stands out as a ledge. If a weak lower layer is removed, the overlying strong bed becomes an overhang. It may remain briefly, then collapse along joints. The result can be episodic retreat: apparently stable periods interrupted by large falls rather than smooth annual movement.
Permeable material allows water to pass through connected pores or fractures. An underlying relatively impermeable layer restricts downward flow. Rainwater may collect above the boundary as a perched water table and emerge along a spring line. Increased pore-water pressure reduces frictional resistance between particles or along failure surfaces; additional water can also increase the load. A saturated weak layer can then support a slump or slide. Permeability and mechanical resistance are different properties: a permeable rock is not automatically weak.
In a hypothetical cliff with strong permeable sandstone over weak, less permeable clay, waves erode the clay at the foot while rainfall accumulates above it. Undercutting removes support; groundwater weakens a possible failure surface. The sandstone can collapse or move as part of a larger slide. A cliff profile with ledges, recesses and failed blocks records interacting marine and subaerial processes, rather than the effect of waves alone.
Succession stabilises dunes and marshes
Vegetation can increase sediment storage by slowing movement and holding loose particles together, although storms can overwhelm this protection.
On a sandy shore, dry beach sand is transported inland by wind when a supply is exposed. Obstacles and pioneer plants trap sand, initiating embryo dunes. Species such as marram tolerate burial and use roots and rhizomes to bind sand. Trapped sediment allows a foredune to grow; plants then trap further sand. Inland dunes are generally less exposed to salt spray and burial. As organic matter accumulates and soils develop, a wider range of plants can establish, followed in suitable conditions by scrub or woodland. This succession is an ordered change in a community through time, not a promise that every dune reaches an identical final vegetation.
In a sheltered estuary, fine sediment settles on tidal flats. Pioneer salt-tolerant plants colonise where inundation and sediment conditions permit. Stems slow currents and trap particles; roots bind the substrate. Accretion raises the surface, reducing how often it is flooded, and changes in salinity and drainage permit different plant communities. Vegetated salt marsh therefore combines ecological succession with physical sediment accumulation. Tidal creeks remain important pathways for water and sediment.
There are two useful stabilising mechanisms: roots increase cohesion, and vegetation reduces flow or wind velocity so deposition exceeds removal. Trampling, grazing or vegetation loss can expose loose sand or marsh sediment. Storm scour, drought, insufficient sediment supply and rapid relative sea-level rise can still cause loss. Dunes and marshes are living, changing stores; they do not halt every form of coastal retreat.
Vegetation changes sediment movement, and sediment accumulation changes the conditions for vegetation. This interaction helps build and stabilise dunes and salt marshes.
Observing and measuring coastal change
A field sketch records the arrangement of visible evidence, while repeat geospatial measurements quantify how far a consistently defined coastal feature has moved.
For contrasting cliff and dune or marsh landscapes, make a selective field sketch, not an artistic copy. Record location, date, viewpoint and approximate scale. Mark the cliff top, foot, rock layers, ledges, debris or vegetation zones. Label observations first, then annotate possible processes: “recessed clay layer” is an observation; “preferential erosion of weak clay” is an interpretation. Include evidence against the interpretation where relevant. Work from safe viewpoints and never enter an unstable cliff face or disturb habitats to improve a sketch.
In a GIS, georeference dated maps or aerial images to a common coordinate system and digitise the same feature, such as the cliff edge. Measure displacement along fixed transects approximately perpendicular to the coast. A tidal waterline is an unsuitable substitute for a cliff edge. Check positional accuracy, image scale and whether shadows conceal the edge.
Use consistent distance and time units, for example metres divided by years gives metres per year.
An original illustrative transect shows 12 m of cliff-edge retreat from 2000 to 2020: . It then retreats 4 m from 2020 to 2021: . The 2000–2021 average is , not the unweighted average of 0.60 and 4.0. A storm-year rate and a multi-decade rate describe different temporal scales. Neither proves the next year's rate.
If each mapped cliff edge has about ±1 m positional uncertainty, a conservative displacement uncertainty is up to ±2 m. That is large beside a one-year 4 m change, but proportionately smaller beside 12 m over twenty years. Multiple transects reveal spatial variation; one cliff line averaged across a whole region can conceal hotspots.