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Sunday, 19 December 2010

Understanding the Downland Landscape

My next project is to try and understabnd why there are substantial landscape / landform / soil / hydrology differences between the Downland landscape of Salisbury Plain and the chalk downlands of Eastern England.  To start with, I found this fascinating summary of the Eastern England situation.  More to follow!


Pics:  above, Dunstable Downs, and below, Therfield Heath


http://www.geo-east.org.uk/special_projects/landscapes.htm
CHALK EAST
Valuing Chalk landscape and geodiversity in the East of England

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The Chalk Belt

The Chalk outcrops at the surface in the 'chalk belt' running from the Chiltern Hills to the north Norfolk coast, and also in places where rivers have cut down through superficial 'drift' deposits (e.g. the Glaven, Wensum, Yare, Gipping, Stour and Stort valleys). It also outcrops in the Grays area of south Essex, where some of the Chalk of Kent is up-folded north of the river Thames.
Away from its outcrop, the influence of the Chalk may be seen in the form of chalky glacial deposits. Chalk is a major component of the tills laid down by ice sheets during the Anglian glaciation some 450,000 years ago. Flints, lumps of chalk and associated fossils are found throughout the 'chalky boulder clay' which mantles many parts of the region. 


Understanding the Chalk

The Chalk has many variations in rock type. It includes clay-rich horizons, known as marl-bands, and hardgrounds, which are horizons of harder Chalk which developed during pauses of sedimentation on the sea floor; it also includes bands of flint nodules, which formed from a silica-rich ooze on the sea floor.

The Lower Chalk is typically grey in colour due to a high proportion of clay, and has many marl-bands, whereas the Upper Chalk is typically white and has a higher proportion of flint. The most significant bands of harder rock are named, for example the Totternhoe Stone and Melbourn Rock. Alternating layers of softer and harder chalk have influenced the shape of the region's landscape.

The Chalk has been divided up into a number of time horizons or ‘zones’ according to the characteristic fossils which occur in them. Using these zonal fossils it is possible to follow the continuity of Chalk strata as they outcrop across the country. Hardgrounds, marl bands and flint bands are also useful for correlating strata across wide areas.

  
The Chalk since the Cretaceous

The Chalk has undergone many changes since it was laid down horizontally on the Cretaceous seafloor. In early Tertiary times (45 to 60 million years ago), in places many of its uppermost layers were eroded and then covered by a succession of marine sands and clays, including the Reading Beds and London Clay. In the mid-Tertiary (about 30 million years ago) it was uplifted and folded as part of the same Earth movements which formed the Alps; later it was tilted towards the deepening basin of the North Sea.

In the later Tertiary, Britain was part of a continental land-mass, and underwent steady erosion which uncovered the Chalk from beneath later sediments, and in places removed further layers. The Earth’s climate progressively cooled at the end of this time, ushering in the ice ages of the Pleistocene period during which the Chalk was directly eroded by ice sheets in most parts of the region, and its surface layer was disturbed by frost action. Today the Chalk is                  mantled by a veneer of superficial 'drift' deposits such as glacial boulder clay and erosional residues such as clay-with-flints.


Chalk landscape, landforms and soils

The Chalk is the 'backbone' of the physical landscape in the East of England. It forms a ‘chalk belt’ which crosses the region, including the northern Chiltern hills and their north-eastwards extension as the East Anglian Heights, and continuing through West Suffolk into Breckland and West Norfolk. It forms some of the most beautiful and inspiring landscapes in the region


The North Chilterns  

The chalk hills of the Chilterns are designated as an Area of Outstanding Natural Beauty, and form the highest ground in the region (up to 244m (802ft) near Tring). Physical features include:        
 • a prominent escarpment with steps and platforms (as seen at Dunstable, Totternhoe, Sundon,
        Sharpenhoe, Hexton);
      • major valleys with chalk rivers draining south-eastwards (such as the Bulbourne, Gade and Ver);
      • a plateau capped with deposits of clay-with-flints (the tops are often wooded);
      • an extensive dip slope with many dry valleys;
      • seasonal or intermittent streams, known as bournes.
This is an ancient landscape; the drainage pattern may have taken shape over a million years ago.

For more information see the Chilterns National Character Area and Chilterns Natural Area profiles.


The East Anglian Chalk  

The chalk escarpment of the Chilterns continues north-eastwards through Cambridgeshire, Hertfordshire, Essex and Suffolk and becomes the East Anglian Heights. Is highest point is 147m (482ft) at Chrishall, Essex. Physical features include:
    • a distinctive open, rolling landscape fronted by gentle, stepped escarpments (as seen at Ashwell, Therfield Heath, Heydon, Wandlebury);
    • shallow dry valleys (as at Kelshall, Heydon);
    • spring-lines where water emerges from the chalk aquifer at impermeable layers, such as the Melbourn Rock (as at Ashwell, Fowlmere, Fulbourn, Chippenham);
    • an extensive dip slope mantled with glacial till and an extensive dip slope mantled with glacial till and dissected by valleys with streams which may cut down to chalk bedrock;
    • seasonal streams (for example the River Kennett at Moulton).
The Anglian glaciation has played an important role in the shaping this landscape: its ice sheets eroded and surmounted the escarpment, draping it and the dip-slope with till ('boulder clay'), breaching it to form through-valleys such as the River Cam, and progressively lowering the escarpment towards the north-east.

For more information see the East Anglian Chalk National Character Area and East Anglian Chalk Natural Area profiles.


