Raised beaches and estuaries 2011Presentation Transcript
Britain’s Atlantis under the North SeaJonathan Leake and Joanna CarpenterThe Times, 2007SCIENTISTS studying the North Sea have found the remnants of a lost landscape, complete with humansettlements, under up to 450ft of water.They have mapped lakes, hills, salt marshes, coastlines and rivers, all now covered in water and silt butwhich were once the homes and hunting grounds of early modern humans.It was inundated more than 5,000 years ago as the ice melted after the last ice age, raising sea levels at arate that some scientists say will now happen again because of climate change.“What is emerging from our research is a prehistoric landscape larger than Britain itself,” said ProfessorVince Gaffney of the Institute of Archaeology and Antiquity at Birmingham University, who led theresearch.He will reveal details of his findings at the British Association’s Festival of Science in York next week. Hewill describe how his mapping project has found the remains of a great lake, known as the Outer SilverPit, lying 100 miles east of what is now the mouth of the River Humber in Yorkshire. The lake drainedwhat were then the greatest rivers in northern Europe, including the precursors of the Ouse, the Tweedand the Elbe.Just to the east of the lake lay the rolling Dogger Hills - now submerged but which have become thefoundations for the notorious Dogger Bank sandbank. They also inspired the archeologists’ name for thelost landscape: Doggerland.Such features emerged from seismic data collected by oil companies hunting reserves of oil and gas inthe North Sea.Gaffney and his colleagues realised that although the surveys had been designed to study rock stratahundreds or thousands of feet below the seabed, the same data could be used to look at the upper layerstoo.“The coasts, rivers, marshes and hills we found were, for thousands of years, parts of a landscape thatwould have been familiar to hundreds of thousands of people and countless species of animals,” saidGaffney. “Now it is all gone.”http://www.timesonline.co.uk/tol/news/science/article2368630.ece http://www.nature.com/news/2008/080709/full/454151a.html?s=news_rss
Long – term Sea Level Change Sea levels have been fairly static since the last ice age but many changes took place during the Quaternary glaciation that reflected both advance and retreat of ice. A typical sequence would run as follows. For each stage, what effect would there be on sea level? What landforms might be created?
Stage 1 – The climate begins to get colder, marking the onset of a new glacial period, increasing the amount of precipitation falling as snow. Eventually this snow turns into glacial ice which acts as a store for water and slows down the hydrological cycle.
Stage 2: The weight of ice causes the land surface to sink. Such a movement is said to be isostatic and it moderates the eustatic sea level change in some areas. Stage 3: The climate gets warmer. Eventually ice masses on the land begins to melt. This starts to replenish the main store and sea levels begin to rise
Stage 4: As the ice is removed from some land areas they begin to move back up to their previous levels (isostatic readjustment). If the isostatic movement is faster than the eustatic, emergent features are produced such as raised beaches.
RAISED BEACHES – ISOSTATIC UPLIFT(i.e. the localised change in sea level,relative to the land)There are many examples of these featuresthroughout Britain, particularly along theWest coast - this is because the areaexperienced the greatest weight of iceduring the last Ice Age (about 10,000 yearsago).During an Ice Age, the massive weight of icebearing down on a landmass caused it tosink. Over time, as the earths temperaturerose and the weight of ice decreased, areasof land began to slowly rise back out of thesea. http://www.bbc.co.uk/scotland/education/geog/coastline/standard/physical/features/sea_level/?topic=raisedbeach
Eustatic sea level change (i.e. worldwide sea level change)The global sea level has fluctuated widely in the recent geologicalpast. It stood 4-6 meters above the present during the lastinterglacial period, 125,000 years ago, but was 120 m lower at thepeak of the last ice age, around 20,000 years ago. http://www.giss.nasa.gov/research/briefs/gornitz_09/
An estuary is ‘the area of a river mouth which is affected by sea tides’. TheRiver Fowey Estuary in South West England was created by eustatic sea level change.
Raised beaches When sea levels fall, more of the coastline is revealed. Beaches, no longer combed by wave are left stranded and exposed above the new sea level. These features are called raised beaches. The lower part will often show signs of marine erosion, and a small cliff may be formed. Any former cliff-line is also left stranded. No longer undercut by the sea if becomes a degraded cliff. Over time it will often become clothed in shrubs and bushes – a sure sign of physical inactivity.
Raised Beaches Common around the coasts of western Scotland where three levels have been recognised at 8m, 15m and 30m. Differential uplift? On the west of the Isle of Arran there is a raised coastline with relict cliffs, arches, stacks and caves including the King’s Cave (see next slide). The beach is around 4 or 5m above present sea level. This suggests that the sea has fallen to its present level but we know that sea levels have raised considerable since the end of the last ice age. So how did they get to their present level?
Scotland: Caves, formed by the action of thesea, now exist above sea level on a raisedbeach Norway: Raised wave-cut platform
Tectonic Activity raised beaches – e.g Turakirae Head, New ZealandA series of storm beach ridges at Turakirae Head, near Wellington, indicate successiveuplift of the southern end of the Rimutaka Range during major earthquakes. The ridge veryclose to the shore is the present storm beach ridge. Next inland is the ridge that was raised6.4 metres in 1855. Further inland is a ridge dated at about 2,300 years ago, and the faintridge near the base of the hills is dated at about 5,000 years ago. http://www.teara.govt.nz/en/historic-earthquakes/3/5
EstuariesAn estuary is ‘thearea of a rivermouth which isaffected by seatides’. TheRiver FoweyEstuary in SouthWest Englandwas created byeustatic sea levelchange.
Estuary Formation HOW DOES THIS DIFFER FROM A DELTA? The Lower Course of a river valley has been RAISED VALLEY SIDES DROWNED by a rise in sea level or a fall in the USING YOUR ATLAS land level. FIND AN ESTUARY (LOOK AT THE RIVER THAMES)
Estuaries The effect of a relative rise in sea level is to flood the coast. Deltas, spits and beaches will all disappear under water. Broad river estuaries known as rias are formed as the flood plains are inundated. They frequently have a number of branches as tributary valleys are also flooded. The land often rises steeply directly from the water’s edge, which may be marked by a small cliff feature. If the land has also been glaciated a fjord is formed.
Chesapeake Bay Estuary in the United States is an example of a drowned rivervalley (known as a ‘Ria’) which has formed an estuary.