Coastal landforms evolve overtime

Sediment cells

Legacy of the last ice age

Rias

Landforms overtime

Evolution of landforms at Westward Ho!

Eustatic vs isostatic

Time periods

10,000 years BP (before present) = Holocene
115,000 - 2,000,000 BP = Pleistocene

We have periods of transgression (warming) and regression (cooling).
We are currently in a transgression period.

1) Temperature decreases, glaciers and ice sheets advance and sea level falls, ecstatically, global (water is stored in ice)
2) Ice sheets thicken and the weight causes the land to lower, isostatically, local
3) Ice melts, sea level rise eustatically
4) continued melting causes pressure release and the land rises isostatially

Scottish Highlands and coastal areas to a lesser extent are rebounding (rising, becoming emergent), +0.5mm/yr
East and south coasts are sinking, this effect is amplified by global warming producing a rise in sea level, -1.5mm/yr

Transgression = sea level rise, more land covered up
Regression = sea level fall, less land covered up

Eustatic changes in sea level

  • worldwide rise or fall in sea level
  • absolute change in volume of water stored in oceans
  • thermal expansion of water in oceans
  • development of mid-ocean ridges
  • changes in volume of water mainly caused by glaciation (sea level falls), or climate warming interglacials (sea level rises)

Isostatic and tectonic changes in sea level

  • localised rise or fall in relative sea level
  • volume of water in ocean does not change
  • relative sea level change caused by 1) isostatic subsidence (sea level seems to rise) or isostatic rebound (sea level seems to fall) of land. main cause is the loading & unloading of mass during glaciation and deglaciation
    2) tectonic movements e.g. faulting

Interglacial, 150,000 years ago

  • warming climate
  • sea level 8m above present
  • cliff retreat, wave erosion processes, storm beach attrition and weathering & mass movements

Glacial period, devensian, 20,000 years ago

  • cooling climate
  • eustatic sea level fall
  • marine terrace, raised beach & abandoned cliff
  • glacial & periglacial processes exposure & weathering of raised beach, soilfluxion, mass movement, soil creep, frost shattered debris in clay, frost 'cracked pebbles', degraded cliff weathering & mass movement of cliff, cryoturbation (folding)

Features of Westward Ho!

  • abandoned cliff
  • raised beach
  • peri-glacial deposits
  • raised wave cut platform (8m)
  • rounded beach cobbles
  • angled frost shattered debris in clay

In North Devon

Submergent

Emergent

Fjords

Raised beach

  • present-day beach below
  • 'new' cliffs
  • raised wave cut platform
  • 'old' cliffs with caves
  • fossil cliffs and caves now 'high and dry' above sea level

Relict landforms

  • marine -> subariel
  • dry rivers

Changes associated with global cooling

  • average atmospheric temp fall causing ice growth & sea level fall
  • thermal contraction of oceans
  • density of water increases causing decreasing sea volumes
  • decreasing storm activity
  • decreasing energy into coastal landscape
  • extension of coastlines
  • exposure of the continental shelf (coastal margin)
  • isostatic loading beneath ice sheets

King's Cave Arran

  • abandoned cliff and sea caves
  • relic wave cut notch
  • raised beach

Morston cliff, Norfolk

  • sandy glacial tills
  • raised beach

Changes associated with global warming

  • average atmospheric rise causing ice melting & sea level rise
  • thermal expansion of ocean
  • density of water decreases causing increasing sea volumes
  • increasing storm activity
  • increasing energy into coastal landscapes
  • retreat of coastlines
  • flooding of the continental shelf (coastal margin)
  • isostatic unloading beneath ice sheets
  • location of erosion change

Drowned (submerged) forests

  • caused by eustatic rises in sea level

Shingle beaches / barrier islands

  • as sea level rises, sediment is swept onshore to form these
  • Chesil beach (barrier island) formed as sea level rose since last glaciation since Flandarian transgression

Rias with major estuaries

  • river valleys are drowned to form these steep sided branching estuaries

Mudflats

  • rivers change base level & deposit their load

Fjords

  • drowing of glacial U shaped valley

Salt marshes

Rejuvenation (emergent)

  • river entry into the sea
  • sea level drops and river cuts into flood plain creating a new flood plain, a waterfall and river bluffs
  • river keeps cutting back and waterfall turns into rapids further back
  • sea level falls, giving regraded profiles (another balance between erosion & deposition) with knick points

