2B.2B Concordant and Discordant Coast Morphology

Geological Structure influences coastal morphology: Dalmatian and Haff type concordant coasts and headlands and bays on Discordant coasts. 

Morphology is the shape of landscape features, and is influenced by geological structure (headlands and bays for discordant, Dalmatian and Haff for concordant).

​Concordant Coasts

Concordant coasts form where geological structure is such that different rock strata or folds are aligned parallel to the coastline. A combination of geological structure and sea level riseproduces the morphology of landforms aligned parallel to the coastline. 

South Dorset Coast

• A concordant coastline with resistant Portland Limestone forming a protective stratum parallel to the sea. 
• Less resistant Purbeck limestone and Wealden Clay lie behind the Portland, with resistant chalk further north.
• Portland limestone erodes very slowly, retreating landwards by marine undercutting and collapse to form a straight W-E coastline. 
• At points where the Portland is weaker, erosion has broken through and then rapidly eroded out the softer strata laterally, creating a series of coves, e.g. Lulworth Cove and Stair Hole, with narrow openings, widening laterally parallel to the coast. 
• In places such as Worbarrow Bay and St Oswald's Bay, lateral widening of coves led to them joining into a single bay, with remnants of the outer Portland left as a line of stumps parallel to the coast, e.g. Bull's Head in St Oswald's Bay.
• Straight coastline now formed by a concordant band of constant chalk. 

Dalmatian Coast of Croatia

• On the Adriatic Sea
• A concordant coastline produced by the geological structure of folds parallel to the coast. 
• Tectonic forces produced by the collision of African and Eurasian plates compressed Carboniferous Limestone during the Alpine Orogeny 50 million years ago. 
• Created up folded ridges (anticlines) and down folded valleys (synclines) aligned parallel to the coast. 
• Sea level rise at the end of the Devensian Glacial overtopped the low points of the anticlines and the sea flooded synclines. 
• This produces lines of narrow islands parallel to the coast formed by projecting sections of anticlines. 
• Lines of islands separated by narrow sea channels parallel to the coast (sounds)

Haff Coastlines

• These form where deposition produces unconsolidated geological structures parallel to the coastline.
• During the Devensian glacial the sea level was about 100 m lower than today as water was retained in huge ice sheets. 
• Meltwater rivers on land beyond the ice front deposited thick layers of sand and gravels onto outwash plains (sandurs)
• In the Holocene Interglacial constructive waves pushed the ride of sands and gravel landwards as sea levels rose. 
• Sand ridge formed bars across some bays and river mouths, with trapped river water forming a lagoon behind (callled haffs in Poland on the Baltic Sea)
• For example the Neman Haff behind the bar running from the Kaliningrad in Russia to the Lithuanian coast at the mouth of the river Neman. 
• Chesil Beach in Dorset was formed this way. Shingle ridge reconnected island of Portland Bill to land (a tombolo)

Might be needed:

• Spits were formed in river mouths where the river was powerful enough to break through the sand ridge to the sea.
• Paired spits formed parallel to the coast with haff lagoon behind at the mouth of the River Vistula in Gdansk. 
• At the mouth of the River Oder near Szczecin, Stettiner Haff formed where sand ridge connected islands, leaving river to emerge through three narrow outlets. 
• Elsewhere a ridge of sand forms a line of barrier islands, sand banks and mud flats parallel to the coast, e..g Friesian Islands off the North Sea coast of the Netherlands, Germany and western Denmark.   

​Discordant Coasts

Discordant coasts forms where geological structure is such that different rock strata of folds are aligned at an angle to the coastline. 

• Rock strata that are less resistant (due to the rock unit's lithology and structure) erode rapidly to form indented bays.
• More resistant strata erode only slowly, and are left projecting into the sea as headlands. 
• The relative resistance of rock types influences the degree of indentation of bays.

The morphology of discordant coasts alters the distribution of wave energy and rate of erosion through wave refraction. Where the wind is blowing directly onshore and the wave front is parallel to the coastline, the section of wave approaching the headland will encounter shallow water before the wave front approaching the indented bay. The waves approaching the headland slow and wave height increases. The wave front refracts, becoming curved. Convex in bays, dispersing energy, and concave at headlands, concentrating energy. Refraction increases the rate of erosion at headlands and reduces it at bays, generally decreasing the degree of indentation. 

Swanage Bay

• on the Isle of Purbeck in East Dorset
• formed by the erosion of less resistant Wealden Clays
• More resistant Jurassic Portland Limestone forms the Peveril Point headland to the south, projecting out by 1 km. 
• Resistant Cretaceous Chalk forms the Foreland headland, projecting 2.5 km to the north.
• However, structure is not the only factor influencing the indentation of Swanage Bay, since Swanage bay faces east, and is sheltered from the prevailing south westerly wind and highest energy waves. 

 
Bantry Bay

• In Cork in the south west of the Republic of Ireland
• Formed from less resistant Carboniferous Limestone
• Beara Peninsula to the north formed from more resistant Devonian Old Red Sandstone and projects 35 km into the Atlantic Ocean. 
• Sheep's Head Peninsula to the south formed from more resistant coarse sandstone, projects out 21 km. 
• The high degree of indentation of Bantry Bay is not solely influenced by the relative resistance of rock types, but also the orientation of strata SW-NE means that they directly face high energy Atlantic waves driven by the prevailing SW wind. 
• The Bay is also a product of sea level rise, since river erosion cut a low-relief river valley into Carboniferous Limestone, allowing the sea to flood inland and creating a ria at the start of the Holocene.