What controls relief?

The difference in height at different places on the world's surface is referred to as 'relief': mountainous areas are found to the north and west of England while more hilly areas are in the south and east of the country. Other parts of the country are broad lowland areas of subdued relief. But why are there these different areas of relief in the first place?

Geology and relief

The map has the main types of rocks in Britan added. The 'relief opacity slider' will show the relief as well as the rock type, and should help you to see the relationship between relief and the underlying geology. Some rocks are hard and resistant to weathering and erosion while others are softer and easily worn away. Where a hard rock like sandstone sits next to a soft rock such as clay, the first will form the uplands and the second will form the lowlands.

The hardness of the rock is only part of the answer: the permeability of the rock and their structure also play a role.

Geology vs relief map

 

Relief map

Sedimentary rocks

  CENOZOIC (Tertiary and marine Pleistocene (Pleistocene glacial drift not shown))

  MESOZOIC (Cretaceous. Chalk)

  MESOZOIC (Cretaceous. Clays and sandstones)

  MESOZOIC (Jurassic. Limestone)

  MESOZOIC (Jurassic. Mainly clays)

  MESOZOIC (Triassic. Mudstones, sandstones and conglomerates)

  UPPER PALAEOZOIC (Permian. Mainly limestones with marls and sandstones)

  UPPER PALAEOZOIC (Carboniferous. Sandstones, shales and coal seams)

  UPPER PALAEOZOIC (Carboniferous. Limestone)

  UPPER PALAEOZOIC (Devonian. Sandstones ('Old Red Sandstone'), shales, conglomerates slates and limestones)

  LOWER PALAEOZOIC (Silurian. Shales, mudstones, greywackes, some limestones)

  LOWER PALAEOZOIC (Ordovician. Mainly shales and mudstones)

  LOWER PALAEOZOIC (Cambrian. Mainly shales, slate and sandstones)

  LATE PRECAMBRIAN (Precambrian. Mainly sandstones, conglomerates and siltstones)

Igneous rocks

  INTRUSIVE IGNEOUS ROCK

  EXTRUSIVE IGNEOUS ROCK (volcanic)

Effects of hardness and permeability on relief

Click on the camera icons for more information.

Example locations of different types of relief The Cheviot Hills near Alwinton, Northumberland Whin Sill at High Force, Teeside. Murton Pike, Cumbria Shap, Cumbria North York Moors. Selwicks Bay at Flamborough Head. Gibb Tor, south-west of Buxton. Nettleton Dale, Lincolnshire Burway Hill, Long Mynd Charnwood Forest, Leicestershire. Church Stretton. Titterstone Clee Hill, Shropshire Malvern Hills Cleeve Cloud Gloucestershire Cressage Bridge, The Wrekin M40 near Lewknor, Oxfordshire Albury Downs, North Downs. The Quantock Hills near Bicknoller Exmoor Video of cross section Devil's Dyke, near Brighton, South Downs Bodmin Moor East Okement Farm, Dartmoor

What effect does the hardness of the rocks have on relief?

red camera Igneous rocks

Igneous rocks can be very hard. Granite, for example, is very resistant to weathering and erosion, and so forms the high rugged landscape in Bodmin Moor, Dartmoor and Shap. Granite is eventually broken down by chemical weathering.

Dolerite is another kind of igneous rock. An intrusion of this rock makes the Whin Sill, which forms high, steep sided hills in northern England. Hadrian’s Wall, which separated the Roman Empire from the Picts to the north, follows these steep hills.

In northern England, close to the Scottish Southern Uplands, the Cheviot Hills are composed of a great thickness of hard, ancient lavas and intrusions of igneous rocks, covered by a thick layer of peat. The Wrekin, near the Welsh borders is also made of hard volcanic rocks.

Igneous rocks in the Lake District vary from granite, which forms deep underground, to ancient lavas and ash ejected from volcanoes.

purple camera Metamorphic rocks

Metamorphic rocks are 'changed rocks'. Rocks that are buried deep within the crust or squeezed under Earth's movements and at plate boundaries become hard. For example, clay can be squeezed intensely and changed into slate. The clay would have been soft and susceptible to erosion, but after changing to slate, it becomes hard and resistant to erosion. Slate forms the mountains of the Lake District.

Other examples of hard metamorphic rocks can be seen in the Long Mynd, Shropshire, where Precambrian muds and sands have become slates and quartzites, and in Charnwood Forest in Leicestershire, where ancient Precambrian slates (as well as volcanic rocks) stick up above the much younger Triassic mudstones. Mudstones are both soft and impermeable so the rivers and streams that flow over them cause rapid erosion. The hard Precambrian rock stands up as hills and the mudstones form the lowlands around them.

Precambrian gneiss, a hard, highly metamorphosed rock, forms the Malvern Hills.

gold camera Sedimentary rocks

Sedimentary rocks may be hard enough to form hills and mountains. Some sedimentary rocks such as the sandstones and grits of the Quantock Hills and Clee Hills are very hard and resist weathering and erosion. Carboniferous sandstones and grits in parts of the southern Pennines (e.g. the Forest of Bowland, Kinder Scout and Ilkley Moor) form high exposed moorland areas. Exmoor is another example where hard sandstones form high hills.

Limestones also make low hills where they sit alongside mudstones. The Cotswolds, for example, are hard limestones rising above Triassic and Jurassic mudstones. The limestone makes a long chain of hills and the softer and easily eroded mudstones form the lowlands. Near Wenlock, Silurian limestone ridges are separated by vales made of less resistant mudstone. Finally, the North York Moors comprise hard Jurassic limestone that rises up above the softer mudstone of the Vale of York and Vale of Pickering.

green camera What effect does permeability have on relief?

The permeability of a rock is important because even fairly soft rocks can form hills if water is not able to flow over them easily. Most of the chalk in southern England, for example, is not very hard, but rainfall percolates through it very rapidly and water is not able to flow over its surface. This is why the chalk is not easily eroded and forms chains of hills. Many of the hills in southern England (like the North Downs, South Downs and the Chiltern Hills) and in eastern England (e.g. the Lincolnshire Wolds and the Yorkshire Wolds) are made of chalk.

Clays, such as those found in the Weald, the Vale of Belvoir, Trent Valley and the Vale of Aylesbury are impermeable. Rainwater cannot sink through, so it flows over the surface as streams and rivers, causing erosion. Not only is the clay worn away because it is soft, but it is worn away even more rapidly because it is impermeable too.

What effect does structure have on relief?

Sometimes the structure of the rocks is important in determining relief. Between about 15 and 35 million years ago, when the African Plate pushed against the European Plate, a high mountain chain, the Alps, was thrown up in France, Switzerland, Austria and Italy. This 'Alpine' phase of mountain building did not make large mountains in Britain (the colliding plates were too far away), but it did buckle the chalk. The North and South Downs originally formed one sheet of chalk that stretched across south-eastern England. However, Alpine mountain building threw it up into a large anticline. The weakened chalk in the anticline was eroded away, leaving two chains of hills on its sides (the North and South Downs).

The mountain building also buckled the area where London is today downward in the form of a syncline. Although the chalk can be seen in the hills that form the two sides of the syncline (the Chiltern Hills and North Downs), it is now hidden many metres below London.

The line marked A to B on the map crosses the folds descibed above, from the Chiltern Hills in the north to the South Downs in the south. Move your mouse over the line and click to activate a video animation showing how the folds were formed and how they influence the relief in the south-east of England.