Breckland and West Norfolk  

The 'chalk belt' continues through West Suffolk into Breckland and West Norfolk, although it is often disguised by Ice Age glacial deposits. 500,000 years ago there were probably chalk hills here as high as the Chilterns today, but 450,000 years ago the Anglian ice sheets flowed over them, eroded and covered them with till in many places. However the influence of the Chalk on the landscape can be seen in subtle ways. Physical features include:
    • low, rolling uplands, often mantled by a veneer of glacial till (as seen at Swaffham, Great Massingham, Docking);
    • periglacial patterned ground, where frost acting on varying chalky and sandy subsoils produced soil stripes and polygons (as at Brettenham Heath, Grimes Graves, Brancaster);
    • chalk streams, where rivers are supplied by chalk springs or run directly over chalk bedrock (for example the rivers Nar, Wissey, Thet);
    • Breckland meres, where the Chalk is an aquifer for a group of natural lakes in Breckland with a distinct seasonal recharge regime (as at Langmere, Ringmere, Fowlmere);
    • periglacial pingo ponds and palsas, where frost acting on springs in the Chalk bedrock during the Ice Age has given rise to clusters of ramparted ponds (as at Thompson, Foulden and East Walton Commons);
    • dolines, basin-shaped depressions caused by solution and collapse of the Chalk bedrock (the Devil's Punchbowl at Croxton is a good example).

 For more information see the Breckland National Character Area and Breckland Natural Area profiles; also the North West Norfolk National Character Area and North Norfolk Natural Area profiles.  


Other Chalk landscapes in the East of England  

The Chalk influences landscapes and landforms in other parts of the region, where it lies close to the surface beneath younger strata and 'drift' deposits. For example:
    • valleys may have cut down through younger deposits to expose chalk bedrock in the middle sections of their course, as in the Wensum and Gipping  valleys;
    • chalk springs may bubble up in valleys, as along the Glaven valley (Norfolk);
    • solution hollows (dolines) may form where solution and collapse of the Chalk has taken place beneath thin superficial deposits; these may take the form of circular depressions in farmland;
    • solution hollows close to valley sides may have become enlarged by thermokarst processes in the Ice Age to form natural lakes; examples include Quidenham Mere (Norfolk) and Bosmere (Suffolk);
    • streams may be absorbed into swallow-holes where overlying deposits become thin, as at Water End (Herts), or where they flow off boulder clay, as  at Coddenham (Suffolk).


Soils in the Chalk landscape

Soils result from weathering and biological processes operating on the underlying geological parent material. In the 'chalk belt' the chalk bedrock has played a formative role, along with various superficial 'drift' deposits. It is typically free-draining, except where it has clay-rich horizons, or is covered with clayey 'drift'.


Soils in the 'chalk belt' include:

    • rendzinas - shallow, silty, grey, well-drained soils; formed over chalk bedrock on escarpments and hillsides;
    • calcareous brown earths - loamy, well-drained, chalk-rich soils; formed over chalky material on gently undulating land;
    • palaeo-argillic brown earths - fine silty or loamy soils with flints and a reddened subsoil; formed over plateau drift, clay-with-flints or glacio-fluvial drift and till.
In the Breckland area, chalky sub-soils are covered with a sandy layer of glacio-fluvial and aeolian drift. Their interaction under periglacial conditions during the Ice Age has produced patterned ground of stripes and polygons in many places.

4 comments:

Constantinos Ragazas said...

Brian,

I keep coming back to this post again and again because something captivates me in these landscape photos. The contrast is stark. I assume the top photo is from the eastern downlands while the one bellow that is from the western downlands somewhere. If that is so, than perhaps these strikingly different landscapes may be telling us something very significant. I have been thinking about this for days now and this is what I conclude.

The top photo shows the effects on a landscape left by water rushing over it. I have seen such landscape before in the lake basin where my hometown is situated. Rushing water will carry with it most all of the topsoil leaving behind just the bedrock. Vegetation takes much longer to start growing and the topsoil collects in low places and crevices where it is more green, as this photo shows.

The bottom photo I believe shows the effects on a landscape left by the melting of a 'local ice' cover slowly over a period of time. The effect will be gradual and less severe than that of rushing water draining over the land. The topsoil will erode less but more uniformly throughout the area and vegetation will grow evenly and uniformly throughout, as this photo shows.

What is your explanation of this difference?

Kostas

BRIAN JOHN said...

Sorry Kostas -- if you look, at the text it's quite clear that both these landscapes are in the EASTERN chalklands. The top one is the chalk scarp and the lower one shows typical rolling downland scenery. Nothing to do with either rushing water or ice...

Constantinos Ragazas said...

Brian,

Thanks for correcting me. I had the geography of these photos wrong. I was mislead by your following quote:

“My next project is to try and understand why there are substantial landscape / landform / soil / hydrology differences between the Downland landscape of Salisbury Plain and the chalk downlands of Eastern England.“

What is your explanation for how such landscape differences may have occurred?

Happy New Year!

Kostas

BRIAN JOHN said...

Most of the chalk landforms can be found in all of the chalk landscapes -- rolling dry downlands, dry valleys or combes, solutional or collapse hollows, and the escarpment itself. I think most of the big differences are down to structure and bedrock lithology -- the chalk beds dip gently down towards the SE, but there are some other structures related to Alpine earth movements. Salisbury Plain is unusual -- a "high chalk plateau" relatively far to the west. I'm trying to dig up more info on the slope deposits and soils.