Cross-section

  • U-shaped glacial trough
  • water, very deep
  • steeper sided
  • fairly straight and narrow
  • shallower section at mouth called a threshold
  • presence of hanging valleys and waterfalls

river valleys are drowned to form these steep sided branching estuaries

Dalmatian coasts

  • coastal mountain range is transformed into a chain of islands
  • several valleys are submerged and from long, narrow inlets parallel to the coast
  • so flooded it is no longer a ria

Cross-section

  • very wide channel of river
  • broad V-shape valley
  • less deep
  • gentle sloping

drowned glacial valleys

Interlocking spurs were removed by as a U-shaped valley was carved out by the glacier which formerly occupied the valley

Skerries may be seen at the entrance to the fjord if the threshold is especially shallow (rocky islands)

Norfolk legacy

  • melting of Devensian ice sheets deposited tills (deposited on previous ice deposits and the chalk plateau)
  • sub-glacial melt water rivers flowed from West to East depositing unconsolidated sands & gravels (form NE cliffs)
  • river deposited major sands and gravels offshore (aggregated)
  • river rejuvination carved valleys which create low lying areas, now liable to flooding (Norfolk Broads & Sea Pailing)
  • forests became submerged
  • rapid sea level rise drove sand & gravel onshore forming shingle ridges, rias formed in river valleys
  • sea level rise pushed sand bars onshore blocking river forming lagoons
  • little ice age exposes of offshore & initiated sand dune formation
  • climate warming increases energy & rapid erosion continues

Evolution of Norfolk during Flandrian transgression
1) legacy of Devensian Glaciation 20,000BP

  • margin of ice sheet
  • former cliff line halts ice
  • river rejuvenated and incise valleys e.g. Glaven
  • glacial meltwater rivers deposit unconsolidated sands and gravels over the flood plains
  • river valleys formed by downcutting
  • sea level drop 100m
  • exposed continental shelf
  • wind blown sands
    2) 10,000-7,000BP, climate warming and sea level rise
  • forests established
  • rising sea rework sands into offshore bars
  • ice sheets retreat and melt
  • flooded river valleys (rias) form
    3) 10,000-7,000 climate warming and sea level rise
  • rising seas push shingle onshore forming ridges
  • ice sheets retreat and melt
  • river mouths blocked by sand bars, lagoons forms
  • forests are drowned
  • 6000BP Norfolk coastline, established single ridges formed
    4) 7,000 - present BP
  • Ira forms in Yare valley 2500-1600BP
  • 600BP shingle ridges e.g. Scott Head Island
  • little ice age c.500BP expose sands and intimated dune formation on ridges and barrier islands formation 500BP
  • estuaries fill with sediment 300BP
  • spits and salt marshes initiate 300BP
  • erosion of soft unconsolidated cliffs cause loss of many settlements

Sediment / littoral cells

  • a stretch of coastline within which sediment is sourced, transported & deposited, a closed system
  • the cell is essentially closed to sediment from other cells but some transfer may take place, the cell is bounded by coastal features i.e. headlands or different longshore drift directions

Sediment cell components

INPUTS - source
inputs from within the cell

  • eroding cliffs
  • eroding depositional features
  • e.g. beaches
  • river / estuaries sediment
  • offshore bars

The concept of sediment cells

  • tides & currents transport material along the UK coastline
  • the coastline is divided up into a series of sediment cells (as a management strategy)
  • each cell is bounded by a sediment sink into which sediment sink is transported
  • in the centre of the cell sediment is being removed or stored

TRANSFER PROCESS - transported
sediment transported within the cell

  • longshore drift
  • tidal
  • currents

SINK - deposited
sediment is deposited in sinks (long term stores). The sediment may well remain geographically part of the cell but not interact with longshore drift currents & waves is could be termed an OUTPUT

  • depositional features e.g. beaches, bar, dunes
  • submarine canyons

Weybourne to Sherringham cliffs

  • weakly consolidated (easily eroded)
  • sands, gravels and clays (Devensian glacial deposits)
  • chalk resistant rock at bottom of cliff
  • differential erosion
  • no scree
  • well sorted beach (below cliffs)